MAX16826ATJ+T - Maxim Integrated - Datasheet.Directory

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

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General Description

The MAX16826 high-brightness LED (HB LED) driver is

designed for backlighting automotive LCD displays and

other display applications such as industrial or desktop

monitors and LCD televisions. The MAX16826 integrates

a switching regulator controller, a 4-channel linear cur-

rent sink driver, an analog-to-digital converter (ADC),

and an I2C interface. The IC is designed to withstand

automotive load dump transients up to 40V and can

operate under cold crank conditions.

The MAX16826 contains a current-mode PWM switching

regulator controller that regulates the output voltage to

the LED array. The switching regulator section is config-

urable as a boost or SEPIC converter and its switching

frequency is programmable from 100kHz to 1MHz.

The MAX16826 includes 4 channels of programmable,

fault-protected, constant-current sink driver controllers

that are able to drive all white, RGB, or RGB plus amber

LED configurations. LED dimming control for each chan-

nel is implemented by direct PWM signals for each of the

four linear current sinks. An internal ADC measures the

drain voltage of the external driver transistors and the

output of the switching regulator. These measurements

are then made available through the I2C interface to an

external microcontroller (µC) to enable output voltage

optimization and fault monitoring of the LEDs.

The amplitude of the LED current in each linear current-

sink channel and the switch-mode regulator output volt-

age is programmed using the I2C interface. Additional

features include: cycle-by-cycle current limit, shorted

LED string protection, and overtemperature protection.

The MAX16826 is available in a thermally enhanced,

5mm x 5mm, 32-pin thin QFN package and is specified

over the automotive -40°C to +125°C temperature range.

Applications

LCD Backlighting:

Automotive Infotainment Displays

Automotive Cluster Displays

Industrial and Desktop Monitors

LCD TVs

Automotive Lighting:

Adaptive Front Lighting

Low- and High-Beam Assemblies

Benefits and Features

•4 Channels of Programmable, Fault-Protected,

Constant-Current Source-Driver Controllers for All

White, RGB, or RGB Plus Amber LED Configurations

•Drives One to Four LED Strings

•Individual PWM Dimming Inputs per String

•Very Wide Dimming Range

•Configurable as a boost or SEPIC Converter with

Programmable 100kHz to 1MHz Switching Frequency

for Design Flexibility

•Integrated Boost/SEPIC Controller

•External MOSFETs Allow Wide-Range LED

Current with Multiple LEDs per String

•200kHz to 2MHz Programmable Switching

Frequency for Optimizing Size vs. Efficiency

•External Switching-Frequency Synchronization

•I2C Allows the Current-Sink Drain Voltages to be

Read to Minimize Power Dissipation and Detect LED

String Fault Conditions

•7-Bit Internal ADC for LED Voltage Monitoring and

Optimization

•Dynamic Adjustment of LED String Currents and

Output Voltage

•Standby Mode

•Protection Features Improve Reliability

•LED Short and Open Detection

•Overvoltage and Overtemperature Protection

Typical Application Circuit and Pin Configuration appear at

end of data sheet.

Simplified Diagram

MAX16826

DR4

DR1

DL1

IN DL

BOOST LED DRIVER

CS FB

GND

VIN

DIM1

DIM2

SDA

SCL

DIM3

DIM4

I2C

INTERFACE

DIMMING

INPUTS

CS1

DL4

CS4

Ordering Information appears at end of data sheet.

19-4047; Rev 7; 3/16

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

Maxim Integrated | 2www.maximintegrated.com

Absolute Maximum Ratings

Electrical Characteristics

(VIN = 12V, R19 = 2kΩ, C33 = 2200pF, R17 = 1.27kΩ, CDL_ = 0.01µF, TJ= -40°C to +125°C, unless otherwise noted. Typical values

are at TA= +25°C.) (Note 1)

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional

operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to

absolute maximum rating conditions for extended periods may affect device reliability.

IN to GND (Continuous) .........................................-0.3V to +30V

IN Peak Current (≤400ms) ...............................................300mA

IN Continuous Current ........................................................50mA

PGND to GND .......................................................-0.3V to +0.3V

All Other Pins to GND...............................................-0.3V to +6V

DL Peak Current (< 100ns)....................................................±3A

DL Continuous Current .....................................................±50mA

DL1, DL2, DL3, DL4 Peak Current ..................................±50mA

DL1, DL2, DL3, DL4 Continuous Current ........................±20mA

VCC Continuous Current .....................................................50mA

All Other Pins Current .......................................................±20mA

Continuous Power Dissipation (TA= +70°C)

32-Pin Thin QFN (derate 34.5mW/°C above +70°C)

Multilayer Board ..........................................................2759mW

Operating Temperature Range .........................-40°C to +125°C

Junction Temperature......................................................+150°C

Storage Temperature Range .............................-65°C to +150°C

Lead Temperature (soldering, 10s) ........…………………+300°C

Soldering Temperature (reflow) .......................................+260°C

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Power-Supply Voltage VIN VSYNC = 3V 4.75 24 V

Quiescent Current IIN DL_ = unconnected; R19, C33 = open 5 10 mA

Shutdown Current IIN,SD VSYNC = 0V 20 75 µA

Standby Current IIN,SB I2C standby activated 3 mA

I2C-COMPATIBLE I/O (SCL, SDA)

Input High Voltage VIH 1.5 V

Input Low Voltage VIL 0.5 V

Input Hysteresis VHYS 25 mV

Input High Leakage Current IIH VLOGIC = 5V -1 +1 µA

Input Low Leakage Current IIL VLOGIC = 0V -1 +1 µA

Input Capacitance CIN 10 pF

Output Low Voltage VOL IOL = 3mA 0.4 V

Output High Current IOH VOH = 5V 1 µA

I2C-COMPATIBLE TIMING

Serial Clock (SCL) Frequency fSCL 400 kHz

BUS Free Time Between STOP

and START Conditions tBUF 1.3 µs

START Condition Hold Time tHD:STA 0.6 µs

STOP Condition Setup Time tSU:STO 0.6 µs

Clock Low Period tLOW 1.3 µs

Clock High Period tHIGH 0.6 µs

Data Setup Time tSU:DAT 0.3 µs

Data In Hold Time tHD:DATIN 0.03 0.9 µs

Data Out Hold Time tHD:DATOUT 0.3 µs

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

Maxim Integrated | 3www.maximintegrated.com

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Maximum Receive SCL/SDA Rise

Time tRCB = 400pF 300 ns

Minimum Receive SCL/SDA Rise

Time tRCB = 400pF 60 ns

Maximum Receive SCL/SDA Fall

Time tFCB = 400pF 300 ns

Minimum Receive SCL/SDA Fall

Time tFCB = 400pF 60 ns

Transmit SDA Fall Time tFCB = 400pF, IO = 3mA 60 250 ns

Pulse Width of Suppressed Spike tSP 50 ns

INTERNAL REGULATORS (IN, VCC)

VCC Output Voltage VVCC

0V < IVCC < 30mA (Note 2),

4.75V < VIN < 24V, DL, DL1 to DL4

unconnected

4.5 5.25 5.65 V

VCC Undervoltage Lockout VVCC_UVLO VCC rising 4.5 V

VCC Undervoltage Lockout

Hysteresis VVCC_HYS 135 175 205 mV

IN Shunt Regulation Voltage IIN = 250mA 24.05 26.0 27.5 V

PWM GATE DRIVER (DL)

Peak Source Current 2A

Peak Sink Current 2A

DL High-Side Driver Resistance IDL = -100mA 2.25 Ω

DL Low-Side Driver Resistance IDL = +100mA 1.30 Ω

Minimum DL Pulse Width 40 ns

PWM CONTROLLER, SOFT-START (FB, COMP, OVP)

FB shorted to COMP; MAX16826 only 1.230 1.250 1.260

FB Voltage Maximum VFB,MAX FB shorted to COMP; MAX16826B only 1.23 1.25 1.27 V

FB shorted to COMP; MAX16826 only 862 876 885

FB Voltage Minimum VFB,MIN FB shorted to COMP; MAX16826B only 735 750 765 mV

FB shorted to COMP; MAX16826 only 2.94

FB Voltage LSB FB shorted to COMP; MAX16826B only 3.9 mV

FB Input Bias Current IFB 0V < VFB < 5.5V -100 0 +100 nA

Feedback-Voltage Line

Regulation

Level to produce VCOMP = 1.25V,

4.5V < VVCC < 5.5V ±0.25 %/V

Soft-Start Current ISS VCSS = 0.5VVCC 3.2 6.0 10.4 µA

OVP Input Bias Current IOVP 0V < VOVP < 5.5V -100 0 +100 nA

Slope Compensation ISLOPE 19 26 32 µA/µs

Electrical Characteristics (continued)

(VIN = 12V, R19 = 2kΩ, C33 = 2200pF, R17 = 1.27kΩ, CDL_ = 0.01µF, TJ= -40°C to +125°C, unless otherwise noted. Typical values

are at TA= +25°C.) (Note 1)

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

Maxim Integrated | 4www.maximintegrated.com

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

ERROR AMPLIFIER (FB, COMP)

Open-Loop Gain AOL 80 dB

Unity-Gain Bandwidth BW 2 MHz

Phase Margin PM 65 Degrees

Sourcing, VCOMP = 3V 1.9

Error-Amplifier Output Current ICOMP Sinking, VCOMP = 2V 0.9 mA

COMP Clamp Voltage VCOMP VFB = 0V 3.25 4.5 V

COMP Short-Circuit Current ICOMP_SC 12 mA

PWM CURRENT LIMIT (CS)

Cycle-by-Cycle Current-Limit

Threshold VCL VDL = 0V 187 200 217 mV

Cycle-by-Cycle Current-Limit

Propagation Time To DL tPROP, CL 10mV overdrive 80 ns

Gross Current-Limit Threshold VGCL VCSS = 0V 250 270 280 mV

Gross Current-Limit Propagation

Time To DL tPROP,GCL 10mV overdrive 80 ns

Input Bias Current 0V < VCS < 5.5V -100 0 +100 nA

PWM OSCILLATOR (RTCT)

RTCT Voltage Ramp (Peak to

Peak) VRAMP 5.5V < VIN < 24V 1.60 1.65 1.80 V

RTCT Voltage Ramp Valley VRAMP_VALLEY 5.5V < VIN < 24V 1.11 1.20 1.27 V

Discharge Current IDIS VRTCT = 2V 7.8 8.4 9.1 mA

Frequency Range fOSC 5.5V < VIN < 24V 100 1000 kHz

SYNCHRONIZATION (SYNC/ENABLE)

Input Rise/Fall Time 200 ns

Input Frequency Range 100 1000 kHz

Input High Voltage 1.5 V

Input Low Voltage 0.5 V

Input Minimum Pulse Width 200 ns

Input Bias Current 0V < VSYNC < 5.5V -100 0 +100 nA

Delay to Shutdown VSYNC = 0V 13 32 65 µs

LED DIMMING (DIM1–DIM4)

Input High Voltage VDIM,MAX 1.5 V

Input Low Voltage VDIM,MIN 0.5 V

Minimum Dimming Frequency fDIM tON = 2µs (Note 3) 45 Hz

Input Bias Current IDIM 0V < VDIM_ < 5.5V -100 0 +100 nA

Electrical Characteristics (continued)

(VIN = 12V, R19 = 2kΩ, C33 = 2200pF, R17 = 1.27kΩ, CDL_ = 0.01µF, TJ= -40°C to +125°C, unless otherwise noted. Typical values

are at TA= +25°C.) (Note 1)

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

Maxim Integrated | 5www.maximintegrated.com

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

ADC (DR1–DR4, OVP)

Maximum Error EMAX ±50 mV

ADC Single Bit Acquisition

Latency (Note 4) 2 µs

DR Channel Sample Time tDR,SMPL 190 ms

OVP Channel Sample Time tOVP,SMPL 20 µs

Full-Scale Input Voltage VFS 1.215 1.24 1.2550 V

Least Significant Bit VLSB 9.76 mV

DR Input Bias Current IDR 0V < VDR_ < 5.5V -100 0 +100 nA

DRAIN FAULT COMPARATORS (DR1–DR4) (Shorted LED String Comparator)

Drain Fault Comparator

Threshold VDFTH Voltage to drive DL1–DL4 low 1.4 1.52 1.63 V

Drain Fault Comparator Delay tDFD 10mV overdrive 1 µs

LINEAR REGULATORS (DL1–DL4, CS1–CS4)

Transconductance Gm ∆I = -500µA 75 mS

Maximum Output Current IDL Sourcing or sinking 15 mA

CS1–CS4 Input Bias Current ICS 0V < VCS < 5.5V -100 0 +100 nA

CS_ = DL_, FB DAC full scale;

MAX16826 only 306 316 324

CS1–CS4 Regulation Voltage

Maximum VCS,MAX CS_ = DL_, FB DAC full scale;

MAX16826B only 308 318 328

CS_ = DL_, FB DAC minus full scale;

MAX16826 only 90 97 105

CS1–CS4 Regulation Voltage

Minimum VCS,MIN CS_ = DL_, FB DAC minus full scale;

MAX16826B only 90 99 109

CS1–CS4 Regulation Voltage LSB VCS,LSB CS_ = DL_, FB DAC 1-bit transition 1.72 mV

Electrical Characteristics (continued)

(VIN = 12V, R19 = 2kΩ, C33 = 2200pF, R17 = 1.27kΩ, CDL_ = 0.01µF, TJ= -40°C to +125°C, unless otherwise noted. Typical values

are at TA= +25°C.) (Note 1)

Note 1: All devices are 100% production tested at TJ= +25°C and TJ= +125°C. Limits to -40°C are guaranteed by design.

Note 2: ICC includes the internal bias currents and the current used by the gate drivers to drive DL, DL1, DL2, DL3, and DL4.

Note 3: Minimum frequency to allow the internal ADC to complete at least one measurement. tON is the on-time with the LED current

in regulation.

Note 4: Minimum LED current pulse duration, which is required to correctly acquire 1 bit.

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

Maxim Integrated | 6www.maximintegrated.com

SUPPLY CURRENT

vs. SUPPLY VOLTAGE

MAX16826 toc01

SUPPLY VOLTAGE (V)

SUPPLY CURRENT (mA)

20164 8 12

024

CDL = 4700pF

SUPPLY CURRENT

vs. OSCILLATOR FREQUENCY

MAX16826 toc02

OSCILLATOR FREQUENCY (kHz)

SUPPLY CURRENT (mA)

900800700600500400300200

100 1000

CDL = 4700pF

C33 FROM 680pF TO 8200pF

SUPPLY CURRENT

vs. TEMPERATURE

MAX16826 toc03

TEMPERATURE (°C)

SUPPLY CURRENT (mA)

110

603510-15

-40

CDL = 4700pF

OSCILLATOR FREQUENCY

vs. SUPPLY VOLTAGE

MAX16826 toc04

SUPPLY VOLTAGE (V)

OSCILLATOR FREQUENCY (kHz)

20.316.612.99.2

310

320

330

340

350

360

300

5.5 24.0

OSCILLATOR FREQUENCY

vs. TEMPERATURE

MAX16826 toc05

TEMPERATURE (°C)

OSCILLATOR FREQUENCY (kHz)

110

3510-15

240

280

320

360

400

200

-40

LED OUTPUT CURRENT

vs. TEMPERATURE

MAX16826 toc06

TEMPERATURE (°C)

LED OUTPUT CURRENT (mA)

80604020

137

139

141

143

145

135

0100

VCS = 0.32V

Typical Operating Characteristics

(VIN = 12V, R19 = 2kΩ, C33 = 2200pF, R17 = 1.27kΩ, CDL_ = 0.01µF. TA= +25°C, unless otherwise noted.)

LED OUTPUT CURRENT

vs. INPUT VOLTAGE

MAX16826 toc07

INPUT VOLTAGE (V)

LED OUTPUT CURRENT (mA)

18126

120

150

024

DIM INPUT TO ILED OUTPUT WAVEFORM

MAX16826 toc08

2µs/div

5V/div

100mA/div

0mA

VDIM

ILED

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

Maxim Integrated | 7www.maximintegrated.com

ENABLE AND DISABLE RESPONSE

MAX16826 toc09

40ms/div

5V/div

100mA/div

0mA

VSYNC/EN

ILED

VCC VOLTAGE

vs. LOAD CURRENT

MAX16826 toc10

LOAD CURRENT (mA)

VCC VOLTAGE (V)

40302010

5.1

5.2

5.3

5.4

5.5

5.0

050

VCC VOLTAGE

vs. TEMPERATURE

MAX16826 toc11

TEMPERATURE (°C)

VCC VOLTAGE (V)

80604020

5.1

5.2

5.3

5.4

5.5

5.0

0100

Typical Operating Characteristics (continued)

(VIN = 12V, R19 = 2kΩ, C33 = 2200pF, R17 = 1.27kΩ, CDL_ = 0.01µF. TA= +25°C, unless otherwise noted.)

VCC VOLTAGE

vs. SUPPLY VOLTAGE

MAX16826 toc12

SUPPLY VOLTAGE (V)

VCC VOLTAGE (V)

20161284

024

SHUNT VOLTAGE

vs. SHUNT CURRENT

MAX16826 toc13

SHUNT CURRENT (mA)

SHUNT VOLTAGE (V)

20015010050

24.5

25.0

25.5

26.0

26.5

27.0

24.0

0250

SHUNT VOLTAGE

vs. TEMPERATURE

MAX16826 toc14

TEMPERATURE (°C)

SHUNT VOLTAGE (V)

11085603510-15

-40

SHUNT REGULATOR LOAD DUMP RESPONSE

MAX16826 toc15

200ms/div

20V/div

10V/div

VSUPPLY

VSHUNT

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

Maxim Integrated | 8www.maximintegrated.com

Pin Description

PIN NAME FUNCTION

1 PGND Power Ground

2, 3 GND Analog Ground

4 RTCT

Timing Resistor and Capacitor Connection. A resistor, R19 (in the Typical Application Circuit), from VCC to

RTCT and a capacitor C33, from RTCT to GND set the oscillator frequency. See the Oscillator section to

calculate RT and CT component values.

5 SYNC/EN

Synchronization and Enable Input. There are three operating modes:

SYNC/EN = LOW: Low current shutdown mode with all circuits shut down except shunt regulator.

SYNC/EN = HIGH: All circuits active with oscillator frequency set by RTCT network.

SYNC/EN = CLOCKED: All circuits active with oscillator frequency set by SYNC clock input. Conversion

cycles initiate on the rising edge of external clock input. The frequency programmed by R19/C33 must be

10% lower than the input SYNC/EN signal frequency.

6 CSS

Soft-Start Timing Capacitor Connection. Connect a capacitor from CSS to GND to program the required soft-

start time for the switching regulator output voltage to reach regulation. See the Soft-Start (CSS) section to

calculate CCSS.

7 COMP Switching Regulator Compensation Component Connection. Connect the compensation network between

COMP and FB.

8FB

Switching Regulator Feedback Input. Connect FB to the center of a resistor-divider connected between the

switching regulator output and GND to set the output voltage. FB is regulated to a voltage set by an internal

9 OVP

Switching Regulator Overvoltage Input. Connect OVP to the center of a resistor-divider connected between the

switching regulator output and GND. For normal operation, configure the resistor-divider so that the voltage at

this pin does not exceed 1.25V. If operation under load dump conditions is also required, configure the resistor-

divider so that the voltage at OVP is less than 1.25V.

10 RSC

Slope Compensation Resistor and PWM Comparator Input Connection. Connect a resistor, R17, from RSC to

the switching current-sense resistor to set the amount of the compensation ramp. See the Slope Compensation

(RSC) section for calculating the value.

11 SDA I2C Serial Data Input/Output

12 SCL I2C Serial Clock Input

13 DIM1

LED String 1 Logic-Level PWM Dimming Input. A high logic level on DIM1 enables the current sink to operate at

the maximum current as determined by its sense resistor and internal register value. A low logic level disables

the current source.

14 DIM2

LED String 2 Logic-Level PWM Dimming Input. A high logic level on DIM2 enables the current sink to operate at

the maximum current as determined by its sense resistor and internal register value. A low logic level disables

the current source.

15 DIM3

LED String 3 Logic-Level PWM Dimming Input. A high logic level on DIM3 enables the current sink to operate at

the maximum current as determined by its sense resistor and internal register value. A low logic level disables

the current source.

16 DIM4

LED String 4 Logic-Level PWM Dimming Input. A high logic level on DIM4 enables the current sink to operate at

the maximum current as determined by its sense resistor and internal register value. A low logic level disables

the current source.

17 CS1 LED String 1 Current-Sense Input. CS1 is regulated to a value set by an internal register. The regulation voltage

can be set between 97mV and 316mV.

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

Maxim Integrated | 9www.maximintegrated.com

Pin Description (continued)

PIN NAME FUNCTION

18 DL1

LED String 1 Linear Current Source Output. DL1 drives the gate of the external FET on LED String 1 and has

approximately 15mA source/sink capability. Connect a minimum capacitor of 4700pF from DL1 to GND to

compensate the internal transconductance amplifier as well as program the rise and fall times of the LED currents.

19 DR1

LED String 1 External FET Drain Voltage Sense. The internal ADC uses this input to measure the drain to GND

voltage of the current sink FET. Drain voltage measurement information can be read back from the I2C

interface. Connect a voltage-divider to scale drain voltage as necessary.

20 CS2 LED String 2 Current-Sense Input. CS2 is regulated to a value set by an internal register. The regulation voltage

can be set between 97mV and 316mV.

21 DL2

LED String 2 Linear Current Source Output. DL2 drives the gate of the external FET on LED String 2 and has

approximately 15mA source/sink capability. Connect a minimum capacitor of 4700pF from DL2 to GND to

compensate the internal transconductance amplifier, as well as program the rise and fall times of the LED

currents.

22 DR2

LED String 2 External FET Drain Voltage Sense. The internal ADC uses this input to measure the drain to GND

voltage of the current sink FET. Drain voltage measurement information can be read back from the I2C

interface. Connect a voltage-divider to scale drain voltage as necessary.

23 CS3 LED String 3 Current-Sense Input. CS3 is regulated to a value set by an internal register. The regulation voltage

can be set between 97mV and 316mV.

24 DL3

LED String 3 Linear Current Source Output. DL3 drives the gate of the external FET on LED String 3 and has

approximately 15mA source/sink capability. Connect a minimum capacitor of 4700pF from DL3 to GND to

compensate the internal transconductance amplifier, as well as program the rise and fall times of the LED currents.

25 DR3

LED String 3 External FET Drain Voltage Sense. The internal ADC uses this input to measure the drain to GND

voltage of the current sink FET. Drain voltage measurement information can be read back from the I2C

interface. Connect a voltage-divider to scale drain voltage as necessary.

26 CS4 LED String 4 Current-Sense Input. CS4 is regulated to a value set by an internal register. The regulation voltage

can be set between 97mV and 316mV.

27 DL4

LED String 4 Linear Current Source Output. DL3 drives the gate of the external FET on LED String 4 and has

approximately 15mA source/sink capability. Connect a minimum capacitor of 4700pF from DL4 to GND to

compensate the internal transconductance amplifier, as well as program the rise and fall times of the LED currents.

28 DR4

LED String 4 External FET Drain Voltage Sense. The internal ADC uses this input to measure the drain to GND

voltage of the current sink FET. Drain voltage measurement information can be read back from the I2C

interface. Connect a voltage-divider to scale drain voltage as necessary.

29 IN

Power Supply. IN is internally connected to a 26V shunt regulator that sinks current. In conjunction with an

external resistor it allows time-limited load dump events as high as 40V to be safely handled by the IC. Bypass

IN to GND with a minimum 10µF capacitor.

30 CS Current-Sense Input

31 VCC

Gate Driver Regulator Output. Bypass VCC to GND with a minimum 4.7µF ceramic capacitor. Gate drive current

pulses come from the capacitor connected to VCC. Place the capacitor as close as possible to VCC. If IN is

32 DL Switching Regulator Gate Driver Output

—EP

Exposed Pad. Connect the exposed pad to the ground plane for heatsinking. Do not use this pad as the only

ground connection to the IC.

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

Maxim Integrated | 10www.maximintegrated.com

Detailed Description

The MAX16826 HB LED driver integrates a switching

regulator controller, a 4-channel linear current sink dri-

ver, a 7-bit ADC, and an I2C interface. The IC is

designed to operate from a 4.75V to 24V input voltage

range and can withstand automotive load dump tran-

sients up to 40V.

The current-mode switching regulator controller is con-

figurable as a boost or SEPIC converter to regulate the

voltage to drive the four strings of HB LEDs. Its program-

mable switching frequency (100kHz to 1MHz) allows the

use of a small inductor and filter capacitors. The four

current sink regulators use independent external current-

sense resistors to provide constant currents for each

string of LEDs. Four DIM inputs allow a very wide range

of independent pulsed dimming to each LED string. An

internal 7-bit ADC measures the drain voltage of the

external driver transistors to enable output voltage opti-

mization and fault monitoring of the LEDs. The

MAX16826 is capable of driving four strings of LEDs.

The number of LEDs in each string is only limited by the

topology of choice, the rating of the external compo-

nents, and the resolution of the ADC and internal DAC.

Simplified Block Diagram

VCC

PGND

OVP

COMP

RTCT

GND

OVT

CURRENT-

MODE

PWM

BLOCK

SYNC/EN

CSS

DR4

7-BIT ADC

AND

SHORTED

STRING

FAULT

DECTECTION

DR3

DR2

DR1

LINEAR

CURRENT-

SINK

DRIVERS

CS4

CS3

CS2

CS1

DL4

DL3

DL2

DL1

I2C

STATE

MACHINE

DOUBLE-

BUFFERED

AND DACS

DIM4

DIM3

OVT

DIM2

DIM1

SDA

SCL

26V

SHUNT

GND

OVT

REF

VCC

GND

RSC

MAX16826

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

Maxim Integrated | 11www.maximintegrated.com

The MAX16826 provides additional flexibility with an

internal I2C serial interface to communicate with a

microcontroller (µC). The interface can be used to

dynamically adjust the amplitude of the LED current in

each LED string and the switch-mode regulator output

voltage. It can also be used to read the ADC drain volt-

age measurements for each string, allowing a µC to

dynamically adjust the output voltage to minimize the

power dissipation in the LED current sink FETs. The I2C

interface can also be used to detect faults such as LED

short or open.

Modes of Operation

The MAX16826 has six modes of operation: normal

mode, undervoltage lockout (UVLO) mode, thermal

shutdown (TSD) mode, shutdown (SHDN) mode,

standby (STBY) mode, and overvoltage protection

(OVP) mode.

The normal mode is the default state where each cur-

rent sink regulator is maintaining a constant current

through each of the LED strings. Digitized voltage feed-

back from the drains of the current sink FETs can be

used to establish a secondary control loop by using an

external µC to control the output of the switching stage

for the purpose of achieving low-power dissipation

across these FETs.

UVLO mode occurs when VVCC goes below 4.3V. In

UVLO mode, each of the linear current sinks and the

switching regulator is shut down until the input voltage

exceeds the rising UVLO threshold.

TSD mode occurs when the die temperature exceeds

the internally set thermal limit (+160°C). In TSD mode,

each of the linear regulators and the switching regulator

is shut down until the die temperature cools by 20°C.

SHDN mode occurs when SYNC/EN is driven low. In

SHDN mode, all internal circuitry with the exception of

the shunt regulator is deactivated to limit current draw

to less than 50µA. SHDN mode disengages when

SYNC/EN is driven high or clocked.

STBY mode is initiated using the I2C interface. In STBY

mode, each of the linear current sinks and the switching

regulator is shut down. STBY mode is also deactivated

using the I2C interface. In STBY mode, the internal VCC

regulator and the shunt regulator remain active. Whenever

the MAX16826 enters a mode that deactivates the switch-

ing regulator, the soft-start capacitor is discharged so that

soft-start occurs upon reactivation.

OVP mode occurs when the voltage at OVP is higher than

the internal reference. In OVP mode, the switching regula-

tor gate-drive output is latched off and can only be

restored by cycling enable, power, or entering standby

mode.

Switching Preregulator Stage

The MAX16826 features a current-mode controller that

is capable of operating in the frequency range of

100kHz to 1MHz. Current-mode control provides fast

response and simplifies loop compensation.

Output voltage regulation can be achieved in a two-

loop configuration. A required conventional control loop

can be set up by using the internal error amplifier with

its inverting input connected to FB. The bandwidth of

this loop is set to be as high as possible utilizing con-

ventional compensation techniques. The noninverting

input of this amplifier is connected to a reference volt-

age that is dynamically adjustable using the I2C inter-

face. The optional slower secondary loop consists of

the external µC using the I2C interface reading out the

voltages at the drains of the current sink FETs and

adjusting the reference voltage for the error amplifier.

To regulate the output voltage, the error amplifier com-

pares the voltage at FB to the internal 1.25V (adjustable

down by using the I2C interface) reference. The output

of the error amplifier is compared to the sum of the cur-

rent-sense signal and the slope compensation ramp at

RSC to control the duty cycle at DL.

Two current-limit comparators also monitor the voltage

across the sense resistor using CS. If the primary cur-

rent-limit threshold is reached, the FET is turned off and

remains off for the reminder of the switching cycle. If

the current through the FET reaches the secondary cur-

rent limit, the switching cycle is terminated and the soft-

start capacitor is discharged. The converter then

restarts in soft-start mode preventing inductor current

runaway due to the delay of the primary cycle-by-cycle

current limit. The switching regulator controller also fea-

tures an overvoltage protection circuit that latches the

gate driver off if the voltage at OVP exceeds the inter-

nal 1.25V reference voltage.

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

Maxim Integrated | 12www.maximintegrated.com

Shunt Regulator

The MAX16826 has an internal 26V (typ) shunt regula-

tor to provide the primary protection against an auto-

motive load dump. When the input voltage is below

26V, the shunt voltage at IN tracks the input voltage.

When the input voltage exceeds 26V, the shunt regula-

tor turns on to sink current, and the voltage at IN is

clamped to 26V. During a load dump, the input voltage

can reach 40V, and the shunt regulator through the

resistor connected to IN is forced to sink large amounts

of current for up to 400ms to limit the voltage that

appears at IN to the shunt regulation voltage. The sink-

ing current of the shunt regulator is limited by the value

of resistor (R1 in Figure 1) in series with IN. There are

two criteria that determine the value of R1: the maxi-

mum acceptable shunt current during load dump, and

the voltage drop on R1 under normal operating condi-

tions with low battery voltage. For example, with typical

20mA input current in normal operation, 250mA load

dump current limit, 40V maximum load dump voltage,

the R1 value is:

where VINMIN is the minimum operating voltage and

VINREG is the minimum acceptable voltage at IN.

Use the following equation to verify that the current

through R1 is less than 250mA under a load-dump con-

dition:

For stable operation, the shunt regulator requires a min-

imum 10µF of ceramic capacitance from IN to GND.

VCC Regulator

The 5.25V VCC regulator provides bias for the internal

circuitry including the bandgap reference and gate dri-

vers. Externally bypass VCC with a minimum 4.7µF

ceramic capacitor. VCC has the ability to supply up to

50mA of current, but external loads should be mini-

mized so as not to take away drive capability for inter-

nal circuitry. If IN is powered by a voltage less than

5.5V, connect VCC directly to IN.

Switch-Mode Controller

The MAX16826 consists of a current-mode controller

that is capable of operating in the 100kHz to 1MHz fre-

quency range (Figure 2). Current-mode control pro-

vides fast response and simplifies loop compensation.

The error amplifier compares the voltage at FB to 1.25V

and varies the COMP output accordingly to regulate.

The PWM comparator compares the voltage at COMP

with the voltage at RSC to determine the switching duty

cycle. The primary cycle-by-cycle current-limit com-

parator interrupts the on-time if the sense voltage is

larger than 200mV. When the sense voltage is larger

than 270mV, the secondary gross current-limit com-

parator is activated to discharge the soft-start capaci-

tor. This forces the IC to re-soft-start preventing

inductor current runaway due to the delay of the prima-

ry cycle-by-cycle current limit.

The switch-mode controller also features a low current

shutdown mode, adjustable soft-start, and thermal

shutdown protection.

IVV

RmA

LD LD

===

−−26

40 26

100 140

RVV

INMIN INREG

175 55

20 10

100

−−

−

.. Ω

MAX16826

VIN

REFERENCE

Figure 1. Shunt Regulator Block Diagram

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

Maxim Integrated | 13www.maximintegrated.com

OVP

CSS

SWR DAC

I2C BUS

SET

CLR

SET

CLR

6µA

1.25V

COMP

SHDN

STBY

OSCILLATOR

26µA/µs

SYNC

RTCT

RSC

SOFT-START COMPARATOR

OVP COMPARATOR

ERROR AMPLIFIER

PWM COMPARATOR

270mV

200mV

VCC

10µA

MAX16826

CURRENT-

RAMP

GENERATOR

VCC

CURRENT-LIMIT

COMPARATORS

ANALOG

MUX

Figure 2. Switch Regulator Controller Block Diagram

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

Maxim Integrated | 14www.maximintegrated.com

Oscillator

The MAX16826 oscillator frequency is programmable

using an external capacitor (C33 in the

Typical

Application Circuit

) and a resistor (R19) at RTCT. R19 is

connected from RTCT to VCC and C33 is connected

from RTCT to GND. C33 charges through RT until VRTCT

reaches 2.85V. CT then discharges through an 8.4mA

internal current sink until VRTCT drops to 1.2V. C33 is

then allowed to charge through R19 again. The period

of the oscillator is the sum of the charge and discharge

times of C3. Calculate these times as follows:

The charge time is:

tC= 0.55 x R19 x C33

The discharge time is:

where tCand tD is in seconds, R19 is in ohms (Ω), and

C33 is in farads (F).

The oscillator frequency is then:

The charge time (tC) in relation to the period (tC+ tD)

sets the maximum duty cycle of the switching regulator.

Therefore, the charge time (tC) is constrained by the

desired maximum duty cycle. Typically, the duty cycle

should be limited to 95%. The oscillator frequency is

programmable from 100kHz to 1MHz. The MAX16826

can be synchronized to an external oscillator through

SYNC/EN.

Slope Compensation (RSC)

The MAX16826 uses an internal ramp generator for

slope compensation to stabilize the current loop when

the duty cycle exceeds 50%. A slope compensation

resistor (R17 in the

Typical Application Circuit

) is con-

nected between RSC and the switching current-sense

resistor at the source of the external switching FET.

When the voltage at DL transitions from low to high, a

ramped current with a slope of 26µA/µs is generated

and flows through the slope compensation resistor. It is

effectively summed with the current-sense signal. When

the voltage at DL is low, the current ramp is reset to 0.

Calculate R17 as follows:

where VOUT is the switching regulator output and

VINMIN is the minimum operating input voltage.

Current Limit (CS)

The MAX16826 includes a primary cycle-by-cycle, cur-

rent-limit comparator and a secondary gross current-

limit comparator to terminate the on-time or switch

cycle during an overload or fault condition. The current-

sense resistor (R12 in the

Typical Application Circuit

)

connected between the source of the switching FET

and GND and the internal threshold, set the current

limit. The current-sense input (CS) has a voltage trip

level (VCS) of 200mV. Use the following equation to cal-

culate R39:

R12 = VCS/IPK

where IPK is the peak current that flows through the

switching FET. When the voltage across R12 exceeds

the current-limit comparator threshold, the FET driver

(DL) turns the switch off within 80ns. In some cases, a

small RC filter may be required to filter out the leading-

edge spike on the sensed waveform. Set the time con-

stant of the RC filter at approximately 100ns and adjust

as needed.

If, for any reason, the voltage at CS exceeds the 270mV

trip level of the gross current limit as set by a second

comparator, then the switching cycle is immediately

terminated and the soft-start capacitor is discharged.

This allows a new soft-start cycle and prevents inductor

current buildup.

Soft-Start (CSS)

Soft-start is achieved by charging the external soft-start

capacitor (C30 in the

Typical Application Circuit

) at

startup. An internal fixed 6µA current source charges

the soft-start capacitor until VCSS reaches VCC. To

achieve the required soft-start timing for the switching

regulator output voltage to reach regulation, the value

of the soft-start capacitor at CSS is calculated as:

C30 = 6µA x tSS/VREF

where tSS is the required time to achieve the switching

regulator output regulation and VREF is the set FB regu-

lation voltage. When the IC is disabled, the soft-start

capacitor is discharged to GND.

Synchronization and Enable Input

The SYNC/EN input provides both external clock syn-

chronization (if desired) and enable control. When

SYNC/EN is held low, all circuits are disabled and the

IC enters low-current shutdown mode. When SYNC/EN

is high, the IC is enabled and the switching regulator

clock uses the RTCT network to set the operating fre-

quency. See the

Oscillator

section for details. The

SYNC/EN can also be used for frequency synchroniza-

tion by connecting it to an external clock signal from

100kHz to 1MHz. The switching cycle initiates on the

RVV R

OUT INMIN

17 12

34 28 1

=×

−()

ftt

OSC CD

tR C R R

D=× ×

()

−−ln . .19 33 19 281 86 19 487 445

()

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

Maxim Integrated | 15www.maximintegrated.com

rising edge of the clock. When using external synchro-

nization, the clock frequency set by RTCT must be 10%

lower than the synchronization signal frequency.

Overvoltage Protection (OVP)

OVP limits the maximum voltage of the switching regu-

lator output for protection against overvoltage due to

circuit faults, for example a disconnected FB. Connect

OVP to the center of a resistor-divider connected

between the switching regulator output and GND to set

the output-voltage OVP limit. Typically, the OVP output

voltage limit is set higher than the load dump voltage.

Calculate the value of R15 and R16 as follows:

R15 = (VOVP/1.25 - 1) x R16

Or to calculate VOVP:

VOVP = 1.25 x (1 + R15/R16)

where R15 and R16 are shown in the

Typical Application

Circuit

. The internal OVP comparator compares the volt-

age at OVP with the internal reference (1.25V typ) to

decide if an overvoltage error occurs. If an overvoltage

error is detected, switching stops, the switching regula-

tor gate-drive output is latched off, and the soft-start

capacitor is discharged. The latch can only be reset by

toggling SYNC/EN, activating the I2C standby mode, or

cycling power.

The internal ADC also uses OVP to sense the switching

regulator output voltage. Output voltage measurement

information can be read back from the I2C interface.

Voltage is digitized to 7-bit resolution.

Undervoltage Lockout (UVLO)

When the voltage at VCC is below the VCC undervolt-

age threshold (VVCC_UVLO, typically 4.3V falling), the

MAX16826 enters undervoltage lockout. VCC UVLO

forces the linear regulators and the switching regulator

into shutdown mode until the VCC voltage is high

enough to allow the device to operate normally. In VCC

UVLO, the VCC regulator remains active.

Thermal Shutdown

The MAX16826 contains an internal temperature sensor

that turns off all outputs when the die temperature

exceeds +160°C. The outputs are enabled again when

the die temperature drops below +140°C. In thermal

shutdown, all internal circuitry is shut down with the

exception of the shunt regulator.

Linear Current Sources

(CS1–CS4, DL1–DL4)

The MAX16826 uses transconductance amplifiers to con-

trol each LED current sink. The amplifier outputs

(DL1–DL4) drive the gates of the external current sink FETs

(Q2 to Q5 in the

Typical Application Circuit

). The source of

each MOSFET is connected to GND through a current-

sense resistor. CS1–CS4 are connected to the respective

inverting input of the amplifiers and also to the source of

the external current sink FETs where the LED string cur-

rent-sense resistors are connected. The noninverting input

of each amplifier is connected to the output of an internal

DAC. The DAC output is programmable using the I2C inter-

face to output between 97mV and 316mV. The regulated

string currents are set by the value of the current-sense

resistors (R28 to R31 in the

Typical Application Circuit

) and

the corresponding DAC output voltages.

LED PWM Dimming (DIM1–DIM4)

The MAX16826 features a versatile dimming scheme for

controlling the brightness of the four LED strings.

Independent LED string dimming is accomplished by dri-

ving the appropriate DIM1–DIM4 inputs with a PWM sig-

nal with a frequency up to 100kHz. Although the

brightness of the corresponding LED string is proportional

to the duty cycle of its respective PWM dimming signal,

finite LED current rise and fall times limit this linearity

when the dim pulse width approaches 2µs. Each LED

string can be independently controlled. Simultaneous

control of the PWM dimming and the LED string currents

in an analog way over a 3:1 range provides great flexibili-

ty allowing independent two-dimensional brightness con-

trol that can be used for color point setup and brightness

control.

Analog-to-Digital Converter (ADC)

The MAX16826 has an internal ADC that measures the

drain voltage of the external current sink driver FETs

(Q2 to Q5 in the

Typical Application Circuit

) using

DR1 - DR4 and the switching regulator output voltage

using OVP. Fault monitoring and switching stage out-

put-voltage optimization is possible by using an exter-

nal microcontroller to read out these digitized voltages

through the I2C interface. The ADC is a 7-bit SAR (suc-

cessive-approximation register) topology. It sequential-

ly samples and converts the drain voltage of each

channel and VOVP. An internal 5-channel analog MUX

is used to select the channel the ADC is sampling.

Conversions are driven by an internally generated

1MHz clock and gated by the external dimming sig-

nals. After a conversion, each measurement is stored

into its respective register and can be accessed

through the I2C interface. The digital circuitry that con-

trols the analog MUX includes a 190ms timer. If the

ADC does not complete a conversion within this 190ms

measurement window then the analog MUX will

sequence to the next channel. For the ADC to complete

one full conversion, the cumulative PWM dimming on-

time must be greater than 10µs within the 190ms mea-

surement window. The minimum PWM dimming on-time

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

Maxim Integrated | 16www.maximintegrated.com

is 2µs, so the ADC requires at least 5 of these minimum

pulses within the 190ms measurement window to com-

plete a conversion. During PWM dimming, LED current

pulse widths of less than 2µs are possible, but the ADC

may not have enough sampling time to complete a con-

version in this scenario and the corresponding data may

be incomplete or inaccurate. Therefore, adaptive volt-

age optimization may not be possible when the LED

current-pulse duration is less than 2µs. The LED current

pulse duration is shorter than the pulse applied at the

DIM_ inputs because of the LED turn-on delay.

Faults and Fault Detection

The MAX16826 features circuitry that automatically

detects faults such as overvoltage or shorted LED string.

An internal fault register at the address OAh is used to

record these faults. For example, if a shorted LED string

is detected, the corresponding fault register bit is set and

the faulty output is shut down.

Shorted LED strings are detected with fast comparators

connected to DR1–DR4. The trip threshold of these

comparators is 1.52V (typ). When this threshold is

exceeded, the shorted string is latched off and the cor-

responding bit of register OAh is set.

After the internal ADC completes a conversion, the

result is stored in the corresponding register and can

be read out by the external µC. The µC then compares

the conversion data with the preset limit to determine if

there is a fault.

When an LED string opens, the voltage at the corre-

sponding current-sink FET drain node goes to 0V.

However, the ADC can only complete a conversion if

the LED current comes into regulation. If an LED string

opens before the LED current can come into regulation,

the ADC cannot complete a conversion and the MSB

(eighth bit) is set to indicate an incomplete conversion

or timeout condition. Thus, an examination of the MSB

provides an indication that the LED string is open. If the

LED string opens after the LED current is in regulation,

the ADC can make conversions and reports that the

drain voltage is 0V. Therefore, to detect an open condi-

tion, monitor the MSB and the ADC measurement. If the

MSB is set and the CS_ on-time is greater than 2µs, or

if the ADC measures 0 at the drain, then there is an

open circuit.

Table 1. ADC Response

CONDITION ADC RESPONSE

Shorted string fault Load full-scale code into register, no conversions on affected channel until power or enable is

cycled.

Shorted string fault while

converting

Immediately load full-scale code into register and cease conversion effort on this channel until

power or enable is cycled.

ADC register read when it is

being updated

Previous sample is shifted out through the I2C interface and then the register is updated with the

new measurement.

UVLO Immediately terminate conversions, do not update current register.

STBY Immediately terminate conversions, do not update current register.

SHDN Immediately terminate conversions, do not update current register.

FILE UNIT

ADC DAC

POWER

MANAGEMENT

OVP

I2C

SYSTEM

CLOCK

EXTERNAL

EVENTS

Figure 3. Digital Block Diagram

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

Maxim Integrated | 17www.maximintegrated.com

Maxim Integrated

Overview of the Digital Section

Figure 3 shows the block diagram of the digital section in

the MAX16826. The I2C serial interface provides flexible

control of the IC and is in charge of writing/reading

to/from the register file unit. The ADC block is a 7-bit

5-channel SAR ADC. The eighth bit of the ADC data reg-

ister indicates an incomplete conversion or timeout has

occurred. This bit is set whenever the LED current fails to

come into regulation during the DIM PWM on-time. This

indicates there is either an LED open condition or the

CS_ on-time is less than 2µs.

A reason for this among other possibilities is an open

LED string condition. This eighth or MSB bit can be

tested to determine open string faults.

I2C Interface

The MAX16826 internal I2C serial interface provides

flexible control of the amplitude of the LED current in

each string and the switch-mode regulator output volt-

age. It is also able to read the current sink FET drain

voltages, as well as the switching regulator output volt-

age through OVP and thus enable some fault detection

and power dissipation minimization. By using an exter-

nal µC, the MAX16826 internal control and status regis-

ters are also accessed through the standard

bidirectional, 2-wire, I2C serial interface.

The I2C interface provides the following I/O functions

and programmability:

• Current sink FET drain and switching regulator out-

put-voltage measurement. The measurement for

each channel and the regulator output is stored in

its respective register and can be accessed

through the I2C interface. The SAR ADC measures

the drain voltage of each current sink FET sequen-

tially. This uses one 8-bit register for each channel

to store the measurement made by the 7-bit SAR

ADC and 1 bit to indicate a timeout during the ADC

conversion cycle.

• Adjustment of the switching regulator output. This is

used for adaptive voltage optimization to improve

overall efficiency. The switching regulator output is

downward adjustable by changing its reference

voltage. This uses a 7-bit register.

• Adjustment of the reference voltage of the current-

sink regulators. The reference voltage at the nonin-

verting input of each of the linear regulator drive

amplifiers can be changed to make adjustments in

the current of each LED string for a given sense

resistor. The output can be adjusted down from a

maximum of 316mV to 97mV in 1.72mV increments.

• Fault reporting. When a shorted string fault or an

overvoltage fault occurs, the fault is recorded.

• Standby mode. When a one is entered into the

standby register the IC goes into standby mode.

The 7-bit I2C address is 58h and the 8-bit I2C address

is B1h for a read operation and B0h for a write opera-

tion. Address the MAX16826 using the I2C interface to

read the state of the registers or to write to the registers.

Upon a read command, the MAX16826 transmits the

data in the register that the address register is pointing

to. This is done so that the user has the ability to confirm

the data written to a register before the output is

enabled. Use the fault register to diagnose any faults.

Serial Addressing

The I2C interface consists of a serial data line (SDA)

and a serial clock line (SCL) to achieve bidirectional

communication between the master and the slave. The

MAX16826 is a slave-only device, relying upon a mas-

ter to generate a clock signal. The master initiates data

transfer to and from the MAX16826 and generates SCL

to synchronize the data transfer (Figure 4).

SCL

SDA

tRtF

tBUF

START

CONDITION

STOP

CONDITION

REPEATED START CONDITION

START CONDITION

tSU,STO

tHD,STA

tSU,STA

tHD,DAT

tSU,DAT

tLOW

tHIGH

tHD,STA

Figure 4. 2-Wire Serial Interface Timing Detail

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

Maxim Integrated | 18www.maximintegrated.com

I2C is an open-drain bus. Both SDA and SCL are bidi-

rectional lines, connected to a positive supply voltage

using a pullup resistor. They both have Schmitt triggers

and filter circuits to suppress noise spikes on the bus to

ensure proper device operation.

A bus master initiates communication with the

MAX16826 as a slave device by issuing a START con-

dition followed by the MAX16826 address. The

MAX16826 address byte consists of 7 address bits and

a read/write bit (R/W). After receiving the proper

address, the MAX16826 issues an acknowledge bit by

pulling SDA low during the ninth clock cycle.

START and STOP Conditions

Both SCL and SDA remain high when the bus is not

busy. The master signals the beginning of a transmis-

sion with a START (S) condition by transitioning SDA

from high to low while SCL is high. When the master

has finished communicating with the MAX16826, it

issues a STOP (P) condition by transitioning SDA from

low to high while SCL is high. The bus is then free for

another transmission (Figure 4). Both START and STOP

conditions are generated by the bus master.

Bit Transfer

Each data bit, from the most significant bit to the least

significant bit, is transferred one by one during each

clock cycle. During data transfer, the SDA signal is

allowed to change only during the low period of the

SCL clock and it must remain stable during the high

period of the SCL clock (Figure 5).

Acknowledge

The acknowledge bit is used by the recipient to hand-

shake the receipt of each byte of data (Figure 6). After

data transfer, the master generates the acknowledge

clock pulse and the recipient pulls down the SDA line

during this acknowledge clock pulse, such that the

SDA line stays low during the high duration of the clock

pulse. When the master transmits the data to the

MAX16826, it releases the SDA line and the MAX16826

takes the control of SDA line and generates the

acknowledge bit. When SDA remains high during this

9th clock pulse, this is defined as the not acknowledge

signal. The master then generates either a STOP condi-

tion to abort the transfer, or a repeated START condi-

tion to start a new transfer.

START

CONDITION

(S)

DATA LINE STABLE

DATA VALID

DATA ALLOWED

TO CHANGE

STOP

CONDITION

(P)

SCL

SDA

Figure 5. Bit Transfer

SCL

SDA

BY MASTER

1289

START CONDITION CLOCK PULSE FOR ACKNOWLEDGMENT

SDA

BY SLAVE

Figure 6. Acknowledge

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

Maxim Integrated | 19www.maximintegrated.com

Accessing the MAX16826

The communication between the µC and the MAX16826

is based on the usage of a set of protocols defined on

top of the standard I2C protocol definition. They are

exclusively write byte(s) and read byte(s).

Write Byte(s)

The write byte protocol is as follows:

1) The master sends a START condition.

2) The master sends the 7-bit slave address followed

by a write bit (low).

3) The addressed slave asserts an ACK by pulling

SDA low.

4) The master sends an 8-bit command code.

5) The slave asserts an ACK by pulling SDA low.

6) The master sends an 8-bit data byte.

7) The slave acknowledges the data byte.

8) The master generates a STOP condition or repeats

6 and 7 to write next byte(s).

The command is interpreted as the destination address

(register file unit) and data is written in the addressed

location. The slave asserts a NACK at step 5 if the com-

mand is not valid. The master then interrupts the com-

munication by issuing a STOP condition. If the address

is correct, the data byte is written to the addressed reg-

ister. After the write, the internal address pointer is

increased by one. When the last location is reached, it

cycles to the first register.

Read Byte(s)

The read sequence is:

1) The master sends a START condition.

2) The master sends the 7-bit slave address plus a

write bit (low).

3) The addressed slave asserts an ACK on the data

line.

4) The master sends an 8-bit command byte.

5) The active slave asserts an ACK on the data line.

6) The master sends a repeated START condition.

7) The master sends the 7-bit slave address plus a

read bit (high).

8) The addressed slave asserts an ACK on the data

line.

9) The slave sends an 8-bit data byte.

10) The master asserts a NACK on the data line to

complete operations or asserts an ACK and

repeats 9 and 10.

11) The master generates a STOP condition.

The data byte read from the device is the content of the

addressed location(s). Once the read is done, the inter-

nal pointer is increased by one. When the last location is

reached, it cycles to the first one. If the device is busy or

the address is not correct (out of memory map), the

command code is not acknowledged and the internal

address pointer is not altered. The master then inter-

rupts the communication by issuing a STOP condition.

S ACKSLAVE ADDRESS R/W

7 BITS 0

COMMAND

8 BITS

ACK ACK PDATA

COMMAND BYTE: SELECT REGISTER TO WRITE DATA BYTE DATA GOES INTO THE REGISTER

SET BY THE COMMAND BYTE

8 BITS

WRITE BYTE FORMAT

Figure 7. Write Byte Format

S ACKSLAVE ADDRESS R/W

7 BITS 0

ACKSLAVE ADDRESS R/W

7 BITS 1

COMMAND

8 BITS

ACK SR NACK PDATA

COMMAND BYTE: PREPARE DEVICE FOR

FOLLOWING READ

DATA BYTE DATA COMES FROM THE

8 BITS

READ BYTE FORMAT

Figure 8. Read Byte Format

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

Maxim Integrated | 20www.maximintegrated.com

The register file unit is used to store all the control infor-

mation from the SDA line and configure the MAX16826

for different operating conditions. The register file

assignments of the MAX16826 are in Table 2.

Registers 00h to 03h: String Current Programming

These registers are used to program LED string 1 to

LED string 4 current sink values. For each LED string,

CS1–CS4 inputs are connected to the source of the

external current sink FET and internally are connected

to the inverting input of the internal transconductance

amplifier. The noninverting input of this amplifier is con-

nected to the output of an internal DAC programmed

by these registers. As the DAC is incremented, its out-

put voltage decreases from 316mV to 97mV in 1.72mV

steps by the data written in the register 00h to 03h;

thus, the steady-state voltage at CS1–CS4 is given by

the following formula:

VCS1,2,3,4 = 316mV - (1.72mV x RegisterValue[6:0])

For example, if 00h is set to 20h, then the CS1 voltage is:

VCS1 = 316mV - 1.72mV x 32 = 265.3mV

Output Programming

Set the switching regulator output voltage by connect-

ing FB to the center of a resistive voltage-divider

between the switching regulator output and GND. VFB

is regulated to a voltage from 876mV to 1.25V (typ) set

by the register 04h through the I2C interface.

The FB reference voltage can be decreased from 1.25V,

its maximum value, by approximately 2.9mV steps. The

steady-state voltage at FB then is regulated to:

VFB = 1.25V - (2.91mV x 04h[6:0])

Registers 05h to 08h: External Current-Sink

FET Drain Voltage ADC Readings

These registers store the drain voltages of the external

current sink FETs. For each register, bits 6–0 are the

conversion data of the ADC outputs. Bit 7 is used to

show if the conversion is terminated by the ADC (indi-

cated by 0) or if there is an internal timeout (indicated

by 1). If the drain voltage exceeds the preset reference

voltage, the corresponding LED string fault bit is assert-

ed. See the

Faults and Fault Detection

section for more

information on the internal timeout function.

Voltage ADC Output

Bits 6-0 of this register store the voltage present at

OVP. This voltage is a scaled down version of the

switching regulator output voltage. Bit 7 is not used.

This register stores all the external events or fault infor-

mation such as overvoltage and shorted LED string

faults. The fault events are logged only if the system is

not in standby mode and their active states are longer

than one clock cycle. Cycle enable or power to clear the

fault status register. Initiating standby mode using the

I2C interface can also be used to clear the fault status

Table 2. Register File Assignments

ADDRESS R/W USED BIT

RANGE

RESET

VALUE DESCRIPTION

00h R/W [6:0] 00h LED String 1 current programming value.

01h R/W [6:0] 00h LED String 2 current programming value.

02h R/W [6:0] 00h LED String 3 current programming value.

03h R/W [6:0] 00h LED String 4 current programming value.

04h R/W [6:0] 00h Switching regulator output voltage programming value.

05h R [7:0] 00h LED String 1 external FET drain voltage ADC output.

06h R [7:0] 00h LED String 2 external FET drain voltage ADC output.

07h R [7:0] 00h LED String 3 external FET drain voltage ADC output.

08h R [7:0] 00h LED String 4 external FET drain voltage ADC output.

09h R [6:0] 00h OVP voltage, ADC output.

0Ah R [5:0] 00h Fault status register.

0Bh R/W [0] 00h Device standby command.

0Ch R [2:0] — Device revision code.

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

Maxim Integrated | 21www.maximintegrated.com

vate this mode using the I2C interface. Next, perform a

read operation on the fault status register. The old fault

information is reported in this first read operation. The

conclusion of the read operation clears the data con-

tained in the register. Subsequent read o perations con-

firm that the fault status register has been cleared.

The description of this register is as follows:

Bit 0: Overvoltage sense flag. This flag is set if the volt-

age at OVP exceeds 1.25V; switching stops until power

or the enable or standby is cycled.

• Bit 1: Not used.

• Bit 2: LED string 1 shorted flag. A diode short in LED

string 1 has been detected if this bit is set.

• Bit 3: LED string 2 shorted flag. A diode short in LED

string 2 has been detected if this bit is set.

• Bit 4: LED string 3 shorted flag. A diode short in LED

string 3 has been detected if this bit is set.

• Bit 5: LED string 4 shorted flag. A diode short in LED

string 4 has been detected if this bit is set.

When register 0Bh bit 0 is set to 1, the IC enters a low-

current standby mode. In this mode, the system clock is

off and no operation is allowed. Set this bit to 0 to leave

standby mode and back to normal operation mode.

These 3 bits are a hardwired value that identifies the

IC’s revision.

Applications Information

Programming LED Currents

The MAX16826 uses sense resistors (R28, R29, R30,

R31 in the

Typical Application Circuit

) to set the output

current for each LED string. To set the LED current for a

particular string, connect a sense resistor across the

corresponding current-sense input (CS1–CS4) and

GND. For optimal accuracy, connect the low-side of the

current-sense resistors to GND with short traces. The

value needed for the sense resistor for a given current

is calculated with the equation below:

R31 = VCS1/IOUT1

where VCS1 can be set from 97mV to 316mV by the

internal registers through the I2C interface and IOUT1 is

the desired LED string 1 current.

Calculating the Value of Peak

Current-Limit Resistor

The value of R12 sets the peak switching current that

flows in the switching FET (Q1). Set the value of resistor

R12 using the equation below:

R12 = 0.19/(1.2 x IPK)

where IPK is the peak inductor current at minimum input

voltage and maximum load.

Boost Inductor Value

The value of the boost inductor is calculated using the

following equation:

where VINMIN is the minimum input voltage, VOUT is the

desired output voltage, and fSW is the switching fre-

quency, and ∆ILis the peak-to-peak ripple in the boost

inductor. Higher inductor values lead to lower ripple but

at a higher cost and size. Choose an inductor value that

gives peak-to-peak ripple current in the order of 30% to

40% of the average current in the inductor at low-line

and full-rated load. This choice of inductor is a compro-

mise between cost, size, and performance for the boost

converter.

Setting Output Voltage

Set the switch regulator output voltage by connecting

FB to the center of a resistive voltage-divider between

the switching regulator output and GND. VFB is regulat-

ed to a voltage from 0.88V to 1.25V (typ) set by an

internal register through the I2C interface. Choose R13

and R14 in the

Typical Application Circuit

for a reason-

able bias current in the resistive divider and use the fol-

lowing formula to set the output voltage:

VOUT = (1 + R13/R14) x VFB

where VFB is the regulated voltage set by the internal

Adaptive Voltage Optimization

The availability of the digitized switching regulator output

voltage and current sink drain voltages and the ability to

change the switching regulator output voltage provide

the ability to do adaptive voltage optimization. A slow dig-

ital control loop is established with an external µC closing

the loop. Firmware residing in the external µC is tasked to

read each one of the current sink FET drain voltages and

select the minimum value of the four LED strings. The

minimum value is subtracted from the scaled output volt-

age reading, and then the switching regulator output is

forced to maintain the difference required to provide cur-

rent regulation in the current sink FETs.

V VV

V f I

INMIN OUT INMIN

OSWL

=×

()

××

−

UT ∆

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

Maxim Integrated | 22www.maximintegrated.com

Switching Noise Effects

on ADC Readings

Excessive switching noise can corrupt the ADC read-

ings on the current sink MOSFET drains. Proper PCB

layout is critical to minimize this noise. The output diode

should be selected appropriately. The capacitance and

reverse recovery characteristics of the output diode

contribute to ground noise. Diodes with lower capaci-

tance and lower reverse recovery time will result in

lower current spikes at the turn on edge of the boost

switch. The lower current spike will result in lower

ground noise. Another method to reduce the ground

noise is to add a gate resistor in series with the gate of

the switching MOSFET. For most applications the gate

resistor should be in the range of 4.7 to 10ohms. Care

must be taken in selecting the gate resistor value, due

to power dissipation increase on the boost transistor. A

higher resistor value reduces the switching noise, but

power dissipation due to switching losses will increase.

The ground noise should be measured by measuring

the voltage between the ground of the input capacitor

that is farthest from the IC on the PCB and the ground

of the MAX16826. The pins 1,2 and 3 are the ground

pins of the MAX16826. To prevent problems on the

ADC readings of the MAX16826 the ground noise mea-

sured from the IC ground to the input capacitor ground

should be less than 0.5V peak to peak on a wide band-

width scope using a wide bandwidth probe. A wide

bandwidth scope must have a bandwidth greater than

150MHz.

SEPIC Topology

The SEPIC power topology is very useful when the

input voltage is expected to be higher or lower than the

output voltage of the switching regulator stage as

required by the number of LEDs used in a single string.

The SEPIC topology is more complex than the simple

boost topology and it requires the use of two additional

energy storage components, L2 and C25, in Figure 9.

MAX16826

R26

R28

C41

R21R23R25R27

R12

R17

R24

R29

C42

R22

R30

C43

R20

R18

C29

R31

CS1

DL1

C44

GNDGND

PGND

GND

VCC

R19

GND

RTCT

GNDGND GND

CSS

CS4

C33 C32

DL4

CS3

DL3

CS2

DL2

DR1

DR2

DR3

DR4

C30

GNDGND

GND

C27

OVP

C25

COMPRSCIN CSDL

DIMMING INPUTS

SYSTEM INTERFACE

R16

R15

GND

R14

R13

GND

R11

VOUT

GND

C26

VIN

SCL I2C INTERFACE

SDA

SYNC/EN

DIM4

DIM3

DIM2

DIM1

SCL

SDA

ENABLE

DIM

GND

SYSTEM

µC

GND

C28

R35

R32 R33 R34

Figure 9. SEPIC-Based LED Driver

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

Maxim Integrated | 23www.maximintegrated.com

PCB Layout and Routing

Careful PCB layout is important for proper operation.

Use the following guidelines for good PCB layout:

• Minimize the area of the high current-switching loop

of the rectifier diode, switching FET, sense resistor,

and output capacitor to avoid excessive switching

noise. Use wide and short traces for the gate-drive

loop from DL, to the FET gate, and through the cur-

rent-sense resistor, then returning to the IC PGND

and GND.

• Connect high-current input and output components

with short and wide connections. The high-current

input loop is from the positive terminal of the input

capacitor to the inductor, to the switching FET, to

the current-sense resistor, and to the negative ter-

minal of the input capacitor. The high-current output

loop is from the positive terminal of the input capac-

itor to the inductor, to the rectifier diode, to the posi-

tive terminal of the output capacitor, reconnecting

between the output capacitor and input capacitor

ground terminals. Avoid using vias in the high-cur-

rent paths. If vias are unavoidable, use multiple vias

in parallel to reduce resistance and inductance.

• Place the feedback and even voltage-divider resis-

tors as close to FB and OVP as possible. The

divider center trace should be kept short. Placing

the resistors far away causes the sensing trace to

become antennas that can pick up switching noise.

Avoid running the sensing traces near drain con-

nection of the switching FET.

• Place the input bypass capacitor as close to the

device as possible. The ground connection of the

bypass capacitor should be connected directly to

GND with a wide trace.

• Minimize the size of the switching FET drain node

while keeping it wide and short. Keep the drain

node away from the feedback node and ground. If

possible, avoid running this node from one side of

the PCB to the other. Use DC traces as shields, if

necessary.

• Provide large enough cooling copper traces for the

external current sink FETs. Calculate the worst-case

power dissipation and allocate sufficient area for

cooling.

• Refer to the MAX16826 Evaluation Kit for an exam-

ple of proper board layout.

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

Maxim Integrated | 24www.maximintegrated.com

Typical Application Circuit

MAX16826

R26

R28

C41

R21R23R25R27

R12

R17

R24

R29

C42

R22

R30

C43

R20

R18

C29

R31

CS1

DL1

C44

GNDGND

PGND

GND

VCC

R19

GND

RTCT

GNDGND GND

CSS

CS4

C33 C32

DL4

CS3

DL3

CS2

DL2

DR1

DR2

DR3

DR4

C30

GNDGND

C28

GND

C27

OVP

COMPRSCIN CSDL

SYSTEM INTERFACE

R16

R15

GND

R14

R13

GND

R11

VOUT

GND

C26

VIN (40V LOAD

DUMP OK)

SCL

SDA

SYNC/EN

DIM4

DIM3

DIM2

DIM1

SCL

SDA

ENABLE

DIM

GND

SYSTEM

µCDIMMING INPUTS

I2C INTERFACE

R35

R32 R33 R34

BOOST LED DRIVER

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

Maxim Integrated | 25www.maximintegrated.com

Chip Information

PROCESS: BiCMOS

MAX16826

TQFN

(5mm x 5mm)

TOP VIEW

GND

RTCT

SNYC/EN

CSS

COMP

PGND

CS3

DL2

CS2

DL3

DR1

DL1

1 2

DR4

4567

2324 22 20 19 18

DIM2

DIM1

SCL

SDA

GND DR2

31 EP

EXPOSED PAD.

VCC RSC

32 9

DL OVP

DL4

26 15 DIM3

CS4

25 16 DIM4

FB CS1

DR3

PACKAGE TYPE PACKAGE CODE DOCUMENT NO.

32 TQFN-EP T3255-4 21-0140

Pin Configuration

Denotes a lead(Pb)-free/RoHS-compliant package.

EP = Exposed pad.

denotes an automotive qualified part.

Package Information

For the latest package outline information and land patterns (foot-

prints), go to www.maximintegrated.com/packages. Note that a

“+”, “#”, or “-” in the package code indicates RoHS status only.

Package drawings may show a different suffix character, but the

drawing pertains to the package regardless of RoHS status.

Ordering Information

PART TEMP RANGE PIN-PACKAGE

MAX16826ATJ+ -40°C to +125°C 32 TQFN-EP*

MAX16826ATJ/V+ -40°C to +125°C 32 TQFN-EP*

MAX16826AGJ/VY+ -40°C to +125°C 32 QFN-EP*

MAX16826BATJ+ -40°C to +125°C 32 TQFN-EP*

MAX16826BATJ/V+ -40°C to +125°C 32 TQFN-EP*

MAX16826 Programmable, Four-String HB

LED Driver with Output-Voltage

Optimization and Fault Detection

Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent

licenses are implied. Maxim Integrated reserves the right to change the circuitry and specifications without notice at any time. The parametric values (min and

max limits) shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.

For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim Integrated’s website at www.maximintegrated.com.

Revision History

REVISION

NUMBER

REVISION

DATE DESCRIPTION PAGES

CHANGED

0 8/08 Initial release —

1 3/09

Added automotive version, updated Features, EC table, Typical Operating

Characteristics, Switching Preregulator Stage, Oscillator, Analog-to-Digital

(ADC), Faults and Fault Detection sections

1, 2, 5, 6, 11,

14–17, 20

2 12/09 Improve definition of minimum on-time for proper ADC operation 5, 10, 16

3 6/10 Added MAX16826B part 2–5, 25

4 12/11 Added MAX16826AGJ/VY+ to data sheet 25

5 10/13 Added Switching Noise Effects on ADC Readings section 22

6 2/15 Updated Benefits and Features section 1

7 3/16 Added missi ng lead( Pb)-free designations in Ord er ing Information table 25

Mouser Electronics

Authorized Distributor

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Maxim Integrated:

MAX16826BATJ/V+ MAX16826BATJ/V+T MAX16826ATJ+ MAX16826ATJ+C24 MAX16826ATJ+T