ISL1903FAZ - Intersil - Datasheet.Directory

FN8285 Rev 1.00 Page 1 of 19

September 20, 2012

FN8285

Rev 1.00

September 20, 2012

ISL1903

Dimmable Buck LED Driver - AC Mains or DC Input LED Driver

DATASHEET

The ISL1903 is a high-performance, critical conduction mode

(CrCM), single-ended buck LED driver controller. It may be used

with DC input converters, but also supports single-stage

conversion of the AC mains to a constant current source with

power factor correction (PFC). The ISL1903 supports buck

converter topologies, such as isolated forward converters or

non-isolated source return buck converters. Operation in CrCM

allows near zero-voltage switching (ZVS) for improved

efficiency while maximizing magnetic core utilization.

The ISL1903 is compatible with both leading and trailing edge

modulated AC mains dimmers. It provides all of the features

required for high-performance dimmable LED ballast designs.

Applications

• Industrial and commercial LED lighting

• Retrofit LED lamps with triac dimming

• Universal AC mains input LED retrofit lamps

• AC or DC input LED ballasts

Features

• Excellent LED current regulation over line, load, and

temperature

• 0 - 100% dimming with leading-edge (triac) and

trailing-edge dimmers

• Power factor correction for up to 0.995 power factor and

less than 20% harmonic content

• Critical conduction mode (CrCM) operation for

quasi-resonant high efficiency performance

• Supports Universal AC Mains Input

• Configurable for PWM or DC current dimming control of

LEDs

• Monitors FET switching current for load regulation

• Supports isolated and non-isolated buck topologies

• Closed loop soft-start for no overshoot

• OFFREF feature to set dimming off-point to improve fixture

performance matching

• -40°C to +125°C operation

• Pb-free (RoHS compliant)

FIGURE 1A. BOOST-RETURN (BUCK) TOPOLOGY FIGURE 1B. CURRENT vs AC CONDUCTION ANGLE

FIGURE 1. ISL1903 APPLICATION PERFORMANCE

Dimmer

EMI Filter

AC Mains

DHC

DELADJ

GND

VREF

IOUT

OUT

ISL1903

VDD CS+

410

RAMP

VERR

100

100 90 80 70 60 50 40 30 20 10 0

CONDUCTION ANGLE (%)

LED CURRENT (%)

OBSOLETE PRODUCT

NO RECOMMENDED REPLACEMENT

contact our Technical Support Center at

1-888-INTERSIL or www.intersil.com/tsc

FN8285 Rev 1.00 Page 2 of 19

September 20, 2012

ISL1903

Functional Block Diagram - ISL1903

OUT

PWM

COMPARATOR

PWM LATCH

RAMP

CRCM

DETECTOR FMAX

CLAMP

200mV

MASTER

OSCILLATOR

REFERENCE OUT

DUTY CYCLE TO

VOLTAGE CONVERTER

LOW PASS FILTER

DELADJ

TRIANGLE WAVE

GENERATOR

CLK -

+PWMOUT

VERR

600mV

DIMMING PWM

EA1

CRCM OSCILLATOR/PWM/ERROR AMPLIFIERS

LEADING

EDGE

BLANKING

FB1

QUASI-ZVS

DELAY

IOUT

REFINBUFF

PRIMARY OC

CLK

FMIN

CLAMP

PEAK

DETECTOR

1/5

SS/5

1/5

VERR/5

PWM

VERR

BIAS AND

REFERENCE

GENERATOR

GND

VDD

UVLO

VREF

OTP

SHUTDOWN

150°C TO 170°C

BIAS/UVLO/OTP

CS+

ISENSE

IOUT

OFFREF

MINIMUM DIMMING LEVEL

CONTROL

0.25VSS LOW

FAULT LATCH

300ms

SOFT-START

ENABLE

-1.50V

INHIBIT INHIBIT

VERR CLAMP

OVP

REF

REFERENCE SS

AC-PRESENT

REFINBUFF

REFERENCE

SS BUFFER

AC DETECTION

SOFT-START/POR/OVP

DHC

PRIMARY CURRENT

SENSE PROCESSOR

FN8285 Rev 1.00 Page 3 of 19

September 20, 2012

ISL1903

Typical Application - Dimmable Buck LED Driver

CS+

DHC

OFFREF PWMOUT

VREF

OUT

VDD

OC OVP

IOUT

RAMP

GND

DELADJ VERR

EMI

FILTER

MAINS

DIMMER

ISL1903

FN8285 Rev 1.00 Page 4 of 19

September 20, 2012

ISL1903

Typical Application - DC Input Dimmable Buck LED Driver

CS+

DHC

OFFREF PWMOUT

VREF

OUT

VDD

OC OVP

IOUT

RAMP

GND

DELADJ VERR

9V TO 26V

PWM DIMMING INPUT

90Hz TO 140Hz 0 TO 4V

0 TO 100% DUTY CYCLE

ISL1903

FN8285 Rev 1.00 Page 5 of 19

September 20, 2012

ISL1903

Typical Application - Isolated Dimmable Buck LED Driver

CS+

DHC

OFFREF PWMOUT

VREF

OUT

VDD

OC OVP

IOUT

RAMP

GND

DELADJ VERR

EMI

FILTER

MAINS

DIMMER

ISL1903

FN8285 Rev 1.00 Page 6 of 19

September 20, 2012

Pin Configuration

ISL1903

(16 LD QSOP)

TOP VIEW

CS+

DHC

OFFREF PWMOUT

VREF

OUT

VDD

OC OVP

IOUT

RAMP

GND

DELADJ VERR

Pin Descriptions

PIN # SYMBOL DESCRIPTION

1 VDD VDD is the power connection for the IC. To optimize noise immunity, bypass VDD to GND with a ceramic capacitor as close to

the VDD and GND pins as possible.

2 OFFREF Sets the reference level to disable the driver at light loading. The turn-off reference can be set at any level between 0 and 0.6V,

corresponding to 0 to 100% of output loading. This feature is normally used in triac-based wall dimmer applications to disable

the output before the dimmer becomes unstable due to insufficient holding current.

3 VREF The 5.40V reference voltage output having ±100 mV tolerance over line, load and operating temperature. Bypass to GND with

a 0.1µF to 3.3µF low ESR capacitor.

4 IOUT A voltage signal proportional to the peak switching current used to determine the inductor current.

5 CS+ The input for the CrCM current sense circuit. This input monitors the winding current or voltage to determine the critical

conduction operating point.

6 OC The input to the load current sensing circuitry and the peak overcurrent comparator. The signal is sampled at the peak current

level for each switching cycle, amplified, and output on IOUT as a DC signal. It must be scaled before application to the FB pin

of the error amplifier (EA). The overcurrent comparator threshold is set at 600mV nominal. Peak OCP performs cycle-by-cycle

over current protection. OCP includes leading-edge-blanking (LEB), which blocks the signal at the beginning of the OUT pulse

for the duration of the blanking period and when the OUT pulse is low.

7 FB FB is the inverting input to the error amplifier (EA). The feedback signal from IOUT, after being scaled and filtered, is applied

to the error amplifier.

8 DELADJ Sets delay before a new switching cycles starts. This adjustment allows the user to delay the next switching cycle until the

switching FET drain-source voltage reaches a minimum value to allow quasi-ZVS (Zero Voltage Switching) operation. A resistor

to ground programs the delay. Pulling DELADJ to VREF disables the CrCM oscillator.

9 VERR Output of the error amplifiers and the control voltage input to the inverting input of the PWM comparator. VERR cannot source

current and requires an external pull-up resistor to VREF.

10 RAMP This is the input for the sawtooth waveform for the PWM comparator. Using an RC from VREF, a sawtooth waveform is created

for use by the PWM. It is compared to the error amplifier output, Verr, to create the PWM control signal. The RAMP pin is shorted

to GND at the termination of the PWM signal.

11 OVP Input to detect an overvoltage (OV) condition on the output. Since the control variable is output current, a fault that results in

an open circuit will cause excessive output voltage. The circuit hysteresis is a switched current source that is active when the

OV threshold is exceeded.

12 AC Input to sense AC voltage presence and amplitude. A resistor divider from the main FET drain and circuit ground or from an

auxiliary winding on the transformer/inductor is used to detect the AC voltage.

13 GND Signal and power ground connections for this device. Due to high peak currents and high frequency operation, a low impedance

layout is necessary. Ground planes and short traces are highly recommended.

14 DHC An open drain FET used to load the input voltage to pre-load a triac-based dimmer so that adequate holding current is

maintained.

ISL1903

FN8285 Rev 1.00 Page 7 of 19

September 20, 2012

15 PWMOUT The PWM gate drive output for LED dimming. The output level is clamped to ~12V for VDD greater than 12V. PWMOUT has

pull-down capability when UVLO is active or when the IC is not biased. This output is used to drive an external dimming FET.

The PWM operates at ~ 310Hz.

16 OUT The gate drive output for the external power FET. OUT is capable of sourcing and sinking 1A @ VDD = 8V. The output level is

clamped to ~12V for VDD greater than 12V. OUT has pull-down capability when UVLO is active or when the IC is not biased.

Pin Descriptions (Continued)

PIN # SYMBOL DESCRIPTION

Ordering Information

PART NUMBER

(Notes 1, 2, 3)

PART

MARKING

TEMP. RANGE

(°C)

PACKAGE

(Pb-free)

PKG.

DWG. #

ISL1903FAZ 1903 FAZ -40 to +125 16 Ld QSOP M16.15A

ISL1903EVAL2Z Evaluation Board

1. Add “-T*” suffix for tape and reel. Please refer to TB347 for details on reel specifications.

2. These Intersil Pb-free plastic packaged products employ special Pb-free material sets, molding compounds/die attach materials, and 100% matte

tin plate plus anneal (e3 termination finish, which is RoHS compliant and compatible with both SnPb and Pb-free soldering operations). Intersil

Pb-free products are MSL classified at Pb-free peak reflow temperatures that meet or exceed the Pb-free requirements of IPC/JEDEC J STD-020.

3. For Moisture Sensitivity Level (MSL), please see device information page for ISL1903. For more information on MSL please see tech brief TB363.

Related Products

PART NUMBER KEY DIFFERENTIATORS

ISL1901 Isolated and non-isolated single-stage flyback regulator

ISL1902 Isolated and non-isolated single-stage flyback regulator with inrush control and interface features for temperature and

ambient light sensors

ISL1903 Non-isolated single-stage buck regulator using switch current for regulation

ISL1904 Isolated single-stage flyback regulator with primary side current sense regulation

ISL1907 Non-isolated two-stage cascaded boost PFC + buck regulator eliminates dependency on electrolytic capacitors

ISL1908 Isolated two-stage cascaded boost PFC + flyback regulator eliminates dependency on electrolytic capacitors

ISL1903

FN8285 Rev 1.00 Page 8 of 19

September 20, 2012

Absolute Maximum Ratings (Note 4) Thermal Information

Supply Voltage, VDD. . . . . . . . . . . . . . . . . . . . . . . . . . . GND - 0.3V to +28.0V

OUT, PWMOUT, DHC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .GND - 0.3V to VDD

Signal Pins. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . GND - 0.3V to VREF + 0.3V

VREF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .GND - 0.3V to 6.0V

Peak OUT Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.0A

Peak PWMOUT Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.0A

ESD Classification

Human Body Model (Per MIL-STD-883 Method 3015.7). . . . . . . . 2500V

Machine Model (Per EIAJ ED-4701 Method C-111) . . . . . . . . . . . . . 200V

Charged Device Model (Per EOS/ESD DS5.3, 4/14/93). . . . . . . . 1000V

Latch up (Per JESD-78B; Class 1, Level A) . . . . . . . . . . . . . . . . . . . . 100mA

Thermal Resistance (Typical) JA (°C/W) JC (°C/W)

16 Lead QSOP (Notes 5, 6) . . . . . . . . . . . . . 85 44

Maximum Junction Temperature . . . . . . . . . . . . . . . . . . . . . -55°C to 150°C

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

Pb-Free Reflow Profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . see link below

http://www.intersil.com/pbfree/Pb-FreeReflow.asp

Operating Conditions

Temperature Range

ISL1903Fxx . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -40°C to 125°C

Supply Voltage Range (Typical). . . . . . . . . . . . . . . . . . . . . . . . . . 9 to 20 VDC

CAUTION: Do not operate at or near the maximum ratings listed for extended periods of time. Exposure to such conditions may adversely impact product

reliability and result in failures not covered by warranty.

NOTES:

4. All voltages are with respect to GND.

5. JA is measured with the component mounted on a high effective thermal conductivity test board in free air. See Tech Brief TB379 for details.

6. For JC, the “case temp” location is taken at the package top center.

Electrical Specifications Recommended operating conditions unless otherwise noted. Refer to “Functional Block Diagram -

ISL1903” on page 2 and “Typical Application schematics” beginning on page 3. VDD = 17V, RRAMP = 54k, CRAMP = 470pF, TA = -40°C to

+125°C, Typical values are at TA = +25°C; Boldface limits apply over the operating temperature range, -40°C to +125°C.

PARAMETER TEST CONDITIONS

MIN

(Note 7) TYP

MAX

(Note 7) UNITS

SUPPLY VOLTAGE

Supply Voltage --26 V

Start-Up Current, IDD VDD = 5.0V - 100 200 µA

Operating Current, IDD RLOAD, COUT = 0 - 6.0 7.8 mA

UVLO START Threshold 8.15 8.55 8.95 V

UVLO STOP Threshold 6.80 7.10 7.50 V

Hysteresis -1.45- V

REFERENCE VOLTAGE VREF

Overall Accuracy IVREF = 0 -, -10mA, 8V < VDD< 26V 5.30 5.40 5.50 V

Long Term Stability TA = +125°C, 1000 hours (Note 8) - 10 25 mV

Operational Current (Source) 8V < VDD< 26V - - -10 mA

Current Limit VREF = 5.00V, 8V < VDD< 26V -100 --15 mA

Load Capacitance (Note 8) 0.1 -3.3 µF

PEAK CURRENT SENSE (OC)

Current Limit Threshold VERR = VREF, RAMP = 0V 570 595 616 mV

IOUT Amplifier Gain VOC = 0.4V, 8V < VDD< 26V 3.90 4.00 4.13 V/V

IOUT High Level Output Voltage (VOH) VIOUT @ 0µA - VIOUT@ -100µA,

8V < VDD< 26V

--0.1 V

IOUT Low Level Output Voltage (VOL) VIOUT @ 100µA, 8V < VDD< 26V - - 0.1 V

Leading Edge Blanking (LEB) Duration 70 120 146 ns

OC to OUT Delay + LEB TA = +25°C 110 170 200 ns

Input Bias Current VOC = 0.3V -1.0 -1.0 µA

ISL1903

FN8285 Rev 1.00 Page 9 of 19

September 20, 2012

RAMP

RAMP Sink Current Device Impedance IRAMP = 10 mA - - 20 Ω

RAMP to PWM Comparator Offset TA = +25°C 181 235 287 mV

Input Bias Current VRAMP = 0.3V -1.0 -1.0 µA

PULSE WIDTH MODULATOR

PWM Restart Delay Range 8V < VDD< 26V 0.2 -2.0 µs

PWM Restart Cycle Delay RDELADJ = 20.0k, 8V < VDD< 26V 240 280 320 ns

RDELADJ = 210k, 8V < VDD< 26V 2.00 2.20 2.40 s

Maximum Frequency Clamp 8V < VDD< 26V, RAMP = 2V,

RRAMP = 100

0.8 1.0 1.2 MHz

Minimum Frequency Clamp 8V < VDD< 26V, RRAMP = 23K20 25 31 kHz

Minimum On Time 8V < VDD< 26V, FB = 1V, AC = 2V,

RAMP = 0V

173 -246 ns

VERR to PWM Gain 8V < VDD< 26V - 0.200 - V/V

SS to PWM Gain 8V < VDD< 26V - 0.222 - V/V

ERROR AMPLIFIER

Input Common Mode (CM) Range (Note 8) 0-3.4 V

GBWP (Note 8) 1.9 --MHz

VERR VOL IVERR = 6mA, 8V < VDD< 26V --0.950 V

VERR VOH IVERR = 1mA (Ext. pull-up),

SS complete

3.90 4.00 4.20 V

Open Loop Gain (Note 8) 70 --dB

Offset Voltage (VOS) 8V < VDD< 26V -7.5 -7.5 mV

Input Bias Current 8V < VDD< 26V -1.0 -1.0 µA

CURRENT SENSE (CS+)

Zero Current Detection (CrCM) Threshold, Falling 8V < VDD< 26V 618 30 mV

Input Bias Current 8V < VDD< 26V -1.0 -1.0 µA

AC DETECTOR

Input Bias Current 8V < VDD< 26V -50 -50 nA

Detection Threshold, Falling 8V < VDD< 26V, ACPEAK = 100mV 19 32 51 mV

Detection Threshold Hysteresis 8V < VDD< 26V -23 -mV

Input Operating Range 8V < VDD< 26V 0-4.00 V

Clamp Voltage IACDETECT = 1.0mA 6.8 -7.6 V

EA Reference Input Range 8V < VDD< 26V 0 - 0.538 V

Electrical Specifications Recommended operating conditions unless otherwise noted. Refer to “Functional Block Diagram -

ISL1903” on page 2 and “Typical Application schematics” beginning on page 3. VDD = 17V, RRAMP = 54k, CRAMP = 470pF, TA = -40°C to

+125°C, Typical values are at TA = +25°C; Boldface limits apply over the operating temperature range, -40°C to +125°C. (Continued)

PARAMETER TEST CONDITIONS

MIN

(Note 7) TYP

MAX

(Note 7) UNITS

ISL1903

FN8285 Rev 1.00 Page 10 of 19

September 20, 2012

EA Reference vs AC Conduction Angle ILPOUT = 0µA, f = 120Hz (rectified),

8V < VDD< 26V

Duty Cycle () = 98% 523 548 574 mV

Duty Cycle () = 75% 286 318 340 mV

Duty Cycle () = 50% 117 139 156 mV

Duty Cycle () = 25% 16 33 44 mV

Duty Cycle () = 10% 0311 mV

DHC

Low Level Output Voltage (VOL) VDHC = 10mA,

VDD = 8V operating

--600 mV

Turn-off Delay after AC Returns - 4.0 - µs

SOFT-START

Duration 289 389 483 ms

Reference Soft Start Initial Step 11 27 43 mV

OFFREF

Input Bias Current -1.0 -1.0 µA

Operating Range (Excluding Offset) 0-0.5 V

Threshold Hysteresis 33 52 70 mV

Threshold Offset 78 104 129 mV

AC Dropout Disable Delay -32 ms

OUT

High Level Output Voltage (VOH) VOUT @ 0mA - VOUT @ -100mA,

VDD = 8V operating

-0.35 1.2 V

Low Level Output Voltage (VOL) VOUT @ 100mA, VDD = 8V operating -0.7 1.2 V

Rise Time CLOAD = 2.2nF, VDD = 8V,

t90% - t10%

-35 55 ns

Fall Time CLOAD = 2.2nF, VDD = 8V,

t10% - t90%

-25 40 ns

Output Clamp Voltage VDD = 20V, ILOAD = -10µA 10.5 12.0 13.4 V

Unbiased Output Voltage Clamp VDD = 6V, ILOAD = 5mA --1.9 V

PWMOUT

High Level Output Voltage (VOH) VOUT @ 0mA - VOUT @ -10mA,

VDD = 8V operating

-0.8 1.2 V

Low Level Output Voltage (VOL) VOUT @ 10mA,

VDD = 8V operating

-0.8 1.2 V

Rise Time CLOAD = 1nF, VDD = 8V operating,

t90% - t10%

-160 240 ns

Fall Time CLOAD = 1nF, VDD = 8V operating,

t10% - t90%

-160 240 ns

Output Voltage Clamp VDD = 20V, ILOAD = -10µA 10.5 12.0 13.4 V

Electrical Specifications Recommended operating conditions unless otherwise noted. Refer to “Functional Block Diagram -

ISL1903” on page 2 and “Typical Application schematics” beginning on page 3. VDD = 17V, RRAMP = 54k, CRAMP = 470pF, TA = -40°C to

+125°C, Typical values are at TA = +25°C; Boldface limits apply over the operating temperature range, -40°C to +125°C. (Continued)

PARAMETER TEST CONDITIONS

MIN

(Note 7) TYP

MAX

(Note 7) UNITS

ISL1903

FN8285 Rev 1.00 Page 11 of 19

September 20, 2012

Unbiased Output Voltage Clamp VDD = 6V, ILOAD = 3mA --1.9 V

Frequency 291 320 349 Hz

Maximum Duty Cycle REFIN = 0.5V --100 %

Minimum On-Time REFIN = 0V --0.5 s

OVP

OVP Threshold 1.46 1.50 1.54 V

OVP Hysteresis 10 20 27 A

Input Bias Current -1.0 -1.0 A

OVP Clamp Voltage IOVP = 1mA 5.4 -7.0 V

THERMAL PROTECTION

Thermal Shutdown (Note 8) 150 160 170 °C

Hysteresis (Note 8) - 25 - °C

NOTES:

7. Parameters with MIN and/or MAX limits are 100% tested at +25°C, unless otherwise specified. Temperature limits established by characterization

and are not production tested.

8. Limits established by characterization and are not production tested.

Electrical Specifications Recommended operating conditions unless otherwise noted. Refer to “Functional Block Diagram -

ISL1903” on page 2 and “Typical Application schematics” beginning on page 3. VDD = 17V, RRAMP = 54k, CRAMP = 470pF, TA = -40°C to

+125°C, Typical values are at TA = +25°C; Boldface limits apply over the operating temperature range, -40°C to +125°C. (Continued)

PARAMETER TEST CONDITIONS

MIN

(Note 7) TYP

MAX

(Note 7) UNITS

Test Waveforms and Circuits

FIGURE 2. RISE/FALL TIME TEST CIRCUIT FIGURE 3. RISE/FALL TIMES

CS+

DHC

OFFREF PWMOUT

VREF

OUT

VDD

OC OVP

IOUT

RAMP

GND

DELADJ VERR

ISL1903

1nF 2.2nF

470pF

54k

0 TO 1V 120Hz

OUT

PWMOUT tRtF

90%

10%

ISL1903

FN8285 Rev 1.00 Page 12 of 19

September 20, 2012

FIGURE 4. OC +LEB TO OUT DELAY FIGURE 5. AC MAINS TO DHC TIMING

Test Waveforms and Circuits (Continued)

OUT

OC THRESHOLD

LEADING EDGE BLANKING

OC PROPAGATION DELAY

OC + LEB TO OUT DELAY

MAINS

DHC

tDELAY

Typical Performance Curves

FIGURE 6. REFERENCE VOLTAGE vs TEMPERATURE FIGURE 7. EA REFERENCE vs AC SIGNAL DUTY CYCLE

FIGURE 8. DELAY vs DELADJ RESISTANCE FIGURE 9. PWMOUT DUTY CYCLE vs AC SIGNAL DUTY CYCLE

-40 -25 -10 520 35 50 65 80 95 110 125

0.996

0.997

0.998

0.999

1.000

1.001

TEMPERATURE (°C)

NORMALIZED VREF

0 102030405060708090100

100

200

300

400

500

AC CONDUCTION ANGLE (% DUTY CYCLE 120Hz)

EA REFERENCE (mV)

025 50 75 100 125 150 175 200 225

0.5

1.0

1.5

2.0

2.5

DELAY RESISTANCE (kΩ)

DELAY TIME (µs)

0 102030405060708090100

100

AC CONDUCTION ANGLE (% DUTY CYCLE 120Hz)

PWMOUT DUTY CYCLE (%)

ISL1903

FN8285 Rev 1.00 Page 13 of 19

September 20, 2012

Functional Description

Features

The ISL1903 LED driver is an excellent choice for low cost AC

mains powered single conversion LED lighting applications. It

provides active power factor correction (PFC) to achieve high

power factor using critical conduction mode operation, and

incorporates additional features for compatibility with

triac-based dimmers. Furthermore, it senses FET switching

currents to regulate the output current which eliminates the need

to level shift current feedback signal, or for isolated designs, to

cross the isolation boundary to close the feedback control loop.

The ISL1903 includes support for both PWM and DC current

dimming of the output.

Oscillator

The ISL1903 uses a critical conduction mode (CrCM) algorithm to

control the switching behavior of the converter. The ON-time of

the primary power switch is held virtually constant by the low

bandwidth control loop (in PFC applications). The OFF-time

duration is determined by the time it takes the current or voltage

to decay during the flyback period. When the mmf (magneto

motive force) of the transformer decays to zero, the winding

currents are zero and the winding voltages collapse. Either may

be monitored and used to initiate the next switching cycle. The

ISL1903 monitors the CrCM condition using the CS+ signal. It

may be used to monitor either current or voltage.

Additionally, there is a user adjustable delay duration, DELADJ, to

delay the initiation of the next switching cycle to allow the

drain-source voltage of the primary switch to ring to a minimal.

This allows quasi-ZVS operation to reduce capacitive switching

losses and improve efficiency. See “Quasi-Resonant Switching”

on page 17.

By its nature the converter operation is variable frequency. There

are both minimum and maximum frequency clamps that limit

the range of operation. The minimum frequency clamp prevents

the converter from operating in the audible frequency range. The

maximum frequency clamps prevents operating at very high

frequencies that may result in excessive losses.

An individual switching period is the sum of the ON-time, the

OFF-time, and the restart delay duration. The ON-time is

determined by the control loop error voltage, VERR, and the

RAMP signal. As its name implies, the RAMP signal is a linearly

increasing signal that starts at zero volts and ramps to a

maximum of ~VERR/5 - 235mV. RAMP requires an external

resistor and capacitor connected to VREF to form an RC charging

network. If VERR is at its maximum level of VREF, the time

required to charge RAMP to ~850mV determines the maximum

ON-time of the converter. RAMP is discharged every switching

cycle when the ON-time terminates.

The OFF-time duration is determined by the design of the

magnetic element(s), which depends on the required energy

storage/transfer and the inductance of the winding(s). The

transformer/inductor design also determines the maximum

ON-time that can be supported without saturation, so, in reality,

the magnetics design is critical to every aspect of determining

the switching frequency range.

The design methodology is similar to designing a discontinuous

mode (DCM) buck converter with the constraint that it must

operate at the DCM/CCM boundary at maximum load and

minimum input voltage. The difference is that the converter will

always operate at the DCM/CCM boundary, whereas a DCM

converter will be more discontinuous as the input voltage

increases or the load decreases. In PFC applications, the design

is further complicated by the input voltage waveform, a rectified

sinewave.

Once the output power, Po, the output current, Io, the output

voltage, Vo, and the minimum input AC voltage are known, the

inductor design can be started. From the minimum AC input

voltage, the minimum DC equivalent (RMS) input voltage must

be determined. In PFC applications, the converter behaves as if

the input voltage is an equivalent DC value due to the low control

loop bandwidth.

A typical minimum operating frequency must be selected. This is

a somewhat arbitrary determination, but does ultimately

determine the inductor size. The typical frequency is what occurs

when the instantaneous rectified input AC voltage is exactly at

the equivalent DC value. The frequency will be higher when the

instantaneous input voltage is lower, and lower when the

instantaneous input voltage is higher. However, the duty cycle at

the equivalent DC input voltage determines the ON-time for the

entire AC half-cycle. The ON-time is constant due to the low

bandwidth control loop, but the OFF-time and duty cycle vary with

the instantaneous input voltage since the peak switch current

follows V = Ldi/dt.

The typical frequency may require adjustment once the initial

calculations are complete to see if the operating frequency at the

peak of the minimum AC input voltage is acceptable. The peak

current will be 1.41 times higher at the AC peak than at the DC

equivalent (RMS) input voltage. So, while the ON-time is nearly

constant due to the low bandwidth control loop, the OFF-time will

be 1.41 times longer.

The effective AC conduction angle must also be considered when

calculating the inductance. Since no current flows to the load

when the instantaneous input voltage is less than the output

voltage, the equivalent DC input voltage (rms) is duty cycle

modulated by the effective AC conduction angle. This results in

higher currents during the portion of the AC half-cycle when the

converter can deliver power to the load. The switching currents

increase and the frequency of operation decreases. Obviously the

higher the output voltage the greater the impact.

ISL1903

FN8285 Rev 1.00 Page 14 of 19

September 20, 2012

The first calculation required is to determine the required

inductance. The desired inductance can be calculated using

Equation 1.

where Lest is the inductance required to achieve the selected

operating frequency at the peak of the AC voltage waveform.

Note that Equation 1 calculates the required inductance when

operating at the DC equivalent input voltage. It does not take into

account the reduction in conduction angle that occurs when the

instantaneous input voltage is less than the output voltage.

Equation 2 corrects for this.

The maximum ON-time can be found using Equation 3.

The peak current at the end of the ON-time is shown in

Equation 4:

And the OFF-time is shown in Equation 5:

The lowest switching frequency is the reciprocal of the sum of the

ON-time, the OFF-time, and the delay time shown by Equation 6.

The delay time can be approximated if the equivalent

drain-source capacitance (Coss) of the primary switch is known.

This value should also include any parasitic capacitance on the

drain node. These parameters may not be known during the early

stages of the design, but are typically on the order of 300ns to

500ns.

If the lowest frequency does not meet the requirements, then

iterative calculations may be required.

The highest frequency is determined by the shortest ON-time

summed with tdelay. The shortest ON-time occurs at high line and

minimum load, and occurs at or near the AC zero crossing when

the primary (and secondary) current is zero. The minimum

ON-time the ISL1903 can produce is ~200ns, suggesting an

operating frequency above 1MHz. Regardless, the maximum

frequency clamp limits the frequency to about 1MHz.

Once the inductance is determined, the general formulae to

calculate the ON-time and OFF-time at an equivalent DC input

voltage and load are:

It is clear from the equations that there is a linear relationship

between load current and frequency. At some light load the

frequency will be limited by the maximum frequency clamp.

There is an inverse relationship between the input voltage and

frequency and its effect is restricted by the input voltage range of

the application.

It should be noted, however, that Equations 8 and 9 assume full

conduction angle of the AC mains. There are two issues

regarding actual conduction angle. First, there is no power

delivered to the load until the AC mains instantaneous voltage

exceeds the output voltage. Like any buck converter, the input

voltage must be higher than the output voltage. Secondly, when

conduction angle modulating dimmers are used to block a

portion of each AC half-cycle, the switching currents remain

essentially unchanged during the conduction portion of the AC

half-cycle as the conduction angle is reduced. The conduction

angle is reduced, not the amplitude of the waveform envelope.

The result being the steady state frequency behavior will not vary

much as the conduction angle is reduced depending on the

linearity of the conduction angle and the control loop reference

gain. See Figure 7 on page 12.

Soft-Start Operation

Soft-start is not user adjustable and is fixed at ~ 400ms. Both the

duty cycle and control loop reference are affected by soft-start.

Soft-starting both the duty cycle and the reference ensures a well

behaved closed loop soft-start that results in virtually no

overshoot.

TABLE 1. OSCILLATOR DEFINITIONS

VmINrms = Minimum RMS input voltage

VmaxINrms = Maximum RMS input voltage

fmin(avg) = Typical frequency when VIN (instantaneous) =

minimum VIN(rms)

Dmax = Maximum typical duty cycle desired

Dmin = Minimum typical duty cycle

tON(MAX) =f

typ(avg) x Dmax

tON ON-time of the power FET controlled by OUT

tOFF OFF-time duration required for CrCM operation

L = Inductance

Nsp = Transformer turns ratio, Ns/Np

Ip(peak) = Peak switch current within a switching cycle

tdelay = User adjustable delay before the next switching

cycle begins

Lest

VOVIN rms

–

min avg

IO2V

IN rms



------------------------------------------------------------------------------------

=H(EQ. 1)

est

2sin

1–VO

IN rms



------------------------------------







–



--------------------------------------------------------------------------

=H

(EQ. 2)

tON

OL

VIN rms

–

--------------------------------------

=s(EQ. 3)

Ip peak

IN rms

VO

–tON



---------------------------------------------------------------------

=A(EQ. 4)

tOFF

ppeak



-----------------------------

=s(EQ. 5)

fmin

tON tOFF tdelay

----------------------------------------------------

=Hz (EQ. 6)

tdelay

LpCoss Cother

+

-----------------------------------------------------------------

s(EQ. 7)

tOFF

L2I



---------------------

=s(EQ. 8)

tON

L2I



VIN rms

------------------------

=s(EQ. 9)

ISL1903

FN8285 Rev 1.00 Page 15 of 19

September 20, 2012

AC Detection and Reference Generation

The ISL1903 creates a 0 to 0.5V reference for the LED current

control loop by measuring the conduction angle of the AC input

voltage. The reference changes only with conduction angle and is

virtually unaffected by variation in either voltage amplitude or

frequency.

The ISL1903 cannot detect the conduction angle by monitoring

the input voltage directly. The AC voltage does not track the

source voltage on the load side of the dimmer once the input

voltage drops below the output voltage. In the buck topology the

converter stops drawing current from the AC line once the

instantaneous input voltage drops below the output voltage. This

results in commutation of the dimmer triac which leaves a

residual voltage on the input capacitance and impedes/prevents

the detection of the conduction angle. Instead, the conduction

angle is detected indirectly, either by monitoring the drain-source

voltage of the switching FET or by using an auxiliary winding on

the inductor. When the dimmer is blocking, the switching FET,

although switching, has no drain-source voltage and transfers no

power. If an auxiliary winding is present, it is not energized.

Referring to Figure 10, capacitor C1 is added to filter the scaled

switching waveform of the FET drain-source voltage, and delays

the detection of the loss of AC voltage. This has the effect of

masking the conduction angle reduction caused by the buck

topology as well as that due to variation in maximum dimmer

conduction angles between manufacturers.

The AC pin has an input range of 0 to 4V. The peak of the input

signal should range between 1 and 4 volts for best accuracy. The

AC detection circuit measures both the duration of the AC

conduction angle and the half-cycle duration. By comparing the

two every half-cycle, the detection circuit creates a frequency

independent reference that is updated each AC half-cycle.

In the event of an AC outage, the AC mains frequency reference

is lost. The ISL1903 will force the reference to zero volts and

reset the soft-start circuit approximately 35ms after the last AC

zero crossing is detected. If AC is held above its detection

threshold for the same duration, the internal reference is forced

to its maximum of ~0.5V.

AC may be directly coupled to a 90Hz to 130Hz PWM signal to

generate a reference if dimming is desired without using an AC

dimmer.

Primary Current Sensing

The ISL1903 is configured to regulate the output current by

monitoring the primary switch current at the OC pin. The peak

primary switch current is captured, processed, and output on

IOUT as a DC signal that is amplitude modulated in proportion to

the output current. The IOUT amplitude is equivalent to 4x the

peak switch current during the previous ON-time. It must be

scaled before being input to the control loop at the FB pin.

The OC pin also provides cycle-by-cycle overcurrent protection.

The ON-time is terminated if OC exceeds 0.6V nominal. There is

~120ns of leading edge blanking (LEB) on OC to minimize or

eliminate external filtering.

Dimming

The ISL1903 supports both PWM and DC current modulation

dimming. In either case, the control loop determines the average

current delivered to the load. PWM dimming is not

recommended for non-isolated applications requiring PFC. The

PWM dimming method will cause high harmonic content due to

the low PWM dimming frequency.

The usual method of dimming an LED string is to modulate the

DC current through the string. DC current dimming is the lower

cost method, but results in a non-linear dimming characteristic

due to the increasing efficacy of the LEDs as current is reduced.

PWM dimming results in linear dimming behavior.

For PWM dimming, an external FET, controlled by PWMOUT, is

required to gate the drive signal to the switching FET. See “Typical

Application - DC Input Dimmable Buck LED Driver” on page 4 for

an example. When PWMOUT is high, the main switching FET

operates normally. When PWMOUT is low, the main switching

FET gate signal is blocked and the converter is effectively off.

Regardless of the dimming method used, the control loop

determines the average current delivered to the load. It does not

matter if the load current is DC or pulsed, the converter control

loop and output capacitance operate to filter and average the

converter output current independently of the actual load current

waveform.

The dimming PWM and control loop are linked together such that

the PWM duty cycle tracks the main control loop reference

setpoint. If the control loop is set for 50% load, for example, the

dimming PWM duty cycle is set for 50%. The LED current will be

at 100% load for 50% of the time and 0% load for 50% of the

time, which averages to the 50% average load setpoint. See

Figures 7 and 9 for a graphical representation of the relationship

between the control loop reference and PWMOUT duty cycle. If

PWM dimming is used, the control loop bandwidth must be

reduced significantly below the PWM dimming frequency. It

should be noted that the PWMOUT duty cycle is not allowed to go

to zero.

FIGURE 10. AC DETECTION

ISL1903

EMI

FILTER

ISL1903

FN8285 Rev 1.00 Page 16 of 19

September 20, 2012

Control Loop

The control loop configuration is user adjustable with the

selection of the external compensation components. For

applications requiring power factor correction (PFC), a very low

bandwidth integrator is used, typically 20Hz or less. In other

applications, the control loop bandwidth can be increased as

required like any other externally compensated voltage mode

PWM controller.

Referring to Figure 11, the FET switching current flowing through

Rs, is applied to the OC pin of the ISL1903. The peak signal is

sampled, buffered, and output on IOUT with a gain of four. The

voltage on IOUT represents 8x the average load current on a

cycle-by-cycle basis. In PFC applications, IOUT tracks the rectified

AC voltage waveform and must be averaged. For DC input

applications, this is obviously not required.

where IOUT is the average or DC value of IOUT. Prior to applying

IOUT to the EA at the FB pin it must be scaled such that at

maximum output current the signal is equal to the maximum EA

reference level (nominally 0.530V), while also limiting the

maximum peak primary OC signal to less than the overcurrent

threshold of 0.6V.

where IoCL is the output current limit threshold, VOC is the

current limit threshold, and Rs is the current sensing resistor.

Once the value of Rs is determined, Equation 11 can be used to

solve for the maximum level of OC at any steady state current

when Io is substituted for IoCL and solving for Voc.

The EA compensation depends on the bandwidth required for the

application. For PFC applications the BW is necessarily limited to

20Hz or less. For other applications, the BW may be increased as

required up to about 1/5 of the lowest switching frequency

allowed as described in “Oscillator” on page 13. For the low BW

applications a Type I compensation configuration is adequate.

For higher BW applications, a Type II configuration may be

required. Figures 11 and 12 show the Type I and Type II

configurations, respectively.

OVP

The ISL1903 has independent overvoltage protection accessed

through the OV pin. There is a nominal 20µA switched current

source used to create hysteresis. The current source is active only

during an OV fault; otherwise, it is inactive and does not affect

the node voltage. The magnitude of the hysteresis voltage is a

function of the external resistor divider impedance.

If the divider formed by R1 and R2 is sufficiently high

impedance, R3 is not required, and the hysteresis is:

If that does not result in the desired hysteresis then R3 is

needed, and the hysteresis is:

If the OV signal requires filtering, the filter capacitor, Copt, should

be placed as shown in Figure 10. The current hysteresis provides

FIGURE 11. CONTROL LOOP CONFIGURATION

OC - IOUT

PROCESSOR

OUT

OC IOUT

VERR

AC REFERENCE

GENERATOR R1

CFILTER

CFB

RFB

VREF

RPU

LOAD

ISL1903

IOUT 8Rs

Nsp

--------------- Io

=V(EQ. 10)

VOC

IoCL



--------------------

=(EQ. 11)

FIGURE 12. TYPE II EA CONFIGURATION

IOUT

VERR

AC REFERENCE

GENERATOR

CFILTER

CFB1

RFB1

VREF

RPU

ISL1903

CFB2 RFB2

FIGURE 13. OV HYSTERESIS

20µA

VREF

MONITORED

VOLTAGE

COPT

1.5V

Vov ri gsin

1.5 R1 R2+

---------------------------

=V(EQ. 12)

V2010

6–R1=V(EQ. 13)

V2010

6–R1 R3 R1 R2+

---------------------------

+





=V(EQ. 14)

ISL1903

FN8285 Rev 1.00 Page 17 of 19

September 20, 2012

great flexibility in setting the magnitude of the hysteresis voltage,

but it is susceptible to noise due to its high impedance. If the

hysteresis was implemented as a fixed voltage instead, the

signal could be filtered with a small capacitor placed between

the OV pin and signal ground. This technique does not work well

when the hysteresis is a current source because a current source

takes time to charge the filter capacitor. There is no

instantaneous change in the threshold level rendering the

current hysteresis ineffective. To remedy the situation, the filter

capacitor must be separated from the OV pin by R3. The

capacitor and R3 must be physically close to the OV pin.

OFFREF Control

The ISL1903 provides the ability to disable the output based on

the level of the control loop reference, set by the AC conduction

angle on the AC pin. Setting OFFREF to a voltage between 0 and

0.6V determines the threshold voltage that disables the output.

OFFREF allows the designer to disable the output at a

pre-determined load current to prevent undesirable behavior

such as at light loading conditions when there may be

insufficient current to maintain the holding current in a

triac-based dimmer. Setting OFFREF to less than 100mV disables

this feature. OFFREF has a nominal hysteresis of 50mV.

Quasi-Resonant Switching

The ISL1903 uses a critical conduction mode PWM control

algorithm. Near zero voltage switching (ZVS) or quasi-resonant

valley switching, as it is sometimes referred to, can be achieved

in the flyback topology by delaying the next switching cycle after

the transformer current decays to zero (critical conduction

mode). The delay allows the primary inductance and capacitance

to oscillate, causing the switching FET drain-source voltage to

ring down to a minimal. If the FET is turned on at this minimal,

the capacitive switching loss (1/2 CV2) is greatly reduced.

The delay duration is set with a resistor from DELADJ to ground.

Figure 7 presents the graphical relationship between the delay

duration and the value of the DELADJ resistance. The relationship

is linear for resistance values greater than ~ 20 kand can be

estimated using Equation 17.

DHC (Dimmer Holding Current)

The DHC pin provides a method to pre-load a triac-based dimmer

during the period of time when the AC is blocked, with overlap at

each edge of the AC conduction period to ensure adequate

holding current. DHC is an open drain FET used to control an

external resistor to act as the load.

DHC controls a resistor on the external high voltage start-up bias

regulator. See “Typical Application - Dimmable Buck LED Driver”

on page 3 for an example of its usage. Note the series resistor

and diode connecting VDD to the gate of the start-up bias FET. It

is required to keep the device on when the AC voltage is near the

zero-crossing.

Gate Drive

The ISL1903 output (OUT) is capable of sourcing and sinking up

to 1A. The OUT high level is limited to the OUT clamp voltage or

VDD, whichever is lower.

Thermal Protection

Internal die over-temperature protection is provided. An

integrated temperature sensor protects the device should the

junction temperature exceed +160°C. There is approximately

+10°C of hysteresis.

Ground Plane Requirements

Careful layout is essential for satisfactory operation of the device.

A good ground plane must be employed. VDD and VREF should

be bypassed directly to GND with good high frequency

capacitance.

REFIN off OFFREF 0.100–= V(EQ. 15)

REFIN on OFFREF 0.050–= V(EQ. 16)

FIGURE 14. QUASI-RESONANT NEAR-ZVS SWITCHING

Winding Current

FET D-S Voltage

tdelay 73.33 10.2 RDELADJ k+ns (EQ. 17)

FN8285 Rev 1.00 Page 18 of 19

September 20, 2012

ISL1903

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in the quality certifications found at www.intersil.com/en/support/qualandreliability.html

Intersil products are sold by description only. Intersil may modify the circuit design and/or specifications of products at any time without notice, provided that such

modification does not, in Intersil's sole judgment, affect the form, fit or function of the product. Accordingly, the reader is cautioned to verify that datasheets are

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Revision History

The revision history provided is for informational purposes only and is believed to be accurate, but not warranted. Please go to web to make sure you

have the latest revision.

DATE REVISION CHANGE

August 27, 2012 FN8285.1 Page 14: Equation 2 changed from S to H

Equation 4 changed from H to A, and Equation 5 changed from H to S.

Page 16: Equation 11, changed from V to ohms

August 10, 2012 FN8285.0 Initial release.

ISL1903

FN8285 Rev 1.00 Page 19 of 19

September 20, 2012

Package Outline Drawing

M16.15A

16 LEAD SHRINK SMALL OUTLINE PLASTIC PACKAGE (QSOP/SSOP)

0.150” WIDE BODY

Rev 3, 8/12

NOTES:

1. Symbols are defined in the “MO Series Symbol List” in Section 2.2 of Publication Number

95.

2. Dimensioning and tolerancing per ANSI Y14.5M-1994.

3. Package length does not include mold flash, protrusions or gate burrs. Mold flash,

protrusion and gate burrs shall not exceed 0.15mm (0.006 inch) per side.

4. Package width does not include interlead flash or protrusions. Interlead flash and

protrusions shall not exceed 0.25mm (0.010 inch) per side.

5. The chamfer on the body is optional. If it is not present, a visual index feature must be

located within the crosshatched area.

6. Terminal numbers are shown for reference only.

7. Lead width does not include dambar protrusion. Allowable dambar protrusion shall be

0.10mm (0.004 inch) total in excess of “B” dimension at maximum material condition.

8. Controlling dimension: MILLIMETER.

INDEX

AREA

-B-

0.17(0.007) CAMBS

-A-

-C-

SEATING PLANE

0.10(0.004) x 45°

0.25

0.010

GAUGE

PLANE

3.99

3.81

6.20

5.84

40.25(0.010) B

0.89

0.41

0.25 5

8°

0° 1.55

1.40 0.249

0.191

4.98

4.80

31.73

1.55

0.249

0.102

0.31

0.20

0.635 BSC

5.59

4.06

7.11

0.38

0.635

DETAIL “X”

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