FN7981 Rev 2.00 Page 1 of 25
March 20, 2013
FN7981
Rev 2.00
March 20, 2013
ISL1902
Full-Featured, Dimmable AC Mains LED Driver with PFC
DATASHEET
The ISL1902 is a high-performance, critical conduction mode
(CrCM), single-ended controller used for single-stage
conversion of the AC mains to a constant current source with
power factor correction (PFC). This controller may be used in
virtually any single-ended topology, isolated or non-isolated,
including Boost, SEPIC, Flyback, and Forward converters.
Operation in CrCM allows near zero-voltage switching (ZVS) for
improved efficiency while maximizing magnetic core
utilization.
The ISL1902 is compatible with both leading and trailing edge
modulated AC mains dimmers as well as analog signal and
ambient light sensor controlled dimming methods. The LED
string may be dimmed either by modulation of the DC current
or PWM dimming. In-rush current limiting minimizes current
spikes caused by leading edge dimmers and prevents dimmer
malfunction when one or more LED fixtures are connected.
Two control loops are provided to improve transient response
since one loop must have restricted bandwidth to allow PFC.
The second control loop may be configured for higher
bandwidth to respond to input transients quickly and prevent
them from propagating to the load and appearing as
flashing/flickering.
The ISL1902 LED driver controller provides all of the features
required for high-performance dimmable LED ballast designs.
Applications
• Industrial and commercial LED lighting
• Architectural lighting LED drivers
• 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
• Analog control signal dimming
• Configurable for PWM or DC Current Dimming Control of
LEDs
• Dual control loops for PFC and fast transient response
• Compatible with ambient light sensors for uniform
lamp-to-lamp performance
• 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
• Active pre-load to eliminate power-off “afterglow”
• OFFREF feature sets dimming turn-off-threshold to improve
fixture performance matching
• In-rush protection control for each AC half-cycle minimizes
audible noise and eliminates dimmer resonance
• Input or Output Overvoltage Protection (OVP)
• Over-Temperature Protection (OTP)
• Bias Supply Under Voltage Lockout (UVLO)
• -40°C to +125°C operation
• Pb-free (RoHS compliant)
FIGURE 1. TYPICAL APPLICATION - DIMMING PERFORMANCE FIGURE 2. TYPICAL APPLICATION - POWER FACTOR
0
100
200
300
400
500
600
700
800
0 20 40 60 80 100
AC CONDUCTION ANGLE (%)
LED CURRENT (mA)
0.90
0.92
0.94
0.96
0.98
1.00
80 130 180 230
INPUT VOLTAGE (VRMS)
POWER FACTOR
OBSOLETE PRODUCT
NO RECOMMENDED REPLACEMENT
contact our Technical Support Center at
1-888-INTERSIL or www.intersil.com/tsc
ISL1902
FN7981 Rev 2.00 Page 2 of 25
March 20, 2013
Pin Configuration
ISL1902
(24 LD QSOP
TOP VIEW
VDD
PRELOAD
OFFREF
VREF
IOUT
CS+
CS-
REFIN
LPOUT
OC
FB2
FB1
OUT
INRUSH
GND
AC
OVP
LREF
RAMP
VERR
DELADJ
PWMOUT
LOUT
LFB
24
23
22
21
20
19
18
17
16
15
14
13
1
2
3
4
5
6
7
8
9
10
11
12
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 PRELOAD The output control signal to drive an external FET placed in parallel with the LED load. This feature allows the output capacitor to be
quickly discharged to prevent continued low level illumination of the LEDs due to stored energy in the output capacitor.
3 OFFREF Sets the reference level to disable the driver at light loading. The turn-off reference can be set at any level between 0V 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. OFFREF triggers PRELOAD to discharge the output
capacitance with an external FET.
4 VREF The 5.40V reference voltage output having ±100mV tolerance over line, load and operating temperature. Bypass to GND with a 0.1µF
to 3.3µF low ESR capacitor.
5 IOUT The output of the differential current sensing circuit. A pair of resistors and capacitors is placed on this output to form two low pass
filters. IOUT creates the current feedback signals for the control loop and is normally filtered and scaled prior to inputing at FB1 and
FB2 through separate input resistors to allow for different BWs.
6, 7 CS+, CS- The differential inputs for the current sense circuit. This circuit generates a DC feedback signal for the control loop as well as the
input to the CrCM circuit to determine the critical conduction operating point. CS± has a common mode range of -0.3V to 0.5V and
a differential input range of 0V to1.5V
8 REFIN The reference voltage input that sets the control loop reference. Normally connected to LPOUT or an external control reference.
9 LPOUT Output of the digital low-pass filter. The output ranges from 0V to 0.5V in proportion to the AC conduction angle. This output may be
used as is or manipulated (such as when used with an external light sensor or temperature monitor) and applied to REFIN to be used
as the reference for the control loop.
10 OC This is the input to the peak overcurrent comparator. The overcurrent comparator threshold is set at 600mV nominal. Peak OCP is
required for cycle-by-cycle protection. It also protects against low AC line conditions. 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 also while the OUT pulse is low.
11, 12 FB2, FB1 FBx is the inverting input to the error amplifier (EAs). The current feedback signal is applied to EA1 and EA2. EA1 is the primary error
amplifier and is used for steady state operation. EA2 is the secondary control loop for operation during transients. Normally EA1 is
configured for low bandwidth operation, about 20Hz, to obtain power factor correction. EA2 is configured for higher BW to respond
to transients. Both error amplifiers are externally compensated to give the user complete flexibility.
13 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.
14 VERR Output of the error amplifiers and the control voltage input to the inverting input of the PWM comparator. VERR requires an external
pull-up resistor to VREF.
15 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.
16 LFB The inverting input to the uncommitted linear amplifier.
ISL1902
FN7981 Rev 2.00 Page 3 of 25
March 20, 2013
17 LREF The non-inverting input to the uncommitted linear amplifier.
18 LOUT Output of the uncommitted linear amplifier.
19 OVP Input to detect an overvoltage (OV) condition on the output with a nominal threshold of 1.5V. 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.
20 AC Input to sense AC voltage presence and amplitude. A resistor divider from line and neutral/line and circuit ground is used to detect
the AC voltage.
21 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.
22 INRUSH Output to drive an isolation transformer to control an inrush current limiting device. Typically this would be a triac, back-to-back FETs,
or anti-parallel SCRs, etc. The output is a 50% duty cycle ~80kHz square-wave capable of sourcing 10mA. The output is enabled in
conjunction with an AC outage (such as from a wall dimmer). Operation is delayed for ~150µs after AC returns and is enabled until
AC is interrupted again. The INRUSH output is also inhibited during normal AC zero-crossing even at full conduction angle.
23 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 the dimming FET in series with the LED string.
The PWM operates at ~310Hz.
24 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.
#
ISL1902FAZ ISL 1902FAZ -40 to +125 24 Ld QSOP M24.15
NOTES:
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 ISL1902. For more information on MSL please see tech brief TB363.
FN7981 Rev 2.00 Page 4 of 25
March 20, 2013
ISL1902
Functional Block Diagram - ISL1902
OUT
PWM
COMPARATOR
PWM LATCH
+
-
S
R
Q
Q
+
-
RAMP
AC
CrCM
DETECTOR fMAX
Clamp
200mV
+
-
Master
Oscillator
LPOUT
REFIN
DUTY CYCLE TO
VOLTAGE
CONVERTER
LOW PASS FILTER
INRUSH
150us
One Shot
INRUSH CONTROL CIRCUIT
+
-
-
DELADJ
+
-
Triangle Wave
Generator
CLK -
+PWMOUT
VERR
600 mV
VREF
LOUT
LFB
LREF
DIMMING PWM
+
-
EA1
CrCM OSCILLATOR/PWM/ERROR
AMPLIFIERS
LINEAR AMPLIFIER
OC
Leading
Edge
Blanking
FB1
Quasi-ZVS
Delay
IOUT
REFINBUFF
+
-
EA2
FB2
PRIMARY OC
CLK
DELAYED AC-P
fMIN
Clamp
Peak
Detector
1/5
SS/5
1/5
Verr/5
PWM
VERR
AC Present 0-Xing
Blanking
_
+
+
Bias and
Reference
Generator
GND
VDD
UVLO
+
-
VREF
OTP Shutdown
150 - 170 C
BIAS/UVLO/OTP
+
-
CS-
CS+
DIFFERENTIAL ISENSE
IOUT
No AC Counter
CLK
RST
CARRY
-
+
PRELOAD
OFFREF
MINIMUM DIMMING
LEVEL CONTROL
0.25 VSS LOW
FAULT LATCH
+
-
S
R
Q
Q
SS
+
-
300 ms
Soft Start
ENABLE
+
-1.50 V
BG
INHIBIT INHIBIT
VERR CLAMP
OVP
REF
Reference SS
AC-PRESENT
SS
REFINBUFF
Reference SS
Buffer
Q
Q
350 S
One Shot
Q
Q
AC
DETECTION
SOFT-START/POR/OVP
FN7981 Rev 2.00 Page 5 of 25
March 20, 2013
ISL1902
Typical Application - SEPIC Topology with PWM Dimming and Ambient Light Compensation
AMBIENT
LIGHT
SENSOR
EMI
FILTER
AC
MAINS
DIMMER
13
14
15
16
17
18
19
20
21
22
23
24
1
2
4
3
5
6
7
8
9
10
11
12
CS+
INRUSHOFFREF
PWMOUT
VREF
OUT
VDD
OC
OVP
PRELOAD
AC
IOUT
RAMP
LOUT
LREF
LFB
LPOUT
REFIN
CS-
GND
FB2
FB1 DELADJ
VERR
ISL1902
FN7981 Rev 2.00 Page 6 of 25
March 20, 2013
ISL1902
Typical Application - Isolated Flyback with PWM Dimming and Ambient Light
Compensation
9
10
11
12
13
14
15
16
1
2
4
3
5
6
7
8
ISL1904
CS+
DHC
OFFREF PWMOUT
VREF
OUT
VDD
OC OVP
AC
IOUT
RAMP
GND
FB
DELADJ VERR
EMI
FILTER
AC
MAINS
DIMMER
AMBIENT
LIGHT
SENSOR
13
14
15
16
17
18
19
20
21
22
23
24
1
2
4
3
5
6
7
8
9
10
11
12
CS+
INRUSHOFFREF
PWMOUT
VREF
OUT
VDD
OC
OVP
PRELOAD
ACIOUT
RAMP
LOUT
LREF
LFB
LPOUT
REFIN
CS-
GND
FB2
FB1 DELADJ
VERR
ISL1902
FN7981 Rev 2.00 Page 7 of 25
March 20, 2013
ISL1902
Typical Application - Non-Isolated Flyback with PWM Dimming and Ambient Light
Compensation
13
14
15
16
17
18
19
20
21
22
23
24
1
2
4
3
5
6
7
8
9
10
11
12
CS+
INRUSHOFFREF
PWMOUT
VREF
OUT
VDD
OC
OVP
PRELOAD
AC
IOUT
RAMP
LOUT
LREF
LFB
LPOUT
REFIN
CS-
GND
FB2
FB1 DELADJ
VERR
ISL1902
AC
MAINS
DIMMER
EMI
FILTER
AMBIENT
LIGHT
SENSOR
ISL1902
FN7981 Rev 2.00 Page 8 of 25
March 20, 2013
Absolute Maximum Ratings Thermal Information
Supply Voltage, VDD . . . . . . . . . . . . . . . . . . . . . . . . . . . GND - 0.3V to +28.0V
OUT, PWMOUT, INRUSH . . . . . . . . . . . . . . . . . . . . . . . . . . . GND - 0.3V to VDD
Signal Pins (except CS-) . . . . . . . . . . . . . . . . . . . . GND - 0.3V to VREF + 0.3V
Signal Pin CS- . . . . . . . . . . . . . . . . . . . . . . . . . . . . GND - 0.6V 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). . . . . . . . 1500V
Machine Model (Per EIAJ ED-4701 Method C-111) . . . . . . . . . . . . . 150V
Charged Device Model (Per EOS/ESD DS5.3, 4/14/93). . . . . . . . . 750V
Latchup (Per JESD-78B; Class 2, Level A. . . . . . . . . . . . . . . . . . . . . . 100mA
Thermal Resistance (Typical) JA (°C/W) JC (°C/W)
24 Lead QSOP (Notes 4, 5) . . . . . . . . . . . . . 78 34
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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-40°C to +125°C
Supply Voltage Range (Typical). . . . . . . . . . . . . . . . . . . . . . . 9VDC to 20VDC
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. JA is measured with the component mounted on a high effective thermal conductivity test board in free air. See Tech Brief TB379 for details.
5. For JC, the “case temp” location is taken at the package top center.
6. All voltages are with respect to GND.
Electrical Specifications Recommended operating conditions unless otherwise noted. Refer to the Block Diagram on page 4 and
Typical Application schematics starting on page 5. 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 10 14.5 mA
UVLO START Threshold 14.8 15.5 16.1 V
UVLO STOP Threshold 6.80 7.10 7.50 V
Hysteresis 7.50 8.30 9.30 V
REFERENCE VOLTAGE (VREF)
Overall Accuracy IVREF = 0mA to -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 -15 -100 mA
Load Capacitance (Note 8) 0.1 3.3 µF
PEAK CURRENT SENSE (OC)
Current Limit Threshold VERR = VREF, RAMP = 0V 577 600 623 mV
Leading Edge Blanking (LEB) Duration (Note 8) 70 120 150 ns
OC to OUT Delay + LEB TA = +25°C 110 170 200 ns
Input Bias Current VOC = 0.3V -1.0 1.0 µA
RAMP
RAMP Sink Current Device Impedance IRAMP = 10mA 20
RAMP to PWM Comparator Offset 190 235 287 mV
Input Bias Current VRAMP = 0.3V -1.0 1.0 µA
ISL1902
FN7981 Rev 2.00 Page 9 of 25
March 20, 2013
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 µs
Maximum Frequency Clamp RRAMP = 100, RAMP = 2V,
8V < VDD< 26V
0.7 1.0 1.2 MHz
Minimum Frequency Clamp RRAMP = 23k, 8V < VDD< 26V 20 25 31 kHz
Minimum Duty Cycle 8V < VDD< 26V, COMP = 0V 0%
Minimum Non-Zero Output Duration 8V < VDD< 26V 70 100 130 ns
Zero Current (CrCm) Detector Threshold, Falling 8V < VDD< 26V 728mV
VERR to PWM Gain 8V < VDD< 26V 0.200 V/V
SS to PWM Gain 8V < VDD< 26V 0.222 V/V
ERROR AMPLIFIERS (EA1 AND EA2)
Input Common Mode (CM) Range (Note 8) 03.4V
GBWP (Note 8) 1.9 MHz
VERR VOL EA1 IVERR = 6mA, 8V < VDD< 26V 0.950 V
VERR VOL EA2 IVERR = 4mA, 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, FB1 REFIN = 0.5V, FB1 = 2.0V, FB2 = 0V,
8V < VDD< 26V
-1.0 1.0 µA
Input Bias Current, FB2 REFIN = 0.5V, FB2 = 2.0V, FB1 = 0V,
8V < VDD< 26V
-1.0 1.0 µA
DIFFERENTIAL CURRENT SENSE (CS+, CS-)
IOUT Amplifier Gain CS- = 0V, CS+ = 0.1V, 0.3V,
8V < VDD< 26V
2.910 2.970 3.030 V/V
Common Mode (CM) Input Range 8V < VDD< 26V -0.30 0.50 V
Differential Input Range 8V < VDD< 26V 01.5V
Offset Voltage (VOS) 8V < VDD< 26V -36 48 mV
GBWP (Note 8) 8MHz
Slew Rate (Note 8) 45 V/µs
Input Bias Current CS- = 0V, 1.0V
CS+ = 1.0V, 1.5V
8V < VDD< 26V
-1.0 1.0 µA
IOUT High Level Output Voltage (VOH) VIOUT at 0µA - VIOUT at -100µA,
8V < VDD < 26V
0.1 V
IOUT Low Level Output Voltage (VOL) VIOUT at 100µA, 8V < VDD< 26V 0.1 V
AC DETECTOR
Input Bias Current 8V < VDD< 26 -50 50 nA
Detection Threshold, Falling ACPEAK = 100mV, 8V < VDD< 26V 4.5 20 40.5 mV
Detection Threshold Hysteresis 8V < VDD< 26V 6 mV
Input Operating Range 8V < VDD< 26V 04.00V
Electrical Specifications Recommended operating conditions unless otherwise noted. Refer to the Block Diagram on page 4 and
Typical Application schematics starting on page 5. 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
ISL1902
FN7981 Rev 2.00 Page 10 of 25
March 20, 2013
Clamp Voltage IACDETECT = 1.0mA 6.8 7.2 7.6 V
INRUSH
High Level Output Voltage (VOH) VINRUSH at 0mA - VINRUSH at -10mA,
VDD = 8V operating
1.00 V
Low Level Output Voltage (VOL) VINRUSH = 10mA, VDD = 8V
operating
1.00 V
Inrush Duration (see tDELAY Fig. 10) 8V < VDD< 26V 140 180 220 µs
Output Clamp Voltage VDD = 20V, IINRUSH = -10µA 10.5 12 13.4 V
Unbiased Output Voltage Clamp VDD = 6V, ILOAD = 3mA 2.3 V
LOW PASS FILTER
High Level Output Voltage (VOH) VLPOUT at 0µA - VLPOUT at -100µA,
8V < VDD< 26V
0.1 V
Low Level Output Voltage (VOL) VLPOUT at 100µA, 8V < VDD< 26V 0.1 V
Output Range 00.50V
LPOUT vs AC Conduction Angle ILPOUT = 0µA, f = 120Hz (rectified),
8V < VDD< 26V
485 514 543 mVDuty Cycle () = 98%
Duty Cycle () = 75% 273 300 323 mV
Duty Cycle () = 50% 110 130 148 mV
Duty Cycle () = 25% 16 30 41 mV
Duty Cycle () = 10% 019mV
REFIN
Input Common Mode (CM) Range 0VREF-1V
Input Bias Current REFIN = 4.4V -1.0 1.0 µA
Offset Voltage (VOS), Combined FBx + REFIN at EA 8V < VDD< 26V, see Fig. 11 -11 11 mV
LINEAR AMPLIFIER
Input Offset (VOS) LFB = LOUT, LREF = 0.5V -4 4 mV
High Level Output Voltage (VOH) ILOUT = -1mA,
LFB = 0V, LREF = VREF
(VOH at 0mA - VOH at -1mA)
1.0 V
Low Level Output Voltage (VOL) ILOUT = 8mA,
LFB = VREF, LREF = 0V
1.0 V
Input Common Mode (CM) Range 0VREFV
Output Operating Range 0.3 4.3 V
GBWP (Note 8) 1MHz
Open Loop Gain (Note 8) 85 dB
Input Bias Current LREF, LFB LREF = 1.0V, LFB = 1.0V -1.0 1.0 µA
Output Pull-Down Impedance VDD = 6.0V, ILOUT = 100µA10 kΩ
SOFT-START
Duration 282 370 483 ms
Reference Soft-Start Initial Step 21 mV
OFFREF
Input Bias Current OFFREF = 0.5V -1.0 1.0 µA
Electrical Specifications Recommended operating conditions unless otherwise noted. Refer to the Block Diagram on page 4 and
Typical Application schematics starting on page 5. 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
ISL1902
FN7981 Rev 2.00 Page 11 of 25
March 20, 2013
Operating Range (Excluding Offset) 00.5V
Threshold Hysteresis 48 62 76 mV
Threshold Offset 78 104 129 mV
AC Dropout Disable Delay 32 ms
PRELOAD VOH ILOAD = 0mA VREF V
PRELOAD VOL ILOAD = 1mA 1.00 V
OUT
High Level Output Voltage (VOH) VOUT at 0mA - VOUT at -100mA,
VDD = 8V operating
0.35 1.2 V
Low Level Output Voltage (VOL) VOUT at 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%
20 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 at 0mA - VOUT at -10mA,
VDD = 8V operating
0.8 1.2 V
Low Level Output Voltage (VOL) VOUT at 10mA,
VDD = 8V operating
0.8 1.2 V
Rise Time CLOAD = 1nF, VDD = 8V operating,
t90% - t10%
130 240 ns
Fall Time CLOAD = 1nF, VDD = 8V operating,
t10% - t90%
130 240 ns
Output Voltage Clamp VDD = 20V, ILOAD = -10µA 10.5 12.0 13.4 V
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 80 µs
OVP
OVP Threshold 1.46 1.50 1.54 V
OVP Hysteresis 15 20 25 µA
Input Bias Current OVP = 1.0V -1.0 1.0 µA
OVP Clamp Voltage IOVP = 5mA 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 the Block Diagram on page 4 and
Typical Application schematics starting on page 5. 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
ISL1902
FN7981 Rev 2.00 Page 12 of 25
March 20, 2013
Typical Performance Curves
FIGURE 3. REFERENCE VOLTAGE vs TEMPERATURE FIGURE 4. LPOUT vs AC SIGNAL DUTY CYCLE
FIGURE 5. DELAY vs DELADJ RESISTANCE FIGURE 6. PWMOUT DUTY CYCLE vs REFIN
-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
0
100
200
300
400
500
AC CONDUCTION ANGLE (% DUTY CYCLE 120Hz)
LPOUT (mV)
025 50 75 100 125 150 175 200 225
0
0.5
1.0
1.5
2.0
2.5
DELAY RESISTANCE (kΩ)
DELAY TIME (µs)
0
20
40
60
80
100
0 50 100 150 200 250 300 350 400 450 500
REFIN (mV)
PWMOUT DUTY CYCLE (%)
Test Waveforms and Circuits
FIGURE 7. RISE/FALL TIME TEST CIRCUIT FIGURE 8. RISE/FALL TIMES
tRtF
90%
10%
ISL1902
FN7981 Rev 2.00 Page 13 of 25
March 20, 2013
FIGURE 9. OC +LEB TO OUT DELAY FIGURE 10. AC MAINS TO INRUSH TIMING
FIGURE 11. ERROR AMPLIFER INPUT OFFSET TEST CIRCUIT
Test Waveforms and Circuits (Continued)
OC
OUT
OC THRESHOLD
LEADING EDGE BLANKING
OC PROPAGATION DELAY
OC + LEB TO OUT DELAY
PLACEHOLDER
AC
MAINS
INRUSH
tDELAY
tDELAY
13
14
15
16
17
18
19
20
21
22
23
24
1
2
4
3
5
6
7
8
9
10
11
12
CS+
INRUSHOFFREF
PWMOUT
VREF
OUT
VDD
OC
OVP
PRELOAD
AC
IOUT
RAMP
LOUT
LREF
LFB
LPOUT
REFIN
CS-
GND
FB2
FB1 DELADJ
VERR
ISL1902
0.4V
0.1µF
VDD
470pF
54k
5k
10k
90/140µA
90/140µA
ISL1902
FN7981 Rev 2.00 Page 14 of 25
March 20, 2013
Functional Description
Features
The ISL1902 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. The ISL1902 includes support for both
PWM and DC current dimming of the output. Similar to the
ISL1901, the ISL1902 adds additional features to facilitate the
design of higher performance LED drivers.
Oscillator
The ISL1902 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. 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
magnetic element decays to zero (dB/dt=0), the winding
voltages collapse and the winding currents are zero (flyback) or
DC (SEPIC). Either may be monitored and used to initiate the next
switching cycle to achieve CrCM operation. Additionally, there is a
user adjustable threshold, DELADJ, to delay the initiation of the
next switching cycle to allow the drain-source voltage of the
primary switch to ring to a minimum. This allows quasi-ZVS
operation to reduce capacitive switching losses and improve
efficiency.
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
while the maximum frequency clamp 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 - 230mV. 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 windings. The
transformer/inductor design also determines the maximum
ON-time that can be supported without saturation, so, in reality,
the magnetic design is critical to every aspect of determining the
switching frequency range.
THE FLYBACK TOPOLOGY
The design methodology is similar to designing a discontinuous
mode (DCM) flyback transformer 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. For PFC applications, the design
is further complicated by the input voltage waveform; a virtually
unfiltered rectified AC mains sinewave.
Once the output power, PO, the output current, IO, the output
voltage, VO, and the minimum input AC voltage are known, the
transformer design can be started. From the minimum AC input
voltage, the minimum average input voltage must be
determined. The converter behaves as if the input voltage is an
equivalent DC value due to the low control loop bandwidth. PO
determines the amount of energy that must be stored in the
transformer on each switching cycle, but must be corrected for
efficiency. This includes leakage inductance losses, winding
losses, and all secondary side losses. This can be estimated as a
portion of the total efficiency, , or as is typically done, includes
all of the losses.
A typical minimum operating frequency and maximum duty cycle
must be selected. These are somewhat arbitrary in their
selection, but do ultimately determine core 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 lowest 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.
TABLE 1. OSCILLATOR DEFINITIONS
VmINrms = Minimum RMS input voltage
VmaxINrms = Maximum RMS input voltage
ftyp(avg) = Typical frequency when VIN (instantaneous) = VIN(rms)
=Efficiency
Dmax = Maximum typical duty cycle desired
Dmin = Minimum typical duty cycle
tON(MAX) =f
typ(avg) x Dmax
Ls = Secondary inductance
Lp = Primary inductance
Nsp = Transformer turns ratio, Ns/Np
Ip(peak) = Peak primary current within a switching cycle
tON ON-time of the power FET controlled by OUT
tOFF OFF-time duration required for CrCM operation
tDELAY = User adjustable delay before the next switching cycle
begins
PIN
Po
-------
=W(EQ. 1)
ISL1902
FN7981 Rev 2.00 Page 15 of 25
March 20, 2013
The first calculation required is to determine the required
secondary inductance.
The turns ratio Nsp is calculated next.
Knowing the secondary inductance and the turns ratio, the
primary inductance can be calculated.
With this information, the lowest switching frequency, which
occurs at maximum load and at the peak instantaneous input
voltage at the minimum RMS voltage, can be determined. By
setting the maximum duty cycle and picking a typical average
frequency, the ON-time is already known.
The primary peak current at the end of the ON-time is:
The peak secondary current is the peak primary current divided
by the transformer turns ratio.
And the OFF-time is:
The lowest switching frequency is the reciprocal of the sum of the
ON-time, the OFF-time, and the delay time.
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 the required delay is typically on the
order of 300ns to 500ns.
If the lowest frequency does not meet the design requirements,
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
non-zero ON-time is ~100ns, suggesting an operating frequency
above 1MHz. In any event, the maximum frequency clamp would
become active at around 800kHz. Once the primary and
secondary inductances are known, the general formulae to
calculate the ON-time and OFF-time at an equivalent DC input
voltage are:
It is clear from these 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. The
frequency has an inverse relationship to input voltage and has a
less significant affect over a typical operating range.
It should be noted, however, that the above equations assume
full conduction angle of the AC mains. 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 result being that the steady
state frequency behavior will not vary much as the conduction
angle is reduced from full. If an analog control signal is used
instead, the frequency behavior will be as predicted above.
THE SEPIC TOPOLOGY
The SEPIC topology, in simplified form, is shown in Figure 12. The
voltage source indicated may be either DC or rectified AC. The
capacitance of CIN is negligibly small for applications requiring
PFC.
The terminology defined in Table 1 shall be reused, except Ls and
Lp are replaced by L1 and L2 per Figure 12. In steady state
operation, the average voltage across L1 and L2 must be zero. If
this were not true, saturation would occur. Furthermore, this
situation implies the voltage across C1 must be equal to the
input source voltage, VIN. During the ON-time, when switch Q1 is
conducting, the voltage across each inductor is VIN. During the
OFF-time, Q1 is off, and the voltage across each inductor is -VOUT.
Since no DC current may flow through C1, the output current, IO,
must be equal to the average current flowing in L2. Additionally,
IO is also the average current that flows in both inductors during
the OFF-time.
To determine the values of L1 and L2, the operating conditions
must be defined. The lowest operating frequency occurs at
maximum load and minimum input voltage. If operating from
and AC source, the lowest frequency occurs at the instantaneous
Ls
Vo1D
max
–
2
ftyp avg
2I
o

--------------------------------------------
=H(EQ. 2)
Nsp
Vo1D
max
–
VmINrms Dmax

----------------------------------------------------
=(EQ. 3)
Lp
Ls
Nsp
2
-------------
=H(EQ. 4)
tON
Dmax
ftyp avg
-----------------------
=s(EQ. 5)
Ip peak
Vrms 2t
ON

Lp
----------------------------------------
=A(EQ. 6)
Is peak
Ip peak
Nsp
----------------------
=s(EQ. 7)
tOFF
LsIs peak
Vo
--------------------------------
=s(EQ. 8)
fmin
1
tON tOFF tdelay
++
----------------------------------------------------
=Hz (EQ. 9)
tdelay
LpCoss Cother
+
2
-----------------------------------------------------------------
s(EQ. 10)
tOFF
2L
sIo

Vo
-----------------------1
LpNsp Vo

LsVINrms
---------------------------------
+



=s(EQ. 11)
tON
2L
pNsp Io

VINrms
-------------------------------------- 1
LpNsp Vo

LsVINrms
---------------------------------
+



=s(EQ. 12)
FIGURE 12. SEPIC TOPOLOGY
+
_
L1
Q1
C1
L2
D1
COUT
CIN
L
O
A
D
ISL1902
FN7981 Rev 2.00 Page 16 of 25
March 20, 2013
peak of the AC voltage waveform at the lowest RMS input
voltage. Therefore, the lowest DC or equivalent DC (RMS) input
voltage is used as the design point with a corresponding
selection of a minimum desired operating frequency.
During the ON-time, the current in L2 ramps from zero to a peak
value, IP.
where VIN(minRMS) is defined as the minimum DC or RMS input
voltage. During the OFF-time, the current ramps from IP back
down to zero at a rate determined by VOUT.
Since the average value of current in L2 must be the load current,
IO, Equations 13 and 14 can be used to relate the DC or RMS
input voltage values for tON and tOFF to IO.
Equations 15 and 16 may be summed to provide an equivalent
switching period for the equivalent DC (RMS) input and stated
design parameters, and the value for L2 may be calculated. For
DC input applications, the calculation is straight forward.
where tdelay is a constant and defined in the next section,
“Quasi-Resonant Switching”.
When the input voltage is rectified AC, the desired switching
period has to be modified to account for the difference between
the RMS voltage and the instantaneous peak of the AC
waveform. The frequency is lower at the AC peak than at the
equivalent DC (RMS) input voltage.
Using Equation 19 for tS and substituting into Equation 18 yields
the appropriate value for L2 in rectified AC input applications.
As stated previously, both inductor currents flow to the output
during the OFF-time. IO may be solved for by averaging the sum
of both inductor currents during the OFF-time over a complete
switching cycle.
Using Equations 15 and 16 and solving for L1 yields:
The final step in specifying the inductor requirements is to
determine the DC bias on each inductor. Earlier it was assumed
that each inductor current ramps from zero to some peak value
during the ON-time. In reality each inductor has a DC bias current
that does not contribute to the output current and may be
ignored in the previous calculations, but its value is required to
determine the RMS currents in each inductor. The reason the DC
bias exists is that there can be no DC current through C1 (see
Figure 12). The current flowing from L2 into C1 during the ON-
time must equal the current flowing in the opposite direction
from L1 during the OFF-time.
where IC1 is the current through C1 during a complete switching
cycle, IDC is the DC bias current, and TS= TON + TOFF. Equation 22
can also be used on a cycle-by-cycle basis providing
instantaneous values of TON, TOFF and VIN are used. Setting
Equation 22 equal to zero and solving for IDC yields Equation 23,
which represents the DC bias current flowing from L1 through C1
into L2 at the equivalent DC (RMS) input voltage. It may be
thought of as the expected value of bias current. In rectified AC
input applications, the bias current varies as needed each
switching cycle to balance charge on C1 as the AC voltage varies
from valley to peak to valley during each AC half-cycle.
Quasi-Resonant Switching
The ISL1902 uses critical conduction mode PWM control
algorithm. Near zero voltage switching (ZVS) or quasi-resonant
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 minima. If the FET is turned on at this minima, the
capacitive switching loss (1/2 CV2) is greatly reduced.
IP
VIN minRMS
tON
L2
---------------------------------------------------
=A(EQ. 13)
IP
VOUT tOFF
L2
---------------------------------
=A(EQ. 14)
tON
IO2L2
VIN minRMS
------------------------------------
=s(EQ. 15)
tOFF
IO2L2
VOUT
-------------------------
=s(EQ. 16)
tStON tOFF tdelay
++=s(EQ. 17)
L2
tSVOUT VIN minRMS
2I
OUT VOUT VIN minRMS
+
------------------------------------------------------------------------------------------
=H(EQ. 18)
tS2t
ON tOFF
+tdelay
+= s(EQ. 19)
IO
VOUT tOFF
2
2t
ON tOFF
+
------------------------------------------1
L1
-------1
L2
-------
+


=A(EQ. 20)
L1
VIN minRMS
L2
VOUT
------------------------------------------------
=H(EQ. 21)
IC1 IDC
VOUT tOFF
2
2L1t
S

--------------------------------- VIN minRMS
tOFF
2
2L2t
S

------------------------------------------------------
–+= A(EQ. 22)
IDC IO
VIN minRMS
VOUT
–
VIN minRMS
VOUT
+
-----------------------------------------------------------
=A(EQ. 23)
FIGURE 13. SEPIC WAVEFORMS
IL2
IL1
VL1, VL2
VIN
-VOUT
VOLTAGE CURRENT
0
0
Ts
TON TOFF tdelay
t
t
ISL1902
FN7981 Rev 2.00 Page 17 of 25
March 20, 2013
The delay duration is set with a resistor from DELADJ to ground.
Figure 5 on page 12 presents the graphical relationship between
the delay duration and the value of the DELADJ resistance. The
relationship is linear for resistance values greater than ~ 20kΩ
and can be estimated using Equation 24.
Soft-Start Operation
Soft-start is not user adjustable and is fixed at ~350ms. Both the
duty cycle and control loop reference have soft-start. This
ensures a well behaved closed loop soft-start that results in
virtually no overshoot.
Biasing
The ISL1902 has a nominal VDD start and stop threshold of
15.5V and 7.1V, respectively. The wide hysteresis allows resistive
trickle charging from the high voltage input for start-up bias.
Operating bias is then supplied from another source, such as an
auxiliary transformer winding or in the case of a non-isolated
design, directly from the output or from a tap in the LED string.
The VDD bypass capacitance value is critical to a successful
design. Unless there is a DC source available, such as the output,
the VDD capacitance must be able to store enough energy to
provide bias during the AC voltage valleys and, if used with a
dimmer, provide bias when the dimmer is blocking the AC
voltage each half-cycle.
AC Detection and Reference Generation
The ISL1902 creates a 0V to -0.5V reference for the LED current
control loop (EA reference) by directly 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 ISL1902 is compatible with
both leading and trailing edge modulated dimmers.
The ISL1902 detects the conduction angle using a divider
network across the AC line and connected to the AC pin, although
it can also be located after the AC bridge rectifier.
FIGURE 14. QUASI-RESONANT NEAR-ZVS SWITCHING
WINDING CURRENT
FET D-S VOLTAGE
tdelay 73.33 10.2 RDELADJ k+ns (EQ. 24)
FIGURE 15. TRICKLE CHARGE START-UP W/AUX. WINDING
VDD
124
ISL1902
R1
C1
CR1
CR2
RECTIFIED
AC+
RECTIFIED
AC-
FIGURE 16. LINEAR REGULATOR START-UP W/AUX. WINDING
VDD
124
ISL1902
R2
C1
CR2
RECTIFIED
AC+
RECTIFIED
AC-
R1
VR1
Q1
CR1
EMI
FILTER
1
2
4
3
AC
19
20
21
22
23
24
GND
ISL1902
FIGURE 17. AC DETECTION
ISL1902
FN7981 Rev 2.00 Page 18 of 25
March 20, 2013
The advantage to sensing the AC voltage directly, rather than the
rectified voltage, is that there is no error in detecting the AC zero
crossing. If monitored after the AC rectifier bridge, the AC signal
tracks the filter capacitor voltage, which may not discharge in
phase with the AC voltage. This can lead to incorrect detection of
the AC zero crossing. At light load, the filter capacitor may not
fully discharge before the AC voltage begins to increase again,
resulting in no detection of the AC zero crossing at all.
The AC pin and has a usable input range of 0V to 4V. The peak of
the input signal should range between 1V and 4V for
uncompromised accuracy. The AC detection circuit measures
both the duration of the AC conduction angle and half-cycle
duration. By comparing them every half-cycle, the detection
circuit creates a frequency independent reference that is
updated each AC half-cycle.
The reference generated by the AC detection circuit is available
as the LPOUT signal. Here it can be modified, or not, and
connected to REFIN for setting the control loop reference.
Examples of modification include interfacing with an external
transducer, such as an ambient light sensor (ALS) or temperature
sensor (NTC or PTC) to modify the reference based on the sensor
input.
The ISL1902 also supports analog dimming control by allowing
the control loop reference to be connected to REFIN, bypassing
LPOUT completely.
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, or an independent reference may be input to REFIN with
LPOUT not connected.
In the event of an AC outage, the AC mains frequency reference
is lost. The ISL1902 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 more than 35ms, the internal reference is forced to
its maximum of 0.5V.
FIGURE 18. ALTERNATE AC DETECTION
EMI
FILTER
1
2
4
3
AC
19
20
21
22
23
24
GND
ISL1902
FIGURE 19. ALTERNATE CONFIGURATIONS FOR THE CONTROL LOOP
REFERENCE
6
8
9
10
11
12
LPOUT
REFIN
13
14
15
16
ISL1902
5
4
3
VREF
6
8
9
10
11
12
LPOUT
REFIN
13
14
15
16
ISL1902
5
4
3
VREF
6
8
9
10
11
12
LPOUT
REFIN
13
14
15
16
ISL1902
5
4
3
VREF
ANALOG
OR
PWM
CONTROL
FIGURE 20. USING AN AMBIENT LIGHT SENSOR WITH AN AC LINE
DIMMER
6
8
9
10
11
12
LPOUT
REFIN
13
14
15
16
ISL1902
5
4
3
VREF
17
18
19
LFB
LREF
LOUT
ALS
FIGURE 21. USING AN AMBIENT LIGHT SENSOR WITH ANALOG OR
PWM INPUT
6
8
9
10
11
12
LPOUT
REFIN
13
14
15
16
ISL1902
5
4
3
VREF
17
18
19
LFB
LREF
LOUT
ALS
ANALOG
OR
PWM
CONTROL
ISL1902
FN7981 Rev 2.00 Page 19 of 25
March 20, 2013
Current Sensing
The ISL1902 is configured to regulate the output current by
differentially monitoring the output switching current using the
CS+ and CS- pins. The output switching current waveform is
amplified 4x and output on IOUT where it must be scaled and
filtered before being input to the control loop at the FB pin. The
required filter time constant depends on the compensated error
amplifier bandwidth. The filter bandwidth must be higher than
the control loop bandwidth, typically an order of magnitude
higher, but it is generally not necessary to filter the IOUT signal to
form a nearly DC voltage. The compensated error amplifier
performs that function.
The OC pin provides cycle-by-cycle overcurrent protection. The
output FET drive signal OUT 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 ISL1902 supports both PWM and DC current modulation
dimming. 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.
An external FET, controlled by PWMOUT, switches the LED current
on and off to achieve PWM dimming.
In either case, 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 output capacitance and control loop
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
Figure 6 for a graphical representation of the relationship
between REFIN and PWMOUT duty cycle. It should be noted that
the PWMOUT duty cycle is not allowed to go to zero. There is a
minimum on-time that ensures the LED string is not allowed to
become a continuos open circuit.
Aside from tracking the main control loop reference, the PWM
dimming control is open loop, but is nevertheless self regulating.
There is no closed loop feedback to regulate the load current
during PWM conduction as is the case with most PWM dimming
methods. If the average current into and out of the output
capacitor is not equal, the output voltage will change, increasing
or decreasing with the polarity of the charge imbalance. The
forward voltage characteristic of the LEDs will cause the current
to increase or decrease with the change in output voltage until
the average capacitor current returns to zero. Figures 24 and 25
show a simulation schematic and results, respectively,
illustrating the PWM dimming behavior for a 50% loaded
condition. The converter output is idealized and represented as a
50mA DC current source and the PWM is operating at 50% duty
cycle.
FIGURE 22. ALTERNATE METHOD FOR USING PWM INPUT CONTROL
WITH AN AMBIENT LIGHT SENSOR
6
8
9
10
11
12
LPOUT
REFIN
13
14
15
16
ISL1902
5
4
3
VREF
17
18
19
LFB
LREF
LOUT
ALS
20
AC 100/120Hz
PWM CONTROL
FIGURE 23. TEMPERATURE COMPENSATING THE REFERENCE USING
AN NTC
6
8
9
10
11
12
LPOUT
REFIN
13
14
15
16
ISL1902
5
4
3
VREF
17
18
19
LFB
LREF
LOUT
20
AC
R
NTC
FIGURE 24. PWM DIMMING SIMULATION SCHEMATIC
PWM DUTY CYCLE
COUT
FIGURE 25. PWM DIMMING RESULTS
V
Y2
11.68
11.70
11.72
11.74
11.76
11.78
11.80
11.82
11.84
11.86
TIME/ms 1ms/DIV
500 501 502 503 504 505 506
mA
Y1
-0
20
40
60
80
100
PWM LOAD CURRENT
VOUT
CONVERTER IO
ISL1902
FN7981 Rev 2.00 Page 20 of 25
March 20, 2013
The red trace is the current source supplying the output. The blue
trace is the output capacitor voltage. The green trace is the LED
current. When the PWM signal is off, the 50mA current source
charges COUT and the output voltage increases. When the PWM
turns on, 100mA of current flows through the LEDs, with the initial
current slightly higher and the final current slightly lower as COUT
discharges. The peak-to-peak ripple voltage on COUT is ~160mV.
The decrease in the LED current during conduction is determined
by the size of the output capacitor and the LED current.
Linear Amplifier
The linear amplifier block is a fully accessible uncommitted
operational amplifier. It may be used for a variety of purposes,
such as interfacing sensors, direct sensing of LED current, or a
pre-load amplifier. Examples of using the linear amplifier as a
sensor interface are shown in Figures 20 through 23.
The linear amplifier may be used as a pre-load control to provide
a larger dynamic dimming range as well as providing additional
holding current for applications using triac-based dimmers. As
shown in Figure 26, the pre-load can be configured as an active
load that increases linearly as the control loop reference level
(REFIN) decreases. The result is not only is the total load current
decreased as REFIN is lowered, but an increasing portion of the
load current is shunted to the pre-load. At some point, the
preload conducts all of the load current while allowing zero LED
current. This allows the converter to continue operation to
maintain circuit bias with zero LED current. Very high levels of
LED dimming resolution become achievable. Both the maximum
pre-load current and turn-on threshold are adjustable.
Again referring to FIgure 26, the voltage across R5 represents
the current flowing in the pre-load. The maximum level of this
signal is limited by the VOH of the linear amplifier and the gate
threshold voltage of the pre-load FET, Q1. A reasonable
maximum voltage for this signal is 3.0V. Therefore, the
maximum pre-load current, IPL, is 3.0/R5.
where LREF and REFIN are the voltages at the LREF and REFIN
pins, respectively. For purposes of illustration, if R3 and R4 are
equal, Equation 25 simplifies to:
Equation 26 shows that if REFIN is greater than 2x LREF, VO is
non-positive and the pre-load is not conducting. With proper
selection of LREF and R3/R4, the pre-load turn-on threshold and
gain characteristics can be matched to the application
requirements.
where%H is the selected fraction of maximum load when the pre-
load begins to conduct.
As an example, assume the pre-load should begin to operate at
75% of full load, and that the maximum pre-load current, IPL, is
50mA. Using Equation 27 to solve for the ratio of R3/R4 yields a
result of 8. Equation 28 yields a value of 0.333V for LREF.
Remembering the maximum allowed voltage across R5 is 3.0V
yields R5 = 3.0V/50mA = 60 at 150mW. VO is plotted in
Figure 27.
Alternatively, the linear amplifier may be used to control a
second LED string, either to force current sharing, or to control a
colored LED string for color correction. The second string can be
controlled from the same reference as the first LED string
allowing the string currents to track, or it can be controlled from
a separate reference that allows the two strings to work in
opposition, sharing the load current in proportion to each
reference. Figure 28 shows the tracking configuration.
FIGURE 26. LINEAR AMPLIFIER CONFIGURED AS PRE-LOAD
+
VREF
1
2
4
3
5
6
7
8
9
10
11
12
PRELOAD
LOUT
LREF
LFB
LPOUT
REFIN
13
14
15
16
17
18
19
20
21
22
23
24
R1
R2
R3
R4
R5
ISL1902
Q1
VOVR5 LREF 1 R3
R4
--------
+


R3
R4
--------REFIN


–== V(EQ. 25)
VOVR5 2LREFREFIN–== V(EQ. 26)
R3
R4
--------30.5%H–0.5 %H3–
26%H+
%H
---------------------------------------------------------------------------------------------------------------
=(EQ. 27)
LREF
R3
R4
--------0.5 %H 
1R3
R4
--------
+
-------------------------------------- VREF R2
R1 R2+
----------------------
== V(EQ. 28)
FIGURE 27. PRE-LOAD EXAMPLE
0
1.0
1.5
2.0
2.5
3.0
0 50 100 150 200 250 300 350 400 450 500
REFIN (mV)
VO (V)
0.5
ISL1902
FN7981 Rev 2.00 Page 21 of 25
March 20, 2013
The linear amplifier may also be used to measure and scale the
LED current directly rather than using the differential current
sensing inputs, CS+ and CS-, that measure the switching current.
Amplifying the signal allows a smaller sensing resistor value for
improved efficiency. As shown in Figure 29, the voltage across
R4 is scaled by the linear amplifier with a gain of 1 + R2/R1.
Control Loop
The control loop configuration is user adjustable with 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.
The ISL1902 has two error amplifiers that share a common
non-inverting input and a common output. Each EA can sink
current, but has negligible sourcing capability. An external pull-up
resistor to VREF is required. This configuration causes the EA with
lowest output to be dominant. EA1 is the principal error amplifier
and is compensated externally for low bandwidth for PFC
applications. The downside to a low bandwidth amplifier is that it
cannot respond to input transients quickly. This is where the
second EA comes in. It can be configured for a much higher
bandwidth so that transient response is greatly improved. Under
normal operating conditions EA2 is not active. Its feedback
network is set for a higher output than EA1. When an input surge
occurs, EA1 cannot respond rapidly and the surge propagates to
the output. EA2 becomes active when its feedback voltage
exceeds the reference setpoint and acts to reduce the output
transient. The difference in setpoint is accomplished by
weighting the feedback networks to the EAs appropriately.
The voltage on IOUT is a scaled version of the CS+/CS-
differential signal, having been amplified by 4x. When averaged,
it is a scaled representation of the converter output current, IO.
By measuring IOUT in this manner, both average and
instantaneous inductor currents are known. The instantaneous
inductor current information informs the critical conduction
mode (CrCM) oscillator when the switching current has decayed
to zero.
Figure 30 shows a typical configuration for the control loop. The
sensing resistor RS determines the amplitude of the CS+ signal.
At maximum load this signal must be scaled to match the 0.5V
maximum reference. Since IOUT is 4x the amplitude of the CS+
signal, a simple resistor divider with filtering is required to scale
IOUT prior to connecting to the FB input.
where AIOUT is the IOUT buffer gain (nominally 4x), ADIVIDER is
the gain of the external resistor divider on IOUT (R2/(R1 + R2)),
VREF is the maximum reference level (0.5V), and IO is the
maximum output current. In most applications, RS will be sized
to minimize power dissipation while providing adequate signal
level. The minimum value of the IORS product is 125mV, required
to achieve 0.5V on IOUT.
FIGURE 28. SECOND LED STRING CONTROL
+
1
2
4
3
5
6
7
8
9
10
11
12
LOUT
LREF
LFB
LPOUT
REFIN
13
14
15
16
17
18
19
20
21
22
23
24
R3
R5
ISL1902
Q1
C1
FIGURE 29. DIRECT LED CURRENT SENSING
+
6
7
8
9
10
11
12
LOUT
LREF
LFB
FB1 13
14
15
16
17
18
19
R2
R4
ISL1902
R1 R3 = R1||R2
RS
VREF
AIOUT ADIVIDER IO

------------------------------------------------------------
=(EQ. 29)
ISL1902
FN7981 Rev 2.00 Page 22 of 25
March 20, 2013
In applications requiring PFC, the fast loop bandwidth can be set
to react to line transients without affecting steady state
operation. For example, the slow loop requires IOUT to be filtered
with a time constant of 50ms to 100ms. The fast loop (to be
effective), requires less filtering on IOUT and requires a
bandwidth three orders of magnitude (1000x) higher with a 60%
divider weighting compared to the slow loop (taking into account
the peak-to-average ratio of a sinusoid).
OVP
The ISL1902 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 17. The current hysteresis provides
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 ISL1902 provides the ability to disable the output based on
the level of the control loop reference, REFIN. 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
predetermined 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.
FIGURE 30. CONTROL LOOP CONFIGURATION
DIFFERENTIAL
CS - IOUT
PROCESSOR
+
_
CS-
IOUT
FB1
AC REFERENCE
GENERATOR R1
R2
CF1
CFB2
RFB1
VREF
RPU
ISL1902
RS
VERR
LPOUT
REFIN
+
_FB2
RFB2
CFB1
CF2
R3
R4
CS+
EA1
EA2
FIGURE 31. IOUT FILTERED WAVEFORMS (100Hz)
FILTERED IOUT FAST LOOP
FILTERED IOUT SLOW LOOP
TIME/ms 2ms/DIV
0246810 12 14
NORMALIZED VOLTS (V)
0
0.2
0.4
0.6
0.8
1.0
1.2
IOUT FILTERING
FIGURE 32. OV HYSTERESIS
+
_
20µA
VREF
10
MONITORED
VOLTAGE
R1
R2
R3
COPT
1.5V
Vov ri gsin
1.5 R1 R2+
R2
---------------------------
=V(EQ. 30)
V2010
6–R1=V(EQ. 31)
V2010
6–R1 R3 R1 R2+
R2
---------------------------
+


=V(EQ. 32)
REFIN off OFFREF 0.100–= V(EQ. 33)
ISL1902
FN7981 Rev 2.00 Page 23 of 25
March 20, 2013
PRELOAD Signal
PRELOAD is a digital signal used to control an external FET that
discharges the output capacitance if AC is low for more than
~30ms, or if REFIN drops below the OFFREF threshold. This
feature prevents the output capacitor from providing load current
for an extended period of time after the converter is disabled.
Otherwise, the output will dim as the output capacitance slowly
discharges through the LEDs. This process can take a significant
amount of time, resulting in “afterglow”, unless supplemental
discharge methods are used. The advantage of PRELOAD over a
non-switched resistive load is efficiency improvement. Examples
of PRELOAD usage may be found on pages 5 through 7 in the
“Typical Applications”.
In-rush Control
The ISL1902 features a AC half-cycle-by-half-cycle in-rush control
signal. Due to the capacitive input of DC/DC converters operating
with a leading edge modulated AC line dimmer, there is an input
current spike every half-cycle when the AC line dimmer turns on,
particularly so when conduction begins near the AC peak. The
current spike is normally attenuated with a resistor in series with
the AC line. The resistor is always present and dissipates power
even at full dimmer conduction.
The ISL1902 provides a control signal, INRUSH, which may be
used to gate an external switch, such as a triac to bypass the
in-rush limiting resistor after the in-rush event is over. The signal
is low when the IC detects the absence of AC line voltage. When
enabled, approximately 150µs after AC voltage is detected,
IN-RUSH outputs an 80kHz square wave. This may be coupled
through a pulse transformer or other isolation device to allow
control of a level shifted device. Examples of using INRUSH can
be found in the “Typical Applications” on pages 5 and 7. Another
example is shown in Figure 33.
Gate Drive
The ISL1902 output is capable of sourcing and sinking 1.5A. The
typical ON-resistance of the outputs is 12Ω. 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 +150°C. There is approximately
+25°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.
FIGURE 33. INRUSH EXAMPLE USING A PHOTO-TRIAC
EMI
FILTER
AC
INRUSH
FN7981 Rev 2.00 Page 24 of 25
March 20, 2013
ISL1902
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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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About Intersil
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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
March 20, 2013 FN7981.2 Initial Release.
ISL1902
FN7981 Rev 2.00 Page 25 of 25
March 20, 2013
Package Outline Drawing
M24.15
24 LEAD SHRINK SMALL OUTLINE PLASTIC PACKAGE (QSOP/SSOP)
0.150” WIDE BODY
Rev 3, 2/13
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-1982.
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 mate-
rial condition.
8. Controlling dimension: MILLIMETER.
INDEX
AREA
24
1
-B-
0.17(0.007) CAMBS
-A-
M
-C-
SEATING PLANE
0.10(0.004)
x 45°
0.25
0.010
GAUGE
PLANE
3.98
3.81
6.19
5.80
40.25(0.010) B
M M
1.27
0.41
0.49
0.26 5
8°
0° 1.54
0.25
0.18
8.74
8.55
31.75
1.35
0.25
0.10
0.30
0.20
7
0.635 BSC
5.59
4.06
7.11
0.635
DETAIL “X”
SIDE VIEW 1
TYPICAL RECOMMENDED LAND PATTERN
TOP VIEW
SIDE VIEW 2
5
0.38
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