MAX8722AEEG-T - Maxim Integrated Products, Inc.

General Description

The MAX8722A integrated backlight controller is opti-

mized to drive cold-cathode fluorescent lamps (CCFLs)

using a full-bridge resonant inverter architecture.

Resonant operation maximizes striking capability and

provides near-sinusoidal waveforms over the entire

input range to improve CCFL lifetime. The controller

operates over a wide input voltage range (4.6V to 28V)

with high power to light efficiency. The device also

includes safety features that effectively protect against

many single-point fault conditions including lamp-out

and short-circuit faults.

The MAX8722A achieves 10:1 dimming range by “chop-

ping” the lamp current on and off using a digital pulse-

width modulation (DPWM) method. The DPWM frequency

can be accurately adjusted with a resistor or synchro-

nized to an external signal. The brightness is controlled

by an analog voltage on the CNTL pin. The device direct-

ly drives the four external n-channel power MOSFETs of

the full-bridge inverter. An internal 5.3V linear regulator

powers the MOSFET drivers, the DPWM oscillator, and

most of the internal circuitry. The MAX8722A is available

in a low-cost, 24-pin QSOP package and operates over a

-40°C to +85°C temperature range.

Applications

Notebook Computer Displays

LCD Monitors

LCD TVs

Features

♦Synchronized to Resonant Frequency

Longer Lamp Life

Guaranteed Striking Capability

High Power to Light Efficiency

♦Wide Input Voltage Range (4.6V to 28V)

♦Input-Voltage Feed-Forward for Excellent Line

Rejection

♦Accurate Dimming Control with Analog Interface

♦10:1 Dimming Range

♦Adjustable Accurate DPWM Frequency with Sync

Function

♦Adjustable Lamp Current Rise and Fall Time

♦Secondary Voltage Limit Reduces Transformer

Stress

♦Lamp-Out Protection with Adjustable Timeout

♦Secondary Overcurrent Protection with

Adjustable Timeout

♦Low-Cost 24-Pin QSOP Package

MAX8722A

Low-Cost CCFL Backlight Controller

________________________________________________________________ Maxim Integrated Products 1

Ordering Information

19-3321; Rev 0; 10/05

For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at

1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.

EVALUATION KIT

AVAILABLE

PART

TEMP RANGE PIN-PACKAGE

PKG

CODE

MAX8722AEEG

-40°C to +85°C

24 QSOP

E24-1

MAX8722A

GND

GH2

LX2

BST2

BST1

LX1

GH1

GL1

VDD

BATT

IFB

VFB

GL2

ISEC

CNTL

ILIM

FREQ

SYNC

PGND

COMP

VIN

VCC

DPWM

TFLT

VCC

SHDN

Minimal Operating Circuit

GND

VCC

VDD

PGNDTFLT

ILIM

BATT

TOP VIEW

GL2

GL1

GH1

LX1FREQ

SYNC

DPWM

CNTL

BST1

BST2

LX2

GH2ISEC

VFB

IFB

COMP

QSOP

MAX8722A

SHDN

Pin Configuration

MAX8722A

Low-Cost CCFL Backlight Controller

2_______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(Circuit of Figure 1. VBATT = 12V, VCC = VDD, VSHDN = 5.3V, TA= 0°C to +85°C.Typical values are at TA= +25°C, unless otherwise noted.)

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.

BATT to GND..........................................................-0.3V to +30V

BST1, BST2 to GND ...............................................-0.3V to +36V

BST1 to LX1, BST2 to LX2 ........................................-0.3V to +6V

CNTL, FREQ, SYNC, VCC, VDD to GND ...................-0.3V to +6V

COMP, DPWM, ILIM, TFLT to GND ............-0.3V to (VCC + 0.3V)

GH1 to LX1 ..............................................-0.3V to (VBST1 + 0.3V)

GH2 to LX2 ..............................................-0.3V to (VBST2 + 0.3V)

GL1, GL2 to GND .......................................-0.3V to (VDD + 0.3V)

IFB, ISEC, VFB to GND................................................-3V to +6V

SHDN to GND...........................................................-0.3V to +6V

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

Continuous Power Dissipation (TA= +70°C)

24-Pin QSOP (derate 9.5mW/°C above +70°C)........761.9mW

Operating Temperature Range ...........................-40°C to +85°C

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

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

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

PARAMETER CONDITIONS

MIN

TYP

MAX

UNITS

VCC = VDD = VBATT 4.6 5.5

BATT Input Voltage Range VCC = VDD = open 5.5

28.0

VBATT = 28V 1 2

BATT Quiescent Current VSHDN = VCC, VIFB = 1V VBATT = VCC = 5V 2 mA

BATT Quiescent Current,

Shutdown SHDN = GND 6 20 µA

VCC Output Voltage, Normal

Operation

VSHDN = 5.5V, 6V < VBATT < 28V,

0 < ILOAD < 10mA

5.25 5.40 5.55

VCC Output Voltage, Shutdown SHDN = GND, no load 3.5 4.6 5.5 V

VCC rising (leaving lockout)

4.58

VCC Undervoltage-Lockout

Threshold VCC falling (entering lockout) 4.0 V

VCC Undervoltage-Lockout

Hysteresis

200

GH1, GH2, GL1, GL2 On-

Resistance, High ITEST = 10mA, VCC = VDD = 5.3V 20 37 Ω

GH1, GH2, GL1, GL2 On-

Resistance, Low ITEST = 10mA, VCC = VDD = 5.3V 10 20 Ω

GH1, GH2, GL1, GL2 Maximum

Output Current 0.3 A

BST1, BST2 Leakage Current VBST_ = 12V, VLX_ = 7V 5 µA

Resonant Frequency Range Guaranteed by design 30 80 kHz

Minimum Off-Time

340 470 600

Maximum Off-Time 24 33 43 µs

Current-Limit Threshold

LX1 to PGND, LX2 to PGND

(Fixed)

ILIM = VCC

180 200 220

VILIM = 0.5V 80

100 120

Current-Limit Threshold

LX1 to PGND, LX2 to PGND

(Adjustable) VILIM = 2.0V

370 400 430

MAX8722A

Low-Cost CCFL Backlight Controller

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS (continued)

(Circuit of Figure 1. VBATT = 12V, VCC = VDD, VSHDN = 5.3V, TA= 0°C to +85°C.Typical values are at TA= +25°C, unless otherwise noted.)

PARAMETER CONDITIONS

MIN TYP MAX

UNITS

Zero-Current Crossing Threshold

LX1 to GND, LX2 to GND 1612mV

Current-Limit Leading Edge

Blanking

240 350 460

IFB Input Voltage Range -2 +2 V

IFB Regulation Point

770 790 810

0 < VIFB < 2V -2 +2

IFB Input Bias Current -2V < VIFB < 0

-150

µA

IFB Lamp-Out Threshold

560 600 640

IFB to COMP Transconductance

0.5V < VCOMP < 4V 60

100 160

µS

COMP Output Impedance 71018MΩ

COMP Discharge Current During

Overvoltage or Overcurrent Fault

VIFB = 800mV, VISEC = 2V

1200

µA

COMP Discharge Current During

DPWM Off-Time CNTL = GND, VCOMP = 2V

100

µA

DPWM Rising to Falling Ratio VIFB = 0 2.5 —

ISEC Overcurrent Threshold

1.15 1.21 1.28

ISEC Input Bias Current 0 < VISEC < 2V

-0.3 +0.3

µA

VFB Input Bias Current -4V < VVFB < +4V -25

+25

µA

VFB Overvoltage Threshold 2.2 2.3 2.4 V

RFREQ = 100kΩ

343

RFREQ = 169kΩ

204 209 214

DPWM Chopping Frequency

RFREQ = 340kΩ

106

DPWM Input Low Voltage SYNC = VCC, RFREQ = 169kΩ0.8 V

DPWM Input High Voltage SYNC = VCC, RFREQ = 169kΩ2.1 V

DPWM Input Hysteresis SYNC = VCC, RFREQ = 169kΩ

100

DPWM Input Bias Current SYNC = VCC, RFREQ = 169kΩ

-0.3 +0.3

µA

DPWM Output Low Resistance SYNC = GND, FREQ = VCC 2.4 kΩ

DPWM Output High Resistance SYNC = VCC, FREQ = VCC 2.4 kΩ

SYNC Input Low Voltage 0.8 V

SYNC Input High Voltage 2.1 V

SYNC Input Hysteresis

100

SYNC Input Bias Current VSYNC = 2V

-0.3 +0.3

µA

SYNC Input Frequency Range 10 50 kHz

CNTL Minimum Duty-Cycle

Threshold

0.20 0.23 0.26

CNTL Maximum Duty-Cycle

Threshold 1.9 2.0 2.1 V

CNTL Input Current 0 < VCNTL < VCC

-0.1 +0.1

µA

DPWM ADC Resolution Guaranteed monotonic 5 Bits

MAX8722A

Low-Cost CCFL Backlight Controller

4_______________________________________________________________________________________

ELECTRICAL CHARACTERISTICS (continued)

(Circuit of Figure 1. VBATT = 12V, VCC = VDD, VSHDN = 5.3V, TA= 0°C to +85°C.Typical values are at TA= +25°C, unless otherwise noted.)

PARAMETER CONDITIONS

MIN

TYP

MAX

UNITS

SHDN Input Low Voltage 0.8 V

SHDN Input High Voltage 2.1 V

SHDN Input Bias Current -1 +1 µA

FREQ Dual Mode Input High

Level

VCC -

0.35

FREQ Input Regulation Level

VCC / 2

FREQ Input Bias Current FREQ = VCC

230

µA

VISEC < 1.25V and VIFB < 600mV; VFLT = 2V

0.95 1.00 1.05

VISEC < 1.25V and VIFB > 600mV; VFLT = 2V -1

TFLT Charge Current

VISEC > 1.25V and VIFB < 600mV; VFLT = 2V

116

µA

TFLT Trip Threshold

3.95 4.10 4.20

Dual Mode is a trademark of Maxim Integrated Products, Inc.

ELECTRICAL CHARACTERISTICS

(Circuit of Figure 1. VBATT = 12V, VCC = VDD, VSHDN = 5.3V, TA= -40°C to +85°C, unless otherwise noted.) (Note 1)

PARAMETER CONDITIONS

MIN

TYP

MAX

UNITS

VCC = VDD = VBATT 4.6 5.5

BATT Input Voltage Range VCC = VDD = open 5.5

28.0

VBATT = 28V 2

BATT Quiescent Current VSHDN = VCC, VIFB = 1V VBATT = VCC = 5V 2 mA

BATT Quiescent Current,

Shutdown SHDN = GND 20 µA

VCC Output Voltage, Normal

Operation

VSHDN = 5.5V, 6V < VBATT < 28V

0 < ILOAD < 20mA

5.25 5.55

VCC Output Voltage, Shutdown SHDN = GND, no load 3.5 5.5 V

VCC rising (leaving lockout)

4.58

VCC Undervoltage-Lockout

Threshold VCC falling (entering lockout) 4.0 V

GH1, GH2, GL1, GL2 On-

Resistance, High ITEST =10mA, VCC = VDD = 5.3V 37 Ω

GH1, GH2, GL1, GL2 On-

Resistance, Low ITEST =10mA, VCC = VDD = 5.3V 20 Ω

BST1, BST2 Leakage Current VBST_ = 12V, VLX_ = 7V 5 µA

Resonant Frequency Range Guaranteed by design 30 80 kHz

Minimum Off-Time

340 600

Maximum Off-Time 24 43 µs

Current-Limit Threshold

LX1 - PGND, LX2 - PGND (Fixed)

ILIM = VCC

180 220

MAX8722A

Low-Cost CCFL Backlight Controller

_______________________________________________________________________________________ 5

ELECTRICAL CHARACTERISTICS (continued)

(Circuit of Figure 1. VBATT = 12V, VCC = VDD, VSHDN = 5.3V, TA= -40°C to +85°C, unless otherwise noted.) (Note 1)

PARAMETER CONDITIONS

MIN TYP MAX

UNITS

VILIM = 0.5V 80

120

Current-Limit Threshold

LX1 - PGND, LX2 - PGND

(Adjustable) VILIM = 2.0V

370 430

Zero-Current Crossing Threshold

LX1 - GND, LX2 - GND 112mV

Current-Limit Leading Edge

Blanking

240 460

IFB Input Voltage Range -2 +2 V

IFB Regulation Point

770 810

0 < VIFB < 2V -2 +2

IFB Input Bias Current -2V < VIFB < 0

-150

µA

IFB Lamp-Out Threshold

560 640

IFB to COMP Transconductance

0.5V < VCOMP < 4V 60

160

µS

COMP Output Impedance 718MΩ

ISEC Overcurrent Threshold

1.15 1.28

VFB Overvoltage Threshold 2.2 2.4 V

DPWM Chopping Frequency RFREQ = 169kΩ

200 218

DPWM Input Low Voltage SYNC = VCC, RFREQ = 169kΩ0.8 V

DPWM Input High Voltage SYNC = VCC, RFREQ = 169kΩ2.1 V

DPWM Output Low Resistance SYNC = GND, FREQ = VCC 2.4 kΩ

DPWM Output High Resistance SYNC = VCC, FREQ = VCC 2.4 kΩ

SYNC Input Low Voltage 0.8 V

SYNC Input High Voltage 2.1 V

SYNC Input Bias Current VSYNC = 2V

-0.3 +0.3

µA

SYNC Input Frequency Range 10 50 kHz

CNTL Minimum Duty-Cycle

Threshold

0.20 0.26

CNTL Maximum Duty-Cycle

Threshold 1.9 2.1 V

SHDN Input Low Voltage 0.8 V

SHDN Input High Voltage 2.1 V

FREQ Dual-Mode Input High

Level

VCC -

0.35

TFLT Trip Threshold

3.95 4.20

Note 1: Specifications to -40°C are guaranteed by design based on final characterization results.

MAX8722A

Low-Cost CCFL Backlight Controller

6_______________________________________________________________________________________

Typical Operating Characteristics

(Circuit of Figure 1. VBATT = 12V, VCC = VDD, VSHDN = 5.3V, TA= +25°C, unless otherwise noted.)

LOW INPUT-VOLTAGE

OPERATION (VBATT = 8V)

MAX8722 toc01

10µs/div

0V A

0V B

A: VIFB, 2V/div

B: VVFB, 2V/div

C: VLX1, 10V/div

D: VLX2, 10V/div

HIGH INPUT-VOLTAGE

OPERATION (VBATT = 20V)

MAX8722 toc02

10µs/div

0V A

0V B

A: VIFB, 2V/div

B: VVFB, 2V/div

C: VLX1, 10V/div

D: VLX2, 10V/div

LINE TRANSIENT RESPONSE

MAX8722 toc03

20µs/div

0V A

0V B

10V

20V

A: VVFB, 2V/div

B: VIFB, 2V/div

C: VLX1, 10V/div

D: VBATT, 10V/div

LINE TRANSIENT RESPONSE

MAX8722 toc04

20µs/div

0V A

0V B

10V

20V

A: VVFB, 2V/div

B: VIFB, 2V/div

C: VLX1, 10V/div

D: VBATT, 10V/div

MINIMUM BRIGHTNESS STARTUP

WAVEFORM (VCNTL = 0)

MAX8722 toc05

2ms/div

0V A

0V B

A: VIFB, 2V/div

B: VVFB, 2V/div

C: VCOMP, 1V/div

D: VSHDN, 5V/div

MINIMUM BRIGHTNESS

DPWM OPERATION (VCNTL = 0)

MAX8722 toc06

2ms/div

0V A

0V B

A: VIFB, 2V/div

B: VVFB, 2V/div

C: VCOMP, 1V/div

D: VDPWM, 5V/div

MAX8722A

Low-Cost CCFL Backlight Controller

_______________________________________________________________________________________ 7

50% BRIGHTNESS DIGITAL

PWM OPERATION (VCNTL = 1V)

MAX8722 toc07

2ms/div

0V A

0V B

A: VIFB, 2V/div

B: VVFB, 2V/div

C: VCOMP, 1V/div

D: VDPWM, 5V/div

DPWM SOFT-START

MAX8722 toc08

40µs/div

0V A

0V B

A: VIFB, 2V/div

B: VVFB, 2V/div

C: VCOMP, 1V/div

D: VDPWM, 5V/div

DPWM SOFT-STOP

MAX8722 toc09

40µs/div

0V A

0V B

A: VIFB, 2V/div

B: VVFB, 2V/div

C: VCOMP, 1V/div

D: VDPWM, 5V/div

SWITCHING FREQUENCY

vs. INPUT VOLTAGE

MAX8722 toc12

INPUT VOLTAGE (V)

SWITCHING FREQUENCY (kHz)

201612

824

4ms/div

SECONDARY OVERCURRENT

PROTECTION AND TIMEOUT

MAX8722 toc11

A: VISEC, 500mV/div

B: VTFLT, 2V/div

200ms/div

LAMP-OUT VOLTAGE LIMITING

AND TIMEOUT

MAX8722 toc10

A: VIFB, 2V/div

B: VVFB, 2V/div

C: VTFLT, 5V/div

Typical Operating Characteristics (continued)

(Circuit of Figure 1. VBATT = 12V, VCC = VDD, VSHDN = 5.3V, TA= +25°C, unless otherwise noted.)

MAX8722A

Low-Cost CCFL Backlight Controller

8_______________________________________________________________________________________

100

200

150

300

250

350

400

100 200150 250 300 350

DPWM FREQUENCY vs. RFREQ

MAX8722 toc13

RFREQ (Ω)

DIGITAL PWM FREQUENCY (Hz)

200

203

209

206

212

215

71310 16 19 22 25

DPWM FREQUENCY

vs. INPUT VOLTAGE

MAX8722 toc14

INPUT VOLTAGE (V)

DIGITAL PWM FREQUENCY (Hz)

RMS LAMP CURRENT

vs. INPUT VOLTAGE

MAX8722 toc15

INPUT VOLTAGE (V)

RMS LAMP CURRENT (mA)

201612

5.85

5.90

5.95

6.00

6.05

6.10

5.80

824

NOMINAL CURRENT SET POINT

100

0 0.8 1.00.4 0.60.2 1.2 1.4 1.6 1.8 2.0

NORMALIZED BRIGHTNESS

vs. CNTL VOLTAGE

MAX8722 toc16

CNTL VOLTAGE (V)

NORMALIZED BRIGHTNESS (%)

-1.0

-0.6

-0.8

-0.2

-0.4

0.2

0.4

4128162024

VCC LINE REGULATION

MAX8722 toc17

INPUT VOLTAGE (V)

VCC ACCURACY (%)

-0.5

-0.4

-0.2

-0.3

-0.1

0426810

VCC LOAD REGULATION

MAX8722 toc18

LOAD CURRENT (mA)

VCC ACCURACY (%)

-0.02

-0.01

0.02

0.01

0.03

0.04

-40 20 40-20 0 60 80 100

VCC ACCURACY vs. TEMPERATURE

MAX8722 toc19

TEMPERATURE (°C)

VCC ACCURACY (%)

Typical Operating Characteristics (continued)

(Circuit of Figure 1. VBATT = 12V, VCC = VDD, VSHDN = 5.3V, TA= +25°C, unless otherwise noted.)

MAX8722A

Low-Cost CCFL Backlight Controller

_______________________________________________________________________________________ 9

PIN NAME FUNCTION

1BATT Supply Input. BATT is the input to the internal 5.4V linear regulator that powers the device. Bypass BATT to

GND with a 0.1µF ceramic capacitor.

2SHDN

Shutdown Control Input. The device shuts down when SHDN is pulled to GND.

3ILIM

Primary Current-Limit Adjustment Input. Connect a resistive voltage-divider between VCC and GND to set

the primary current limit. The current-limit threshold is 1/5 of the voltage at ILIM. Connect it to VCC with a

pullup resistor to select the default current-limit threshold of 0.2V.

4TFLT

Fault Timer Adjustment Pin. Connect a capacitor from TFLT to GND to set the timeout periods for open-

lamp and secondary overcurrent faults.

5CNTL

Brightness Control Input. Varying VCNTL between 0 and 2V varies the DPWM duty cycle (brightness)

between 10% (minimum) and 100% (maximum). The brightness remains at maximum for VCNTL greater

than 2V.

DPWM

Dual-Function DPWM Signal Pin. The DPWM pin can be used either as the DPWM signal output or as a

low-frequency sync input. See the DPWM Dimming Control and DPWM Frequency Setting sections.

7SYNC

DPWM High-Frequency Sync Input. The DPWM chopping frequency can be synchronized to an external

high-frequency signal by connecting FREQ to VCC and SYNC to the external signal source. The DPWM

chopping frequency is 1/128 of the frequency of the external signal.

8FREQ

DPWM Frequency Dual-Mode Adjustment Pin. Connect a resistor from FREQ to GND to set the DPWM

frequency. Connect FREQ to VCC to set DPWM frequency using SYNC.

fDPWM = 209Hz x 169kΩ / RFREQ

9COMP

Transconductance Error-Amplifier Output. A compensation capacitor connected between COMP and GND

sets the rise and fall time of the lamp current in DPWM operation.

10 IFB

Lamp-Current Feedback Input. The average voltage on IFB is regulated to 0.8V by controlling the on-time

of high-side switches. If VIFB falls below 0.6V for a period longer than the timeout period set by TFLT, the

MAX8722A activates the fault latch.

11 VFB

Transformer Secondary Voltage Feedback Input. A capacitive voltage-divider between the high-voltage

terminal of the CCFL tube and GND sets the maximum average lamp voltage during lamp strike and open-

lamp conditions. When the average voltage on VFB exceeds the internal overvoltage threshold, the

controller turns on an internal current sink discharging the COMP capacitor.

12 ISEC

Transformer Secondary Current Feedback Input. A current-sense resistor connected between the low-

voltage end of the transformer secondary and ground sets the maximum secondary current during faults.

When the average voltage on ISEC exceeds the internal overcurrent threshold, the controller turns on an

internal current sink discharging the COMP capacitor.

13 GH2

High-Side MOSFET NH2 Gate-Driver Output

14 LX2

GH2 Gate-Driver Return. LX2 is the input to the current-limit and zero-crossing comparators. The device

senses the voltage across the low-side MOSFET NL2 to detect primary current zero-crossing and primary

overcurrent.

15 BST2

GH2 Gate-Driver Supply Input. Connect a 0.1µF capacitor from LX2 to BST2 and a diode from VDD to BST2

to form a bootstrap circuit.

16 BST1

GH1 Gate-Driver Supply Input. Connect a 0.1µF capacitor from LX1 to BST1 and a diode from VDD to BST1

to form a bootstrap circuit.

17 LX1

GH1 Gate-Driver Return. LX1 is the input to the current-limit and zero-crossing comparators. The device

senses the voltage across the low-side MOSFET NL1 to detect primary current zero-crossing and primary

overcurrent.

Pin Description

MAX8722A

Low-Cost CCFL Backlight Controller

10 ______________________________________________________________________________________

PIN NAME FUNCTION

18 GH1 High-Side MOSFET NH1 Gate-Driver Output

19 GL1 Low-Side MOSFET NL1 Gate-Driver Output

20 GL2 Low-Side MOSFET NL2 Gate-Driver Output

21 PGND Power Ground. PGND is the return for the GL1 and GL2 gate drivers.

22 VDD

Low-Side Gate-Driver Supply Input. Connect VDD to the output of the internal linear regulator (VCC). Bypass

VDD with a 0.1µF capacitor to PGND.

23 VCC 5.3V/10mA Internal Linear-Regulator Output. VCC is the supply voltage for the device. Bypass VCC with a

1µF ceramic capacitor to GND.

24 GND Analog Ground. The ground return for VCC, REF, and other analog circuitry. Connect GND to PGND under

the IC at the IC’s backside exposed metal pad.

Pin Description (continued)

MAX8722A

GND

GH2

LX2

BST2

BST1

LX1

GH1

GL1

VDD

BATT

IFB

VFB

GL2

ISEC

CNTL

ILIM

FREQ

SYNC

PGND

COMP

VIN

VCC

DPWM TFLT

VCC

SHDN

GND

ON/OFF

BRIGHTNESS

SYNC

DPWM

0.1µF

0.47µF

100kΩ

200kΩ

169kΩ

C10

0.01µF

C11

0.22µF

4.7µF

25V

0.47µF

0.1µF

NH1 NH2

NL1 NL2

1µF

T1 CCFL

18pF

3kV

15nF

40.2Ω

150Ω

Figure 1. Typical Operating Circuit of the MAX8722A

MAX8722A

Low-Cost CCFL Backlight Controller

______________________________________________________________________________________ 11

MAX8722A

MUX

DPWM OSCILLATOR

AND DIMMING

CONTROL LOGIC

LINEAR

REGULATOR

BIAS

SUPPLY

FLT

QFLT

BST1

GH1

LX1

BST2

GH2

LX2

VDD

GL1

PGND

GL2

ILIM

BATT

VCC

VFB

2.3V

OVERVOLTAGE

COMPARATOR

PWM

COMPARATOR

ERROR

AMPLIFIER

IFB

FREQ

DPWM

SYNC

CNTL

600mV

1.25V

ISEC

OPEN-LAMP

COMPARATOR

SECONDARY

OVERCURRENT

COMPARATOR

GATE-DRIVER

CONTROL

STATE

MACHINE

FAULT

LATCH

RESET

800mV

COMP

OVER-

CURRENT

UVLO

COMPARATOR

4.2V

PWM CONTROL

LOGIC

RAMP

PRIMARY

OVERCURRENT

AND ZERO-

CROSSING

FAULT DELAY

BLOCK

TFLT

UVLO

1200µA

100µA

OVER-

CURRENT

GND

F.W. RECT

H.W. RECT

SHDN

Figure 2. MAX8722A Functional Diagram

MAX8722A

Typical Operating Circuit

The typical operating circuit of the MAX8722A (Figure

1) is a complete CCFL backlight inverter for TFT-LCD

panels. The input voltage range of the circuit is from 8V

to 24V. The maximum RMS lamp current is set to 6mA,

and the maximum RMS striking voltage is set to 1600V.

Table 1 lists some important components, and Table 2

lists the component suppliers’ contact information.

Detailed Description

The MAX8722A controls a full-bridge resonant inverter

to convert an unregulated DC input into a near-sinu-

soidal, high-frequency AC output for powering CCFLs.

The lamp brightness is adjusted by turning the lamp on

and off with a signal. The brightness of the lamp is pro-

portional to the duty cycle of the DPWM signal, which is

set through an analog voltage on the CNTL pin. Figure

2 shows the functional diagram of the MAX8722A.

Resonant Operation

The MAX8722A drives the four n-channel power

MOSFETs that make up the zero-voltage-switching

(ZVS) full-bridge inverter as shown in Figure 3. Assume

that NH1 and NL2 are turned on at the beginning of a

switching cycle as shown in Figure 3(a). The primary

current flows through MOSFET NH1, DC blocking

capacitor C2, the primary side of transformer T1, and

MOSFET NL2. During this interval, the primary current

ramps up until the controller turns off NH1. When NH1

turns off, the primary current forward biases the body

diode of NL1, which clamps the LX1 voltage just below

ground as shown in Figure 3(b). When the controller

turns on NL1, its drain-to-source voltage is near zero

because its forward-biased body diode clamps the

drain. Since NL2 is still on, the primary current flows

through NL1, C2, the primary side of T1, and NL2.

Once the primary current drops to the minimum current

threshold (6mV/RDS(ON)), the controller turns off NL2.

The remaining energy in T1 charges up the LX2 node

until the body diode of NH2 is forward biased. When

NH2 turns on, it does so with near-zero drain-to-source

voltage. The primary current reverses polarity as shown

in Figure 3(c), beginning a new cycle with the current

flowing in the opposite direction, with NH2 and NL1 on.

The primary current ramps up until the controller turns

off NH2. When NH2 turns off, the primary current for-

ward biases the body diode of NL2, which clamps the

LX2 voltage just below ground as shown in Figure 3(d).

After the LX2 node goes low, the controller losslessly

turns on NL2. Once the primary current drops to the

minimum current threshold, the controller turns off NL1.

The remaining energy charges up the LX1 node until

the body diode of NH1 is forward biased. Finally, NH1

losslessly turns on, beginning a new cycle as shown in

Figure 3(a). Note that switching transitions on all four

power MOSFETs occur under ZVS condition, which

reduces transient power losses and EMI.

A simplified CCFL inverter circuit is shown in Figure

4(a). The full-bridge power stage is simplified and rep-

resented as a square-wave AC source. The resonant

tank circuit can be further simplified to Figure 4(b) by

removing the transformer. CSis the primary series

capacitor, C’Sis the series capacitance reflected to the

secondary, CPis the secondary parallel capacitor, N is

the transformer turns ratio, L is the transformer sec-

ondary leakage inductance, and RLis an idealized

resistance that models the CCFL in normal operation.

Figure 5 shows the frequency response of the resonant

tank’s voltage gain under different load conditions.

Low-Cost CCFL Backlight Controller

12 ______________________________________________________________________________________

DESIGNATION

DESCRIPTION

4.7µF ±20%, 25V X5R ceramic capacitor

Murata GRM32RR61E475K

Taiyo Yuden TMK325BJ475MN

TDK C3225X7R1E475M

C2 1µF ±10%, 25V X7R ceramic capacitor

18pF ±1pF, 3kV, high-voltage ceramic

capacitor

Murata GRM42D1X3F180J

TDK C4520C0G3F180F

Dual silicon switching diode, common

anode, SOT-323

Central Semiconductor CMSD2836

Diodes, Inc. BAW56W

NH1/2, NL1/2

Dual n-channel MOSFETs, 30V, 0.095,

SOT23-6

Fairchild FDC6561AN

T1 CCFL transformer, 1:93 turns ratio

TOKO T912MG-1018

Table 1. List of Important Components

SUPPLIER WEBSITE

Central Semiconductor www.centralsemi.com

Diodes Inc. www.diodes.com

Fairchild Semiconductor

www.fairchildsemi.com

Murata www.murata.com

Sumida www.sumida.com

Taiyo Yuden www.t-yuden.com

TDK www.components.tdk.com

TOKO www.tokoam.com

Table 2. Component Suppliers

MAX8722A

Low-Cost CCFL Backlight Controller

______________________________________________________________________________________ 13

VBATT

(a)

NH1

NL1

OFF

NH2

OFF

NL2

LX2LX1

VBATT

(b)

NH1

OFF

NL1

NH2

OFF

NL2

LX2LX1

VBATT

(c)

NH1

OFF

NL1

NH2

NL2

OFF

LX2LX1

VBATT

(d)

NH1

OFF

NL1

NH2

OFF

NL2

LX2LX1

(BODY DIODE TURNS ON FIRST) (BODY DIODE TURNS ON FIRST)

Figure 3. Resonant Operation

MAX8722A

The primary series capacitor is 1µF, the secondary par-

allel capacitor is 15pF, the transformer turns ratio is

1:93, and the secondary leakage inductance is 260mH.

Notice that there are two peaks, fSand fP, in the fre-

quency response. The first peak, fS, is the series reso-

nant peak determined by the secondary leakage

inductance (L) and the series capacitor reflected to the

secondary (C’S):

The second peak, fP, is the parallel resonant peak

determined by the secondary leakage inductance (L),

the parallel capacitor (CP), and the series capacitor

reflected to the secondary (C’S):

The inverter is designed to operate between these two

resonant peaks. When the lamp is off, the operating

point of the resonant tank is close to the parallel resonant

peak due to the lamp’s infinite impedance. The circuit

displays the characteristics of a parallel-loaded resonant

converter. While in parallel-loaded resonant operation,

the inverter behaves like a voltage source to generate

the necessary striking voltage. Theoretically, the output

voltage of the resonant converter will increase until the

lamp is ionized or until it reaches the IC’s secondary volt-

age limit, without regard to the transformer turns ratio or

the input voltage level. Once the lamp is ionized, the

equivalent load resistance decreases rapidly and the

operating point moves toward the series resonant peak.

While in series resonant operation, the inverter behaves

like a current source.

Lamp-Current Regulation

The MAX8722A uses a lamp-current control loop to

regulate the current delivered to the CCFL. The heart of

the control loop is a transconductance error amplifier.

The AC lamp current is sensed with a resistor connect-

ed in series with the low-voltage terminal of the lamp.

The voltage across this resistor is fed to the IFB input

and is internally full-wave rectified. The transconduc-

tance error amplifier compares the rectified IFB voltage

with a 790mV (typ) internal threshold to generate an

error current. The error current charges and discharges

a capacitor connected between COMP and ground to

create an error voltage (VCOMP). VCOMP is then com-

pared with an internal ramp signal to set the high-side

MOSFET switch on-time (tON).

LCC

2π

Low-Cost CCFL Backlight Controller

14 ______________________________________________________________________________________

SOURCE CCFL

CS1:N

(a)

SOURCE RL

C'S =

(b)

Figure 4. Equivalent Resonant Tank Circuit

FREQUENCY (kHz)

VOLTAGE GAIN (V/V)

80604020

0 100

RL INCREASING

Figure 5. Frequency Response of the Resonant Tank

Transformer Secondary Voltage Limiting

The MAX8722A reduces the voltage stress on the

transformer’s secondary winding by limiting the sec-

ondary voltage during startup and open-lamp faults.

The AC voltage across the transformer secondary

winding is sensed through a capacitive voltage-divider.

The small voltage across the larger capacitor of the

divider is fed to the VFB input and is internally half-

wave rectified. An overvoltage comparator compares

the VFB voltage with a 2.3V (typ) internal threshold.

Once the sense voltage exceeds the overvoltage

threshold, the MAX8722A turns on a 1200µA current

source that discharges the COMP capacitor. The high-

side MOSFET on-time shortens as the COMP voltage

decreases, reducing the transformer secondary’s peak

voltage below the threshold set by the capacitive volt-

age-divider.

Lamp Startup

A CCFL is a gas discharge lamp that is normally driven

in the avalanche mode. To start ionization in a nonion-

ized lamp, the applied voltage (striking voltage) must

be increased to the level required for the start of

avalanche. At low temperatures, the striking voltage

can be several times the typical operating voltage.

Because of the MAX8722A’s resonant topology, the strik-

ing voltage is guaranteed. Before the lamp is ionized, the

lamp impedance is infinite. The transformer secondary

leakage inductance and the high-voltage parallel capac-

itor determine the unloaded resonant frequency. Since

the unloaded resonant circuit has a high Q, it can gener-

ate very high voltages across the lamp.

Upon power-up, two soft-start features acting together

smooth the startup behavior. First, VCOMP slowly rises,

increasing the duty cycle of the high-side MOSFET

switches and providing a measure of soft-start.

Second, the MAX8722A charges VFB to the overvolt-

age threshold (2.3V typ) immediately after the device is

enabled. The DC voltage on VFB is gradually dis-

charged through an internal 300kΩ(typ) resistor during

startup. This feature is equivalent to slowly raising the

overvoltage threshold during startup, so it further

improves the soft-start behavior.

Feed-Forward Control and

Dropout Operation

The MAX8722A is designed to maintain tight control of

the lamp current under all transient conditions. The

feed-forward control instantaneously adjusts the on-

time for changes in input voltage (VBATT). This feature

provides immunity to input-voltage variations and sim-

plifies loop compensation over wide input voltage

ranges. The feed-forward control also improves the line

regulation for short on-times and makes startup tran-

sients less dependent on the input voltage.

Feed-forward control is implemented by increasing

the internal voltage ramp rate for higher VBATT. This

has the effect of varying tON as a function of the in-

put voltage while maintaining approximately the same

signal levels at VCOMP. Since the required voltage

change across the compensation capacitor is minimal,

the controller’s response to input voltage changes is

essentially instantaneous.

DPWM Dimming Control

The MAX8722A controls the brightness of the CCFL by

chopping the lamp current on and off using a low-fre-

quency (between 100Hz and 350Hz) DPWM signal

either from the internal oscillator or from an external sig-

nal source. The CCFL brightness is proportional to the

DPWM duty cycle, which can be adjusted from 9.375%

to 100% by the CNTL pin. CNTL is an analog input with

a usable input voltage range between 0 and 2000mV,

which is digitized to select one of 128 brightness levels.

As shown in Figure 6, the MAX8722A ignores the first

12 steps, so the first 12 steps all represent the same

brightness. When VCNTL is between 0 and 187.5mV,

the DPWM duty cycle is always 9.375%. When VCNTL is

above 187.5mV, a 15.625mV change on CNTL results

in a 0.78125% change in the DPWM duty cycle. When

VCNTL is equal to or above 2000mV, the DPWM duty

cycle is always 100%.

MAX8722A

Low-Cost CCFL Backlight Controller

______________________________________________________________________________________ 15

CONTROL VOLTAGE (mV)

BRIGHTNESS (%)

16001200400 800

100

0 2000

Figure 6. Theoretical Brightness vs. Control Voltage

MAX8722A

In DPWM operation, COMP controls the rise and fall time

of the lamp-current envelope. At the beginning of the

DPWM on-cycle, VCOMP rises linearly, gradually increas-

ing tON, which provides soft-start. At the end of the

DPWM on-cycle, the COMP capacitor discharges linear-

ly, gradually decreasing tON and providing soft-stop.

DPWM Frequency Setting

There are three ways to set the DPWM frequency.

1) The DPWM frequency can be set with an external

resistor. Connect SYNC to GND and connect a

resistor between FREQ and GND. The DPWM fre-

quency is given by the following equation:

The adjustable range of the DPWM frequency is

between 100Hz and 350Hz (RFREQ is between

353kΩand 101kΩ). CNTL controls the DPWM duty

cycle.

2) The DPWM frequency can be clocked by an exter-

nal high-frequency signal. Connect FREQ to VCC

and connect SYNC to the external high-frequency

signal. The DPWM frequency is 1/128 of the fre-

quency of the external signal:

where fEXT is the frequency of the external signal.

The frequency range of the external signal should

be between 13kHz and 45kHz, resulting in a DPWM

frequency range between 100Hz and 350Hz. CNTL

controls the DPWM duty cycle.

3) The DPWM frequency can be synchronized to an

external low-frequency signal. To enable this mode,

connect SYNC to VCC, connect FREQ to GND

through a 100kΩresistor, and connect DPWM to

the external low-frequency signal. The DPWM fre-

quency and duty cycle are equal to those of the

external signal.

The frequency range of the external signal is between

100Hz and 350Hz. In this mode, the brightness control

input CNTL is disabled, and the brightness is propor-

tional to the duty cycle of the external signal.

Table 3 summarizes the three ways of setting the

DPWM frequency.

UVLO

The MAX8722A includes an undervoltage-lockout

(UVLO) circuit. The UVLO circuit monitors the VCC volt-

age. When VCC is below 4.2V (typ), the MAX8722A

disables both high-side and low-side MOSFET drivers

and resets the fault latch.

Low-Power Shutdown

When the MAX8722A is placed in shutdown, all func-

tions of the IC are turned off except for the 5.4V linear

regulator. In shutdown, the linear-regulator output volt-

age drops to about 4.5V and the supply current is 6µA

(typ). While in shutdown, the fault latch is reset. The

device can be placed into shutdown by pulling SHDN

to its logic-low level.

Lamp-Out Protection

For safety, the MAX8722A monitors the lamp-current

feedback (IFB) to detect faulty or open CCFL tubes and

secondary short circuits in the lamp and IFB sense

resistor. As described in the Lamp-Current Regulation

section, the voltage on IFB is internally full-wave recti-

fied. If the rectified IFB voltage is below 600mV, the

MAX8722A charges the TFLT capacitor with 1µA. The

DPWM EXT

=128

fHzkR

DPWM FREQ

/ =×209 169 Ω

Low-Cost CCFL Backlight Controller

16 ______________________________________________________________________________________

FREQ SYNC DPWM DIGITAL PWM

FREQUENCY/DUTY CYCLE

Connect FREQ to GND

through an external resistor.

Connect SYNC to GND.

DPWM is used as the DPWM

signal output.

The resistor value sets the frequency.

CNTL controls the duty cycle.

Connect FREQ to VCC.

Connect SYNC to an

external high-frequency

signal.

DPWM is used as the DPWM

signal output.

The frequency is 1/128 of the

frequency of the external signal. CNTL

controls the duty cycle.

Connect FREQ to GND

through a 100kΩ resistor. Connect SYNC to VCC.Connect DPWM to an external

low-frequency signal.

The frequency and duty cycle are

equal to those of the external signal.

Table 3. DPWM Frequency Setting

MAX8722A latches off if the voltage on TFLT exceeds

4V. Unlike the normal shutdown mode, the linear-regu-

lator output (VCC) remains at 5.4V. Toggling SHDN or

cycling the input power reactivates the device.

During the delay period, the current control loop tries to

maintain lamp-current regulation by increasing the

high-side MOSFET on-time. Because the open-circuit

lamp impedance is very high, the transformer sec-

ondary voltage rises as a result of the high Q-factor of

the resonant tank. Once the secondary voltage

exceeds the overvoltage threshold, the MAX8722A

turns on a 1200µA current source that discharges the

COMP capacitor. The on-time of the high-side MOSFET

is reduced, lowering the secondary voltage, as the

COMP voltage decreases. Therefore, the peak voltage

of the transformer secondary winding never exceeds

the limit set by a capacitive voltage-divider during the

lamp-out delay period.

Primary Overcurrent Protection (ILIM)

The MAX8722A senses transformer primary current in

each switching cycle. When the regulator turns on the

low-side MOSFET, a comparator monitors the voltage

drop from LX_ to GND. If the voltage exceeds the cur-

rent-limit threshold, the regulator turns off the high-side

switch at the opposite side of the primary to prevent

further increasing the transformer primary current.

The current-limit threshold can be adjusted using the

ILIM input. Connect a resistive voltage-divider between

VCC and GND with the midpoint connected to ILIM. The

current-limit threshold measured between LX_ and

GND is 1/5 of the voltage at ILIM. The ILIM adjustment

range is 0 to 3V. Connect ILIM to VCC to select the

default current-limit threshold of 0.2V.

Secondary Current Limit (ISEC)

The secondary current limit provides fail-safe current

limiting in case a failure, such as a short circuit or leak-

age from the lamp high-voltage terminal to ground, pre-

vents the current control loop from functioning properly.

ISEC monitors the voltage across a sense resistor

placed between the transformer’s low-voltage sec-

ondary terminal and ground. The ISEC voltage is inter-

nally half-wave rectified and continuously compared to

the ISEC regulation threshold (1.25V typ). Any time the

ISEC voltage exceeds the threshold, a controlled cur-

rent is drawn from COMP to reduce the on-time of the

bridge’s high-side switches. At the same time, the

MAX8722A charges the TFLT capacitor with a 116µA

current source. The MAX8722A latches off when the

voltage on TFLT exceeds 4V. Unlike the normal shut-

down mode, the linear-regulator output (VCC) remains at

5.4V. Toggling SHDN or cycling the input power reacti-

vates the device.

Linear-Regulator Output (VCC)

The internal linear regulator steps down the DC input

voltage to 5.4V (typ). The linear regulator supplies

power to the internal control circuitry of the MAX8722A

and is also used to power the MOSFET drivers by con-

necting VCC to VDD. The VCC voltage drops to 4.5V in

shutdown.

Applications Information

MOSFETs

The MAX8722A requires four external n-channel power

MOSFETs NL1, NL2, NH1, and NH2 to form a full-

bridge inverter circuit to drive the transformer primary.

The regulator senses the on-state drain-to-source volt-

age of the two low-side MOSFETs NL1 and NL2 to

detect the transformer primary current, so the RDS(ON)

of NL1 and NL2 should be matched. For instance, if

dual MOSFETs are used to form the full bridge, NL1

and NL2 should be in one package. Since the

MAX8722A uses the low-side MOSFET RDS(ON) for pri-

mary overcurrent protection, the lower the MOSFET

RDS(ON), the higher the current limit. Therefore, the

user should select a dual, logic-level n-channel

MOSFET with low RDS(ON) to minimize conduction loss,

and keep the primary current limit at a reasonable level.

The regulator uses zero-voltage switching (ZVS) to soft-

ly turn on each of the four switches in the full bridge.

ZVS occurs when the external power MOSFETs are

turned on when their respective drain-to-source volt-

ages are near zero (see the Resonant Operation sec-

tion). ZVS effectively eliminates the instantaneous

turn-on loss of MOSFETs caused by COSS (drain-to-

source capacitance) and parasitic capacitance dis-

charge, and improves efficiency and reduces

switching-related EMI.

Setting the Lamp Current

The MAX8722A senses the lamp current flowing

through resistor R1 (Figure 1) connected between the

low-voltage terminal of the lamp and ground. The volt-

age across R1 is fed to IFB and is internally full-wave

rectified. The MAX8722A controls the desired lamp cur-

rent by regulating the average of the rectified IFB volt-

age. To set the RMS lamp current, determine R1 as

follows:

RmV

ILAMP RMS

1790

()

=×

MAX8722A

Low-Cost CCFL Backlight Controller

______________________________________________________________________________________ 17

MAX8722A

where ILAMP(RMS) is the desired RMS lamp current and

790mV is the typical value of the IFB regulation point

specified in the Electrical Characteristics table. To set

the RMS lamp current to 6mA, the value of R1 should

be 148Ω. The closest standard 1% resistors are 147Ω

and 150Ω. The precise shape of the lamp-current

waveform, which is dependent on lamp parasitics, influ-

ences the actual RMS lamp current. Use a true RMS

current meter connected between the R1/IFB junction

and the low-voltage side of the lamp to make final

adjustments to R1.

Setting the Secondary Voltage Limit

The MAX8722A limits the transformer secondary voltage

during startup and lamp-out faults. The secondary volt-

age is sensed through the capacitive voltage-divider

formed by C3 and C4 (Figure 1). The voltage on VFB is

proportional to the CCFL voltage. The selection of

the parallel resonant capacitor C3 is described in

the Transformer Design and Resonant Component

Selection section. C3 is usually between 10pF and 22pF.

After the value of C3 is determined, select C4 using the

following equation to set the desired maximum RMS sec-

ondary voltage VLAMP(RMS)_MAX:

where 2.34V is the typical value of the VFB peak voltage

when the lamp is open. To set the maximum RMS sec-

ondary voltage to 1600V using 18pF for C3, use approxi-

mately 15nF for C4.

Setting the Secondary Current Limit

The MAX8722A limits the secondary current even if the

IFB sense resistor (R1) is shorted or transformer sec-

ondary current finds its way to ground without passing

through R1. ISEC monitors the voltage across the

sense resistor R3, connected between the low-voltage

terminal of the transformer secondary winding and

ground. Determine the value of R3 using the following

equation:

where ISEC(RMS)_MAX is the desired maximum RMS

transformer secondary current during fault conditions,

and 1.241V is the typical value of the ISEC peak volt-

age when the secondary is shorted. To set the maxi-

mum RMS secondary current in the circuit of Figure 1

to 22mA, use approximately 40.2Ωfor R3.

Transformer Design and Resonant

Component Selection

The transformer is the most important component of the

resonant tank circuit. The first step in designing the

transformer is to determine the turns ratio (N). The ratio

must be high enough to support the CCFL operating

voltage at the minimum supply voltage. N can be cal-

culated as follows:

where VLAMP(RMS) is the maximum RMS lamp voltage

in normal operation, and VIN(MIN) is the minimum DC

input voltage. If the maximum RMS lamp voltage in nor-

mal operation is 650V and the minimum DC input volt-

age is 8V, the turns ratio should be greater than 90. The

turns ratio of the transformer used in the circuit of

Figure 1 is 93.

The next step in the design procedure is to determine

the desired operating frequency range. The MAX8722A

is synchronized to the natural resonant frequency of the

resonant tank. The resonant frequency changes with

operating conditions, such as the input voltage, lamp

impedance, etc.; therefore, the switching frequency

varies over a certain range. To ensure reliable opera-

tion, the resonant frequency range must be within the

operating frequency range specified by the CCFL

transformer manufacturer. As discussed in the

Resonant Operation section, the resonant frequency

range is determined by the transformer secondary leak-

age inductance L, the primary series DC blocking

capacitor C2, and the secondary parallel resonant

capacitor C3. Since it is difficult to control the trans-

former leakage inductance, the resonant tank design

should be based on the existing secondary leakage

inductance of the selected CCFL transformer. Leakage-

inductance values can have large tolerance and signifi-

cant variations among different batches, so it is best to

work directly with transformer vendors on leakage-

inductance requirements. The MAX8722A works best

when the secondary leakage inductance is between

250mH and 350mH. The series capacitor C2 sets the

minimum operating frequency, which is approximately

two times the series resonant peak frequency. Choose:

LAMP RMS

IN MIN

()

≥

×09

ISEC RMS MAX

31 241

()_

LAMP RMS MAX

234 3

()_

=××

Low-Cost CCFL Backlight Controller

18 ______________________________________________________________________________________

where fMIN is the minimum operating frequency range.

In the circuit of Figure 1, the transformer’s turns ratio is

93 and its secondary leakage inductance is approxi-

mately 300mH. To set the minimum operating frequen-

cy to 45kHz, use 1µF for C2.

The parallel capacitor C3 sets the maximum operating

frequency, which is also the parallel resonant peak fre-

quency. Choose C3 with the following equation:

In the circuit of Figure 1, to set the maximum operating

frequency to 65kHz, use 18pF for C3.

The transformer core saturation should also be consid-

ered when selecting the operating frequency. The pri-

mary winding should have enough turns to prevent

transformer saturation under all operating conditions.

Use the following expression to calculate the minimum

number of turns N1 of the primary winding:

where DMAX is the maximum duty cycle (approximately

0.4) of the high-side switches, VIN(MAX) is the maximum

DC input voltage, BSis the saturation flux density of the

core, and S is the minimal cross-section area of the core.

COMP Capacitor Selection

The COMP capacitor sets the speed of the current loop

that is used during startup, while maintaining lamp cur-

rent regulation, and during transients caused by

changing the input voltage. The typical COMP capaci-

tor value is 0.01µF. Larger values increase the tran-

sient-response delays. Smaller values speed up

transient response, but extremely small values can

cause loop instability.

Other Components

The external bootstrap circuits formed by D1 and

C5/C6 in Figure 1 power the high-side MOSFET drivers.

Connect VDD to BST1/BST2 through dual-diode D1 and

couple BST1/BST2 to LX1/LX2 through C5 and C6. C5

= C6 = 0.1µF or greater.

Layout Guidelines

Careful PC board layout is important to achieve stable

operation. The high-voltage section and the switching

section of the circuit require particular attention. The

high-voltage sections of the layout need to be well sep-

arated from the control circuit. Most layouts for single-

lamp notebook displays are constrained to long and

narrow form factors, so this separation occurs naturally.

Follow these guidelines for good PC board layout:

1) Keep the high-current paths short and wide, espe-

cially at the ground terminals. This is essential for

stable, jitter-free operation and high efficiency.

2) Use a star-ground configuration for power and ana-

log grounds. The power and analog grounds

should be completely isolated—meeting only at the

center of the star. The center should be placed at

the analog ground pin (GND). Using separate cop-

per islands for these grounds may simplify this task.

Quiet analog ground is used for VCC, COMP,

FREQ, TFLT, and ILIM (if a resistive voltage-divider

is used).

3) Route high-speed switching nodes away from sen-

sitive analog areas (VCC, COMP, FREQ, TFLT, and

ILIM). Make all pin-strap control input connections

(ILIM, etc.) to analog ground or VCC rather than

power ground or VDD.

4) Mount the decoupling capacitor from VCC to GND

as close as possible to the IC with dedicated traces

that are not shared with other signal paths.

5) The current-sense paths for LX1 and LX2 to GND

must be made using Kelvin sense connections to

guarantee the current-limit accuracy.

6) Ensure the feedback connections are short and

direct. To the extent possible, IFB, VFB, and ISEC

connections should be far away from the high-volt-

age traces and the transformer.

7) To the extent possible, high-voltage trace clearance

on the transformer’s secondary should be widely

separated. The high-voltage traces should also be

separated from adjacent ground planes to prevent

lossy capacitive coupling.

8) The traces to the capacitive voltage-divider on the

transformer’s secondary need to be widely separated

to prevent arcing. Moving these traces to opposite

sides of the board can be beneficial in some cases.

NDV

BSf

MAX IN MAX

S MIN

()

>×

××

fLCN

MAX

222

( )

≥

×××−π

MIN

≤

×× ×π

MAX8722A

Low-Cost CCFL Backlight Controller

______________________________________________________________________________________ 19

Chip Information

TRANSISTOR COUNT: 2985

PROCESS: BiCMOS

MAX8722A

Low-Cost CCFL Backlight Controller

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

implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

20 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600

Package Information

(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,

go to www.maxim-ic.com/packages.)

QSOP.EPS

21-0055

PACKAGE OUTLINE, QSOP .150", .025" LEAD PITCH