High Voltage CMOS BoostWhite LED Driver
Description
The CAT4237 is a DC/DC step−up converter that delivers anaccurate constant current ideal for driving LEDs. Operation at aconstant switching frequency of 1 MHz allows the device to be usedwith small value external ceramic capacitors and inductor. LEDsconnected in series are driven with a regulated current set by theexternal resistor R1. LED currents up to 40 mA can be supported overa wide range of input supply voltages from 2.8 V to 5.5 V, making thedevice ideal for battery−powered applications. The CAT4237high−voltage output stage is perfect for driving six, seven or eightwhite LEDs in series with inherent current matching in LCD backlightapplications.
LED dimming can be done by using a DC voltage, a logic signal, ora pulse width modulation (PWM) signal. The shutdown input pinallows the device to be placed in power−down mode with “zero”quiescent current.
In addition to thermal protection and overload current limiting, thedevice also enters a very low power operating mode during “OpenLED” fault conditions. The device is housed in a low profile (1mmmax height) 5−lead thin SOT23 package for space criticalapplications.
Features
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51TSOT−23TD SUFFIXCASE 419AE
PIN CONNECTIONS
1SWGNDFB
(Top View)
SHDNVIN
MARKING DIAGRAMS
••••••••••••
Drives 6 to 8 White LEDs in Series from 3 VUp to 87% Efficiency
Low Quiescent Ground Current 0.6 mAAdjustable Output Current (up to 40 mA)High Frequency 1 MHz OperationHigh Voltage Power Switch
Shutdown Current Less than 1 mAOpen LED Low Power Mode
Automatic Shutdown at 1.9 V (UVLO)Thermal Shutdown Protection
Thin SOT23 5−lead (1 mm Max Height)
These Devices are Pb−Free, Halogen Free/BFR Free and are RoHSCompliant
Color LCD and Keypad BacklightingCellular PhonesHandheld DevicesDigital CamerasPDAs
Portable Game Machine
LTYMUDYM
LT = CAT4237TD−T3UD = CAT4237TD−GT3
Y = Production Year (Last Digit)
M = Production Month (1−9, A, B, C)
ORDERING INFORMATION
DeviceCAT4237TD−T3(Note 1)
CAT4237TD−GT3(Note 2)
PackageTSOT−23(Pb−Free)TSOT−23(Pb−Free)
Shipping3,000/Tape & Reel3,000/Tape & Reel
Applications
••••••
1.Matte−Tin Plated Finish (RoHS−compliant).2.NiPdAu Plated Finish (RoHS−compliant)
© Semiconductor Components Industries, LLC, 2010
February, 2010 − Rev. 2
1
Publication Order Number:
CAT4237/D
CAT4237
VIN3 V to4.2 VC1VINL33 mHSWDC20.22 mFVOUT4.7 mFCAT4237OFFONSHDNGNDFBVFB = 300 mVR115 W20 mAL: Sumida CDRH3D16−330
D: Central CMDSH05−4 (rated 40 V)
C2: Taiyo Yuden UMK212BJ224 (rated 50 V)
Figure 1. Typical Application Circuit
Table 1. ABSOLUTE MAXIMUM RATINGS
Parameters
VIN, FB voltageSHDN voltageSW voltage
Storage Temperature RangeJunction Temperature RangeLead Temperature
Ratings−0.3 to +7−0.3 to +7−0.3 to +55−65 to +160−40 to +150
300
UnitsVVV_C_C_C
Stresses exceeding Maximum Ratings may damage the device. Maximum Ratings are stress ratings only. Functional operation above theRecommended Operating Conditions is not implied. Extended exposure to stresses above the Recommended Operating Conditions may affectdevice reliability.
Table 2. RECOMMENDED OPERATING CONDITIONS
Parameters
VIN
SW pin voltage
Ambient Temperature Range6, 7 or 8 LEDs
Range2.8 to 5.50 to 30−40 to +851 to 40
UnitsVV_CmA
NOTE:Typical application circuit with external components is shown above.
3.Thin SOT23−5 package thermal resistance qJA = 135°C/W when mounted on board over a ground plane.
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CAT4237
Table 3. DC ELECTRICAL CHARACTERISTICS
(VIN = 3.6 V, ambient temperature of 25°C (over recommended operating conditions unless otherwise specified))SymbolIQISDVFBIFBILED
Parameter
Operating CurrentShutdown CurrentFB Pin VoltageFB pin input leakageProgrammed LED Current
R1 = 10 WR1 = 15 WR1 = 20 W
Enable Threshold LevelShutdown Threshold Level
28.51914.250.40.8350
ISW = 100 mASwitch Off, VSW = 5 V
3020150.80.71.04501.01150201.935
Conditions
VFB = 0.2 V
VFB = 0.4 V (not switching)VSHDN = 0 V8 LEDs with ILED = 20 mA
285Min
Typ0.60.10.1300
Max1.50.61315131.52115.751.51.36002.05
UnitmAmAmVmAmA
VIHVILFSWILIMRSWILEAK
SHDN Logic HighSHDN Logic LowSwitching FrequencySwitch Current LimitSwitch “On” ResistanceSwitch Leakage CurrentThermal ShutdownThermal Hysteresis
VMHzmAWmA°C°CVV
VUVLOVOV-SW
Undervoltage Lockout (UVLO) ThresholdOvervoltage Threshold
Pin Description
VIN is the supply input for the internal logic. The device iscompatible with supply voltages down to 2.8 V and up to5.5V. It is recommended that a small bypass ceramiccapacitor (4.7 mF) be placed between the VIN and GND pinsnear the device. If the supply voltage drops below 1.9 V, thedevice stops switching.
SHDN is the shutdown logic input. When the pin is tied toa voltage lower than 0.4 V, the device is in shutdown mode,drawing nearly zero current. When the pin is connected to avoltage higher than 1.5 V, the device is enabled.
GND is the ground reference pin. This pin should beconnected directly to the ground place on the PCB.
SW pin is connected to the drain of the internal CMOSpower switch of the boost converter. The inductor and theSchottky diode anode should be connected to the SW pin.Traces going to the SW pin should be as short as possiblewith minimum loop area. An over-voltage detection circuitis connected to the SW pin. When the voltage reaches 35V,the device enters a low power operating mode preventing theSW voltage from exceeding the maximum rating.
FB feedback pin is regulated at 0.3 V. A resistor connectedbetween the FB pin and ground sets the LED currentaccording to the formula:
ILED+
0.3VR1
The lower LED cathode is connected to the FB pin.
Table 4. PIN DESCRIPTIONS
Pin #12345
NameSWGNDFBSHDNVIN
Function
Switch pin. This is the drain of the internal power switch.Ground pin. Connect the pin to the ground plane.Feedback pin. Connect to the last LED cathode.Shutdown pin (Logic Low). Set high to enable the driver.Power Supply input.
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CAT4237
Block Diagram
VINC14.7 mF1 MHzOscillatorVREF300 mV–A1+EnableThermalShutdown& UVLORCCCRS+GND+A2–PWM &LogicN1Over VoltageProtectionDriver33 mHSWC20.22 mFVINLEDCurrentSHDNCurrentSenseFigure 2. Block DiagramDevice Operation
The CAT4237 is a fixed frequency (1 MHz), low noise,inductive boost converter that provides a constant currentwith excellent line and load regulation. The device uses ahigh-voltage CMOS power switch between the SW pin andground to energize the inductor. When the switch is turnedoff, the stored energy in the inductor is released into the loadvia the Schottky diode.
The on/off duty cycle of the power switch is internallyadjusted and controlled to maintain a constant regulatedvoltage of 0.3 V across the feedback resistor connected to thefeedback pin (FB). The value of the resistor sets the LEDcurrent accordingly (0.3 V/R1).
During the initial power-up stage, the duty cycle of theinternal power switch is limited to prevent excessive in-rushcurrents and thereby provide a “soft-start” mode ofoperation.
While in normal operation, the device can deliver up to40mA of load current into a string of up to 8 white LEDs.In the event of an “Open LED” fault condition, where thefeedback control loop becomes open, the output voltage willcontinue to increase. Once this voltage exceeds 35 V, aninternal protection circuit will become active and place thedevice into a very low power safe operating mode whereonly a small amount of power is transferred to the output.This is achieved by pulsing the switch once every 60 ms andkeep it on for about 1 ms only.
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–FBR115 WThermal overload protection circuitry has been includedto prevent the device from operating at unsafe junctiontemperatures above 150°C. In the event of a thermaloverload condition the device will automatically shutdownand wait till the junction temperatures cools to 130°C beforenormal operation is resumed.
Light Load Operation
Under light load condition (under 4 mA) and with inputvoltage above 4.2 V, the CAT4237 driving 6 LEDs, thedriver starts pulse skipping. Although the LED currentremains well regulated, some lower frequency ripple mayappear.
Figure 3. Switching Waveform VIN = 4.2 V,
ILED = 4 mA
CAT4237
TYPICAL CHARACTERISTICS
(VIN = 3.6 V, CIN = 4.7 mF, COUT = 0.22 mF, L = 33 mH with 8 LEDs at 20 mA, TAMB = 25°C, unless otherwise specified.)140120
SUPPLY CURRENT (mA)INPUT CURRENT (mA)1008060402002.73.03.33.63.94.24.54.802.5
3.0
3.5
4.0
4.5
5.0
VFB = 0.4 V(not switching)1.52.0
1.0
0.5
INPUT VOLTAGE (V)INPUT VOLTAGE (V)
Figure 4. Quiescent Current vs. VIN
(Not Switching)
315310FEEDBACK (mV)305300295290285
8 LEDs at 20 mAVOUT = 26 V315310FB PIN VOLTAGE (mV)Figure 5. Quiescent Current vs. VIN
(Switching)
8 LEDs305300295290285
2.73.03.33.63.94.24.54.8051015202530INPUT VOLTAGE (V)OUTPUT CURRENT (mA)
Figure 6. FB Pin Voltage vs. Supply Voltage
1040
SW pin20V/div
Figure 7. FB Pin Voltage vs. Output Current
FREQUENCY (kHz)1020
1000
InductorCurrent100mA/div
980
960
VOUTAC coupled200mV/div
2.7
3.0
3.3
3.6
3.9
4.2
4.5
4.8
0.5 msec/div
INPUT VOLTAGE (V)
Figure 8. Switching Frequency vs. Supply
Voltage
Figure 9. Switching Waveforms
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CAT4237
TYPICAL CHARACTERISTICS
(VIN = 3.6 V, CIN = 4.7 mF, COUT = 0.22 mF, L = 33 mH with 8 LEDs at 20 mA, TAMB = 25°C, unless otherwise specified.)35
CURRENT VARIATION (%)30LED CURRENT (mA)25201510502.53.03.54.04.55.0RFB = 15 WRFB = 20 WRFB = 10 W1.0
0.5
0
−0.5
−1.0
3.03.33.63.94.24.54.8
INPUT VOLTAGE (V)INPUT VOLTAGE (V)
Figure 10. LED Current vs. Input Voltage
(8 LEDs)
9085EFFICIENCY (%)80757065
8 LEDsVOUT ~ 27 V at 20 mAL = 33 mH51015202530VIN = 4.2 VEFFICIENCY (%)908580757065
Figure 11. LED Current Regulation (20 mA)
20 mA15 mAVIN = 3.6 V8 LEDsVOUT ~ 27 V at 20 mAL = 33 mH3.03.54.0INPUT VOLTAGE (V)
4.55.0LED CURRENT (mA)
Figure 12. 8 LED Efficiency vs. Load Current
90
VIN = 4.2 V85EFFICIENCY (%)EFFICIENCY (%)VIN = 3.6 V80757065
7 LEDsVOUT ~ 23 V at 20 mAL = 33 mH510152025308590
Figure 13. 8 LED Efficiency vs. Input Voltage
VIN = 4.2 VVIN = 3.6 V80757065
6 LEDsVOUT ~ 20 V at 20 mAL = 33 mH51015202530LED CURRENT (mA)LED CURRENT (mA)
Figure 14. 7 LED Efficiency vs. Load CurrentFigure 15. 6 LED Efficiency vs. Load Current
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CAT4237
TYPICAL CHARACTERISTICS
(VIN = 3.6 V, CIN = 4.7 mF, COUT = 0.22 mF, L = 33 mH with 8 LEDs at 20 mA, TAMB = 25°C, unless otherwise specified.)
EN5V/div
SWITCH RESISTANCE (W)VOUT10V/div
2.0
1.5
1.0
InputCurrent100mA/
div
0.5
0
50 msec/div
2.53.03.5
INPUT VOLTAGE (V)
4.04.5
Figure 16. Power−up with 8 LEDs at 20 mA
303FEEDBACK VOLTAGE (mV)SHUTDOWN VOLTAGE (V)302301300299298297−50VIN = 3.6 V, 8 LEDsILED = 20 mA050TEMPERATURE (°C)
1001501.0
Figure 17. Switch ON Resistance vs. Input
Voltage
0.8
−40°C25°C0.6
85°C125°C0.4
0.2
3.03.54.0INPUT VOLTAGE (V)
4.55.0Figure 18. FB Pin Voltage vs. Temperature
140MAX OUTPUT CURRENT (mA)1201008060402002.53.03.54.0VOUT = 15 VFigure 19. Shutdown Voltage vs. Input Voltage
VOUT = 20 V4.55.0INPUT VOLTAGE (V)
Figure 20. Maximum Output Current vs. Input
Voltage
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CAT4237
Application Information
External Component SelectionCapacitors
The CAT4237 only requires small ceramic capacitors of4.7mF on the input and 0.22 mF on the output. Under normalcondition, a 4.7 mF input capacitor is sufficient. Forapplications with higher output power, a larger inputcapacitor of 10 mF may be appropriate. X5R and X7Rcapacitor types are ideal due to their stability acrosstemperature range.
Inductor
given current. In order to achieve the best efficiency, thisforward voltage should be as low as possible. The responsetime is also critical since the driver is operating at 1MHz.Central Semiconductor Schottky diode CMDSH05−4(500mA rated) is recommended for most applications.
LED Current Setting
The LED current is set by the external resistor R1connected between the feedback pin (FB) and ground. Theformula below gives the relationship between the resistorand the current:
R1+
0.3V
currentLEDA 33 mH inductor is recommended for most of theCAT4237 applications. In cases where the efficiency iscritical, inductances with lower series resistance arepreferred. Inductors with current rating of 300 mA or higherare recommended for most applications. SumidaCDRH3D16−330 33 mH inductor has a rated current of320mA and a series resistance (D.C.R.) of 520 mW typical.
Schottky Diode
Table 5. RESISTOR R1 AND LED CURRENT
LED Current (mA)
51015202530
R1 (W)603020151210
The current rating of the Schottky diode must exceed thepeak current flowing through it. The Schottky diodeperformance is rated in terms of its forward voltage at a
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CAT4237
Open LED Protection
In the event of an “Open LED” fault condition, theCAT4237 will continue to boost the output voltage withmaximum power until the output voltage reachesapproximately 35 V. Once the output exceeds this level, theinternal circuitry immediately places the device into a verylow power mode where the total input power is limited toabout 4 mW (about 1 mA input current with a 3.6 V supply).The SW pin clamps at a voltage below its maximum ratingof 60 V. There is no need to use an external zener diodebetween Vout and the FB pin. A 50 V rated C2 capacitor isrequired to prevent any overvoltage damage in the openLED condition.
LVIN
C14.7 mFVINSWCAT4237OUTPUT VOLTAGE (V)OFFONSHDNGNDFBVFB = 300 mVR115 W45
33 mHC20.22 mF50
Schottky 100 V
(Central CMSH1−100)
VOUT
2.0
SUPPLY CURRENT (mA)1.5
1.0
0.5
0
2.53.03.54.04.55.0
INPUT VOLTAGE (V)
40
Figure 21. Open LED Protection without Zener
35
30
2.53.03.54.04.55.0
INPUT VOLTAGE (V)
Figure 24. Open LED Output Voltage vs. VIN without
Zener
SW PIN10 V/div
10 msec/div
Figure 22. Open LED Switching Waveforms without
Zener
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CAT4237
Dimming Control
There are several methods available to control the LEDbrightness.
PWM Signal on the SHDN PinVINSHDNGNDFBPWNSignal2.5 V0 V
3.73 kWRAC1SWCAT4237LED brightness dimming can be done by applying a PWMsignal to the SHDN input. The LED current is repetitivelyturned on and off, so that the average current is proportionalto the duty cycle. A 100% duty cycle, with SHDN alwayshigh, corresponds to the LEDs at nominal current. Figure25shows a 1kHz signal with a 50% duty cycle applied to theSHDN pin. The recommended PWM frequency range isfrom 100Hz to 2kHz.
VIN
VFB = 300 mV1 kW3.1 kWRBiR2LEDCurrentR115 W
0.22 mFFigure 26. Circuit for Filtered PWM Signal
A PWM signal at 0 V DC, or a 0% duty cycle, results ina max LED current of about 22 mA. A PWM signal with a93% duty cycle or more, results in an LED current of 0mA.
252015105
Figure 25. Switching Waveform with 1 kHz PWM on
SHDNFiltered PWM Signal
LED CURRENT (mA)0
0102030405060708090100
PWM DUTY CYCLE (%)
A filtered PWM signal used as a variable DC voltage cancontrol the LED current. Figure 26 shows the PWM controlcircuitry connected to the CAT4237 FB pin. The PWMsignal has a voltage swing of 0 V to 2.5 V. The LED currentcan be dimmed within a range from 0 mA to 20 mA. ThePWM signal frequency can vary from very low frequency upto 100 kHz.
Figure 27. Filtered PWM Dimming (0 V to 2.5 V)
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CAT4237
Board Layout
The CAT4237 is a high−frequency switching regulator.The traces that carry the high−frequency switching currenthave to be carefully layout on the board in order to minimizeEMI, ripple and noise in general. The thicker lines onFigure28 show the switching current path. All these traceshave to be short and wide enough to minimize the parasiticinductance and resistance. The loop shown on Figure28corresponds to the current path when the CAT4237 internalswitch is closed. On Figure 29 is shown the current loop,
when the CAT4237 switch is open. Both loop areas shouldbe as small as possible.
Capacitor C1 has to be placed as close as possible to theVIN pin and GND. The capacitor C2 has to be connectedseparately to the top LED anode. A ground plane under theCAT4237 allows for direct connection of the capacitors toground. The resistor R1 must be connected directly to theGND pin of the CAT4237 and not shared with the switchingcurrent loops and any other components.
ClosedOpenFigure 28. Closed−switch Current LoopFigure 29. Open−switch Current Loop
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CAT4237
PACKAGE DIMENSIONS
TSOT−23, 5 LEADCASE 419AE−01
ISSUE O
SYMBOL
AA1A2bcD
E1EEE1eLL1L2
0.300.010.800.300.12
0.152.90 BSC2.80 BSC1.60 BSC0.95 TYP0.400.60 REF0.25 BSC
0.50
0.050.87
DeMINNOMMAX
1.000.100.900.450.20
θ
TOP VIEW
0º8º
A2AqbA1L1LcL2SIDE VIEW
Notes:
(1) All dimensions are in millimeters. Angles in degrees.(2) Complies with JEDEC MO-193.
END VIEW
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CAT4237
Example of Ordering Information (Note 6)
PrefixCATDevice #4237SuffixTD− GT3Company ID(Optional)Product Number4237PackageTD: TSOT−23Lead FinishG: NiPdAuBlank: Matte−Tin (Note 7)Tape & Reel (Note 8)T: Tape & Reel3: 3,000 / Reel4.5.6.7.8.
All packages are RoHS−compliant (Lead−free, Halogen−free).The standard lead finish is NiPdAu.
The device used in the above example is a CAT4237TD−GT3 (TSOT−23, NiPdAu Plated Finish, Tape & Reel, 3,000/Reel).For Matte−Tin package option, please contact your nearest ON Semiconductor Sales office.
For information on tape and reel specifications, including part orientation and tape sizes, please refer to our Tape and Reel PackagingSpecifications Brochure, BRD8011/D.
ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further noticeto any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liabilityarising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages.“Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. Alloperating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. SCILLC does not convey any license under its patent rightsnor the rights of others. SCILLC products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applicationsintended to support or sustain life, or for any other application in which the failure of the SCILLC product could create a situation where personal injury or death may occur. ShouldBuyer purchase or use SCILLC products for any such unintended or unauthorized application, Buyer shall indemnify and hold SCILLC and its officers, employees, subsidiaries, affiliates,and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or deathassociated with such unintended or unauthorized use, even if such claim alleges that SCILLC was negligent regarding the design or manufacture of the part. SCILLC is an EqualOpportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any manner.
PUBLICATION ORDERING INFORMATION
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