FEATURES LT3743 High Current Synchronous Step-Down LED Driver with Three-State Control DESCRIPTION PWM Dimming Provides Up to 3000:1 Dimming Ratio CTRL_SEL Dimming Provides Up to 3000:1 Dimming Ratio Between Any Current nn Three-State Current Control for Color Mixing nn ±6% Current Regulation Accuracy nn 6V to 36V Input Voltage Range nn Average Current Mode Control nn 2µs Maximum Recovery Time Between Any Current Regulation State nn <1µA Shutdown Current nn Output Voltage Regulation and Open-LED Protection nn Thermally Enhanced 4mm × 5mm QFN and 28-Pin FE Package The LT®3743 is a fixed frequency synchronous step-down DC/DC controller designed to drive high current LEDs. The average current mode controller will maintain inductor current regulation over a wide output voltage range of 0V to (VIN – 2V). LED dimming is achieved through analog dimming on the CTRL_L, CTRL_H and CTRL_T pins and with PWM dimming on the PWM and CTRL_SEL pins. Through the use of externally switched load capacitors, the LT3743 is capable of changing regulated LED current levels within several µs, providing accurate, high speed PWM dimming between two current levels. The switching frequency is programmable from 200kHz to 1MHz through an external resistor on the RT pin. nn nn Additional features include voltage regulation and overvoltage protection set with a voltage divider from the output to the FB pin. Overcurrent protection is provided and set by the CTRL_H pin. APPLICATIONS DLP Projectors High Power Architectural Lighting nn Laser Diodes nn nn L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and True Color PWM is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents including 7199560, 7321203, 8120335 and 8901904. TYPICAL APPLICATION 92% Efficient 20A LED Driver EN/UVLO PWM CTRL_SEL 82.5k VIN EN/UVLO PWM CTRL_SEL RT SYNC VREF LT3743 RHOT 499Ω 100k CTRL_L 220nF 1.1µH SW 2.5mΩ 22µF LG M2 RNTC 10k 10nF M3 SENSE+ SENSE– PWMGH SS PWM 5V/DIV 330µF ×3 GND CTRL_T CTRL_SEL 5V/DIV OUTPUT 20A MAXIMUM 4.7µF VCC_INT CTRL_H 100k VIN 10V TO 30V M1 HG CBOOT 2.2nF 4.7µF ×4 1µF SW 20V/DIV 330µF ×3 ILED 10A/DIV M4 PWMGL 51.1k FB VCL VCH 34k 8.2nF 330µF ×3 3743 TA01a 34k 8.2nF VIN = 24V 20µs/DIV 0A TO 2A TO 20A LED CURRENT STEP 3743 TA01b 10.0k M1, M2: SiR462DP M3, M4: Si7234DP 3743fe For more information www.linear.com/LT3743 1 LT3743 ABSOLUTE MAXIMUM RATINGS (Note 1) VIN Voltage.................................................................40V EN/UVLO Voltage.........................................................6V VREF Voltage................................................................3V CTRL_L, CTRL_H, CTRL_T Voltage.............................3V PWM, CTRL_SEL Voltage............................................6V SENSE+ Voltage.........................................................40V SENSE– Voltage.........................................................40V VCH, VCL Voltage..........................................................3V SW Voltage................................................................40V CBOOT.......................................................................46V RT Voltage...................................................................3V FB Voltage....................................................................3V SS Voltage...................................................................6V SYNC Voltage...............................................................6V Storage Temperature Range................... –65°C to 150°C Lead Temperature (Soldering, 10 sec) TSSOP............................................................... 300°C PIN CONFIGURATION TOP VIEW VCC_INT 1 28 LG GND 2 27 GND VIN 3 26 CBOOT EN/UVLO 4 25 SW HG SW CBOOT LG VCC_INT VIN TOP VIEW 28 27 26 25 24 23 22 PWMGL GND 1 EN/UVLO 2 21 GND VREF 5 24 HG VREF 3 20 GND CTRL_T 6 23 PWMGL GND 7 18 PWM CTRL_H 8 CTRL_H 6 17 CTRL_SEL CTRL_L 9 CTRL_L 7 16 SYNC CTRL_T 4 19 PWMGH 29 GND GND 5 SS 8 SS 10 15 RT GND 11 FB 12 VCH VCL SENSE– SENSE+ FB GND 9 10 11 12 13 14 UFD PACKAGE 28-LEAD (4mm × 5mm) PLASTIC QFN TJMAX = 125°C, θJA = 37°C/W EXPOSED PAD (PIN 29) IS GND, MUST BE SOLDERED TO PCB ORDER INFORMATION 29 GND 22 GND 21 PWMGH 20 PWM 19 CTRL_SEL 18 SYNC 17 RT SENSE+ 13 16 VCH SENSE– 14 15 VCL FE PACKAGE 28-LEAD PLASTIC TSSOP TJMAX = 125°C, θJA = 30°C/W EXPOSED PAD (PIN 29) IS GND, MUST BE SOLDERED TO PCB (Note 2) LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT3743EUFD#PBF LT3743EUFD#TRPBF 3743 28-Lead (4mm × 5mm) Plastic QFN –40°C to 125°C LT3743IUFD#PBF LT3743IUFD#TRPBF 3743 28-Lead (4mm × 5mm) Plastic QFN –40°C to 125°C LT3743EFE#PBF LT3743EFE#TRPBF LT3743FE 28-Lead Plastic TSSOP –40°C to 125°C LT3743IFE#PBF LT3743IFE#TRPBF LT3743FE 28-Lead Plastic TSSOP –40°C to 125°C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/. Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix. 2 3743fe For more information www.linear.com/LT3743 LT3743 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VEN/UVLO = 5V, VSYNC = 0V, VCTRL_SEL = 0V, VPWM = 2V, unless otherwise noted. PARAMETER CONDITIONS MIN Input Voltage Range VIN Pin Quiescent Current (Note 3) Non-Switching Operation Shutdown Mode VPWM = VCTRL_SEL = 0V, Not Switching, RT = 40k VEN/UVLO = 0V l EN/UVLO Pin Falling Threshold 1.49 EN/UVLO Hysteresis EN/UVLO Pin Current TYP 6 VIN = 6V, EN/UVLO = 1.45V PWM Pin Threshold MAX UNITS 36 V 1.8 0.1 2.5 1 mA µA 1.55 1.61 V 130 mV 5.5 µA 1.0 V CTRL_SEL Threshold 1.0 V SYNC Pin Threshold 1.0 V CTRL_H and CTRL_L Pin Control Range 0 CTRL_H and CTRL_L Pin Current 1.5 100 V nA Reference Reference Voltage (VREF Pin) l 1.96 2 2.04 l 48 51 54 V Inductor Current Sensing Full Range SENSE+ to SENSE– VCTRL_H = 1.5V, VSENSE– = 6V mV SENSE+ Pin Current VSENSE – = 6V 50 nA SENSE– Pin Current VSENSE+ = VSENSE– = 6V 10 µA +=V SENSE Internal VCC Regulator (VCC_INT Pin) Regulation Voltage l 4.7 5 5.2 V NMOS FET Driver (Note 2) Non-Overlap time HG to LG 100 ns Non-Overlap time LG to HG 60 ns 50 ns Minimum On-Time LG (Note 3) Minimum On-Time HG (Note 3) 80 ns Minimum Off-Time LG (Note 3) 60 ns High Side Driver Switch On-Resistance Gate Pull Up Gate Pull Down VCBOOT – VSW = 5V 2.3 1.3 Ω Ω Low Side Driver Switch On-Resistance Gate Pull Up Gate Pull Down VCC_INT = 5V 2.5 1.3 Ω Ω Switching Frequency fSW RT = 40kΩ RT = 200kΩ l 900 190 1000 200 1070 233 kHz kHz Soft-Start Charging Current 5.5 µA Voltage Regulation Amplifier Input Bias Current 1 Feedback Regulation Voltage nA 200 gm VCTRL_H = 0V, VCTRL_L = 2V, VSENSE+ = 2V l 0.945 1 µA/V 1.025 V 3743fe For more information www.linear.com/LT3743 3 LT3743 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VEN/UVLO = 5V, unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX UNITS PWMG Control Signals CTRL_SEL High to PWMGL Low Delay 10 40 ns CTRL_SEL High to PWMGH High Delay 150 200 ns CTRL_SEL Low to PWMGH Low Delay 30 60 ns 220 ns CTRL_SEL Low to PWMGL High Delay 170 PWMGH and PWMGL Pull-up Impedance 3.2 Ω PWMGH and PWMGL Pull-Down Impedance 1.75 Ω Current Control Loop gm Amp Offset Voltage VSENSE+ = 4V, VSENSE– = 4V Input Common Mode Range VCM(LOW) VCM(HIGH) VCM(HIGH) Measured from VIN to VCM l –3 3 0 2 Output Impedance 375 Differential Gain 475 1.7 Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LT3743E is guaranteed to meet performance specifications from 0°C to 125°C junction temperature. Specifications over the –40°C mV V V 3.5 gm 4 0 MΩ 625 µA/V V/mV to 125°C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LT3743I is guaranteed to meet performance specifications over the –40°C to 125°C operating junction temperature range. Note 3: The minimum on and off times are guaranteed by design and are not tested. 3743fe For more information www.linear.com/LT3743 LT3743 TYPICAL PERFORMANCE CHARACTERISTICS EN/UVLO Pin Current 0.5 1.64 8 0.4 6 0.3 1.58 –50°C 1.52 130°C 1.46 1.40 6 12 18 24 VIN (V) 30 IQ (µA) 10 EN/UVLO PIN CURRENT (µA) EN/UVLO THRESHOLD (V) EN/UVLO Threshold (Falling) 1.70 4 2 0 36 12 18 24 VIN (V) 30 3743 G01 2.01 0 6 12 18 24 VIN (V) 30 8 16 24 VIN (V) 32 VREF Current Limit 1.4 2.00 VIN = 36V 1.99 VIN = 6V TA = 25°C TA = 130°C 1.2 TA = –50°C 1.0 1.97 –50 36 –15 55 20 TEMPERATURE (°C) 90 3743 G04 0.8 125 6 12 18 24 VIN (V) 30 3743 G05 Oscillator Frequency 36 3743 G06 RT Pin Current Limit Soft-Start Pin Current 90 1.5 40 1.6 1.98 TA = 25°C TA = 130°C TA = –50°C 0 3743 G03 ILIMIT (mA) 1.6 0.4 36 VREF Pin Voltage 2.02 VREF VOLTAGE (V) QUIESCENT CURRENT (mA) Quiescent Current (Non-Switching) 0.8 25°C 0 3743 G02 2.0 1.2 130°C 0.2 0.1 25°C 130°C –50°C 6 IQ in Shutdown 7 1.2MHz 80 0.9 0.6 0.3 0 –50 55 20 TEMPERATURE (°C) VIN = 36V 70 60 90 125 3743 G07 40 –50 5 VIN = 6V 4 50 220kHz –15 6 ISS (µA) 900kHz ILIMIT (µA) FREQUENCY (MHz) 1.2 –15 55 20 TEMPERATURE (°C) 90 125 3743 G08 3 –50 –15 55 20 TEMPERATURE (°C) 90 125 3743 G09 3743fe For more information www.linear.com/LT3743 5 LT3743 TYPICAL PERFORMANCE CHARACTERISTICS Internal UVLO VCC_INT UVLO CBOOT-SW UVLO Voltage 5.0 3.00 4.00 2.75 3.75 2.50 3.50 4.0 UVLO (V) VOLTAGE (V) VIN (V) 4.5 2.25 3.25 2.00 3.00 1.75 2.75 3.5 3.0 –50 –15 55 20 TEMPERATURE (°C) 90 1.50 –50 125 –15 20 55 TEMPERATURE (°C) 90 VCC_INT Load Reg at 12V OPEN-LED THRESHOLD (V) REGULATED CURRENT (%) VCC_INT (V) 4.4 50 0 –50 –100 0 10 20 30 40 ILOAD (mA) 50 60 –150 800 900 1000 1100 VFB (mV) 1200 3743 G13 Open-LED Timeout 125 40 30 20 90 0 125 VIN = 6V 1.5 VIN = 36V 1.0 0.5 0 0.5 1.0 2.0 1.5 VCTRL (V) 3743 G16 55 20 TEMPERATURE (°C) MEASURED VIN – VOUT 2.0 10 90 –15 Common Mode Lockout 2.5 CM LOCKOUT (V) VSENSE+ – VSENSE– (mV) OPEN-LED TIMEOUT (µs) 13 11 6 1.1 Regulated Sense Voltage 15 55 20 TEMPERATURE (°C) 1.2 3743 G15 50 –15 1.3 1.0 –50 1300 60 17 9 –50 1.4 3743 G14 19 125 Open-LED Threshold 100 4.8 90 1.5 150 5.2 20 55 TEMPERATURE (°C) 3743 G12 Regulated Current vs VFB 6.0 5.6 –15 3743 G11 3743 G10 4.0 2.50 –50 125 3743 G17 0 –50 –15 55 20 TEMPERATURE (°C) 90 125 3743 G18 3743fe For more information www.linear.com/LT3743 LT3743 TYPICAL PERFORMANCE CHARACTERISTICS 4 4 PMOS 3 2 NMOS HG Driver RDS(ON) 5 3 PMOS 2 1 –15 55 20 TEMPERATURE (°C) 90 0 –50 125 –15 55 20 TEMPERATURE (°C) 90 120 PWMGH TO PWMGL 110 –15 55 20 TEMPERATURE (°C) 90 LG TO HG 60 30 90 HG 60 LG 30 –15 55 20 TEMPERATURE (°C) 90 0 –50 125 –15 55 20 TEMPERATURE (°C) 90 3743 G23 3 125 120 HG TO LG 0 –50 125 300 125 3743 G24 Regulation Accuracy CTRL_H = 1.5V, VIN = 12V 6 2 4 1 2 Regulation Accuracy CTRL_H = 0.75V, VIN = 12V 180 120 HG 60 ACCURACY (%) LG ACCURACY (%) MINIMUM OFF-TIME (ns) 90 Minimum On-Time 90 Minimum Off-Time 0 –50 55 20 TEMPERATURE (°C) 150 3743 G22 240 –15 3743 G21 MINIMUM ON-TIME (ns) 140 PWMGL TO PWMGH NMOS 0 –50 125 Non-Overlap Time 150 NON-OVERLAP TIME (ns) DELAY (ns) Non-Overlap PWM Signal Delay 100 –50 2 3743 G20 150 120 PMOS 1 3743 G19 130 3 NMOS 1 0 –50 LG Driver RDS(ON) 4 RDS(ON) (Ω) 5 RDS(ON) (Ω) RDS(ON) (Ω) PWM Driver RDS(0N) 5 0 –1 55 20 TEMPERATURE (°C) 90 125 3743 G25 –3 –2 –4 –2 –15 0 0 2.5 5.0 7.5 OUTPUT VOLTAGE (V) 10 3743 G26 –6 0 2.5 5.0 7.5 OUTPUT VOLTAGE (V) 10 3743 G27 3743fe For more information www.linear.com/LT3743 7 LT3743 TYPICAL PERFORMANCE CHARACTERISTICS LED Current Waveforms (90% PWM) 0.5A to 5A Overcurrent Threshold LED Current Waveforms (2000:1) 3A to 10A 120 VSENSE+ – VSENSE– (mV) 80 60 ILED 5A/DIV 40 IL 10A/DIV 20 0 CTRL_SEL 5V/DIV CTRL_SEL 5V/DIV SW 20V/DIV 100 ILED 5A/DIV IL 10A/DIV 40µs/DIV 0 0.75 1.5 CTRL_H (V) 2.25 3743 G29 5µs/DIV 3743 G30 3.0 3743 G28 CTRL_SEL 5V/DIV SW 10V/DIV ILED 11.1A/DIV 20µs/DIV CTRL_L 0.2V/DIV SW 10V/DIV CTRL_SEL 5V/DIV ILED 10A/DIV ILED 8A/DIV 3743 G31 20µs/DIV 3743 G32 40µs/DIV VOUT 2V/DIV IL 5A/DIV IL 200mA/DIV 3743 G34 3743 G33 Overvoltage Lockout Operation With Open-Load Condition Common Mode Lockout (VIN = 7V) VOUT 2V/DIV 8 PWM 5V/DIV CTRL_SEL 5V/DIV PWM 5V/DIV Voltage Regulation with 10A Regulated Inductor Current 100µs/DIV LED Current Waveforms (3000:1) Analog Dimming on CTRL_L COUT(LOW) = 22µF, COUT(HIGH) = 1mF LED Current Waveforms (3000:1) 0A to 2A to 20A LED Current Waveforms (3000:1) 2A to 20A IL 200mA/DIV VOUT 2V/DIV 1ms/DIV 3743 G35 40ms/DIV 3743 G36 3743fe For more information www.linear.com/LT3743 LT3743 PIN FUNCTIONS (QFN/TSSOP) GND (Pins 1, 5, 9, 20, 21, Exposed Pad Pin 29/Pins 2, 7, 11, 22, 27, Exposed Pad Pin 29): Ground. The exposed pad must be soldered to the PCB. EN/UVLO (Pin 2/Pin 4): Enable Pin. The EN/UVLO pin acts as an enable pin and turns on the internal current bias core and subregulators at 1.55V. The pin does not have any pull-up or pull-down, requiring a voltage bias for normal part operation. Full shutdown occurs at approximately 0.5V. VREF (Pin 3/Pin 5): Buffered 2V Reference Capable of 0.5mA Drive. CTRL_T (Pin 4/Pin 6): The thermal control input to reduce the regulated current level for both current levels (CTRL_L and CTRL_H). CTRL_H (Pin 6/Pin 8): The CTRL_H pin sets the high level regulated output current and overcurrent. The maximum input voltage is internally clamped to 1.5V. The overcurrent set point is equal to the high level regulated current level set by the CTRL_H pin with an additional 23mV offset between the SENSE+ and SENSE– pins. CTRL_L (Pin 7/Pin 9): The CTRL_L pin sets the low level regulated output current. It is not recommended that the CTRL_L voltage be higher than the CTRL_H voltage. SS (Pin 8/Pin 10): Soft-Start Pin. Place an external capacitor to ground to limit the regulated current during start-up conditions. The SS pin has a 5.5µA charging current. This pin controls both of the regulated inputs determined by CTRL_L and CTRL_H. FB (Pin 10/Pin 12): Feedback Pin for Overvoltage Protection. The feedback voltage is 1V. Overvoltage/Open LED is sensed through the FB pin. When the feedback voltage exceeds 1.3V, the overvoltage lockout prevents switching and connects both output capacitors to discharge the inductor current. SENSE+ (Pin 11/Pin 13): SENSE+ is the inverting input of the average current mode loop error amplifier. This pin is connected to the external current sense resistor, RS. The voltage drop between SENSE+ and SENSE– referenced to the voltage drop across an internal resistor produces the input voltages to the current regulation loop. SENSE– (Pin 12/Pin 14): SENSE– is the noninverting input of the average current mode loop error amplifier. The reference current, based on CTRL_L or CTRL_H flows out of the pin to the output (LED) side of the sense resistor, RS. VCL (Pin 13/Pin 15): VCL provides the necessary compensation for the average current loop stability during low level current regulation. Typical compensation values are 15k to 80k for the resistor and 2nF to 10nF for the capacitor. VCH (Pin 14/Pin 16): VCH provides the necessary compensation for the average current loop stability during high level current regulation. Typical compensation values are 15k to 80k for the resistor and 2nF to 10nF for the capacitor. RT (Pin 15/Pin 17): A resistor to ground sets the switching frequency between 200kHz and 1MHz. When using the SYNC function, set the frequency to be 20% lower than the SYNC pulse frequency. This pin is current limited to 60µA. Do not leave this pin open. SYNC (Pin 16/Pin 18): Frequency Synchronization Pin. This pin allows the switching frequency to be synchronized to an external clock. The RT resistor should be chosen to operate the internal clock at 20% slower than the SYNC pulse frequency. The synchronization range is 240kHz to 1.2MHz. This pin should be grounded when not in use. CTRL_SEL (Pin 17/Pin 19): The CTRL_SEL pin selects between the high current control, CTRL_H and the low current control, CTRL_L. When high, the VCH pin is connected to the error amp output and the PWMGH gate signal is high. When low, the VCL pin is connected to the error amp output and the PWMGL gate signal is high. This pin is used for current level dimming of the LED. This pin should be grounded when not in use. PWM (Pin 18/Pin 20): The input pin for PWM dimming of the LED. When low, all switching is terminated and the output caps are disconnected. This pin should be pulled to VCC_INT when not in use. PWMGH (Pin 19/Pin 21): The PWMGH output pin drives the gate of an external FET to connect one of the switching regulator output capacitors to the load. The driver pull-up impedance is 3.2Ω and pull-down impedance is 1.75Ω. 3743fe For more information www.linear.com/LT3743 9 LT3743 PIN FUNCTIONS (QFN/TSSOP) PWMGL (Pin 22/Pin 23): The PWMGL output pin drives the gate of an external FET to connect one of the switching regulator output capacitors to the load. The driver pull-up impedance is 3.2Ω and pull-down impedance is 1.75Ω. LG (Pin 26/Pin 28): LG is the bottom FET gate drive signal that controls the state of the low side external power FET. The driver pull-up impedance is 2.5Ω and pull-down impedance is 1.3Ω. HG (Pin 23/Pin 24): HG is the top FET gate drive signal that controls the state of the high side external power FET. The driver pull-up impedance is 2.3Ω and pull-down impedance is 1.3Ω. VCC_INT (Pin 27/Pin 1): A regulated 5V output for charging the CBOOT capacitor. VCC_INT also provides the power for the digital and switching subcircuits. Below 6V VIN, tie this pin to the rail. VCC_INT is current limited to ≈50mA. Shutdown operation disables the output voltage drive. SW (Pin 24/Pin 25): The SW pin is used internally as the lower rail for the floating high side driver. Externally, this node connects the two power FETs and the inductor. VIN (Pin 28/Pin 3): Input Supply Pin. Must be locally bypassed with a 1µF low ESR capacitor to ground. CBOOT (Pin 25/Pin 26): The CBOOT pin provides a floating 5V regulated supply for the high side FET driver. An external Schottky diode is required from the VCC_INT pin to the CBOOT pin to charge the CBOOT capacitor when the switch pin is near ground. 10 3743fe For more information www.linear.com/LT3743 LT3743 BLOCK DIAGRAM (QFN Package) VIN VIN 402k 2 INTERNAL REGULATOR AND UVLO 133k 3 2.2nF SYNC 16 15 28 47µF EN/UVLO VREF 2V REFERENCE SYNC OSCILLATOR RT 82.5k – R + S Q PWM COMPARATOR VCC_INT 27 10µF HIGH SIDE CBOOT DRIVER 25 HG 23 SW SYNCRONOUS 24 CONTROLLER LG 26 LOW SIDE DRIVER SENSE+ 0.1µF 2.2µH 10Ω 11 – 33nF CTRL_L + + 5.5µA + – CTRL BUFFER 8 100nF 4 13 34k 8.2nF 14 SENSE– FB CTRL_T VCL 10Ω 12 RS 5mΩ 330µF ×3 10A LED OUTPUT 330µF ×3 40.2k 10 10k SS 90k VOLTAGE REGULATOR AMP + 7 CTRL_H gm AMP gm = 475µA/V RO = 3.5M IOUT = 40µA – 6 CURRENT MIRROR 3k + 1.5V VIN 1V + VCH 34k 8.2nF OPEN-LED COMPARATOR 17 18 CTRL_SEL PWM – 1.3V 2.5Ω 2.5Ω PWMGL PWMGH 22 19 3743 F01 Figure 1. Block Diagram 3743fe For more information www.linear.com/LT3743 11 LT3743 OPERATION The LT3743 utilizes fixed frequency, average current mode control to accurately regulate the inductor current, independently from the output voltage. This is an ideal solution for applications requiring a regulated current source including driving high current LEDs where the forward junction voltage can range from 2V to 6V with a dynamic resistance of 20mΩ to 40mΩ. The control loop will regulate the current in the inductor at an accuracy of 6%. For additional operation information, refer to the Block Diagram in Figure 1. The control loop has two independent reference inputs, determined by the analog control pins, CTRL_H and CTRL_L. When the CTRL_SEL pin is low, the control loop uses the reference determined by the CTRL_L pin and when high, the loop uses the reference determined by the CTRL_H pin. The analog voltage at the CTRL_L and CTRL_H pins is buffered and produces a reference voltage across an internal resistor. The internal buffers have a 1.5V clamp on the output, limiting the analog control range of the CTRL_L and CTRL_H pins from 0V to 1.5V. The average current mode control loop uses the internal reference voltage to regulate the inductor current, as a voltage drop across the external sense resistor, RS. In many applications, a rapid transition between the two regulated current states is desirable to provide background LED color mixing for pure colors in an RGB projector or display. For this purpose, pulse width modulation dimming can be achieved with both the PWM and CTRL_SEL pins. When the PWM pin is low, the regulated current in the inductor is zero and both output capacitors are disconnected. When the PWM pin is high, and the CTRL_SEL pin is low, the regulated current in the inductor is determined by the analog voltage at the CTRL_L pin. When the PWM and CTRL_SEL pins are both high, the regulated current in the inductor is determined by the analog voltage at the CTRL_H pin. The LT3743 uses a unique switched output capacitor topology and two independent compensation networks to transition between the two regulated current states in less than 2µs. When the CTRL_SEL pin is low and the PWM pin is high, the PWMGL output pin is high, switching in the output capacitor for the CTRL_L current level. 12 The CTRL_L output capacitor stores the LED forward voltage drop when the control loop regulates the low current level. When the CTRL_SEL pin changes to the high state, a 150ns delay ensures that the output capacitors are not connected at the same time. After this delay, the output capacitor for the CTRL_H level is switched in when PWMGH goes high and immediately delivers current to the LED. The CTRL_H output capacitor has the voltage drop of the LED with the regulated current determined by the analog voltage at the CTRL_H pin. To achieve minimum transition delay, the inductor is precharged to 70% of the regulation current level just after the PWMGH pin goes high. Conversely, when the PWM pin goes low, the inductor is discharged to 70% of the low current level before normal switching at the low current level commences. The error amplifier for the average current mode control loop also has a common mode lockout that regulates the inductor current so that the error amplifier is never operated out of the common mode range. The common mode range is with an output voltage from 0V to 2V below the VIN supply rail. The overcurrent set point is equal to the high level regulated current level set by the CTRL_H pin with an additional 23mV offset between the SENSE+ and SENSE– pins. The overcurrent is limited on a cycle-by-cycle basis; shutting switching down once the overcurrent level is reached. Overcurrent is not soft started. The output voltage may be limited with a resistor divider from the output back to the FB pin. The reference at the FB pin is 1.0V. If the output voltage level is high enough to engage the voltage loop, the regulated inductor current will be reduced so that the output voltage is limited. If the voltage at the FB pin reaches 1.3V (30% higher than the regulation level), an internal open-LED flag is set, shutting down switching for 13µs and switching in both output capacitors to fully drain the inductor’s current. During start-up, the SS pin is held low until the first time the PWM pin goes high. Once the PWM signal goes high, the capacitor at the SS pin is charged with a 5.5µA current source. The internal buffers for the CTRL_L and CTRL_H signals are limited by the voltage at the SS pin, slowly ramping the regulated inductor current to the current determined by the voltage at the CTRL_H or CTRL_L pins. 3743fe For more information www.linear.com/LT3743 LT3743 APPLICATIONS INFORMATION Programming Inductor Current Inductor Selection The analog voltage at the CTRL_L and CTRL_H pins is buffered and produces a reference voltage, VCTRL, across an internal resistor. The regulated average inductor current is determined by: The recovery time between regulated states is critical to maintaining accurate control of the LED current. For this reason, sizing the inductor to have no less than 30% peak-to-peak ripple will provide excellent recovery time with reasonable ripple. The overcurrent set point is equal to the high level regulated current level set by the CTRL_H pin with an additional 23mV offset between the SENSE+ and SENSE– pins. The saturation current for the inductor should be at least 20% higher than the maximum regulated current. The following equation sizes the inductor to achieve a reasonable recovery time while minimizing the inductor ripple: VCTRL 30 •RS where RS is the external sense resistor and IO is the average inductor current, which is equal to the LED current. Figure 2 shows the LED current vs RS. The maximum power dissipation in the resistor will be: PRS = (0.05V )2 L= RS Table 1 contains several resistors values, the corresponding maximum current and power dissipation in the sense resistor. Figure 3 shows the power dissipation in RS. Table 1. Sense Resistor Values VIN • ( VF ) – ( VF ) 2 0.2 • fS •IO • VIN where VF is the LED forward voltage drop, IO is the maximum regulated current in the inductor and fS is the switching frequency. Using this equation, the inductor will have approximately 10% ripple at maximum regulated current. MAXIMUM LED CURRENT (A) RESISTOR, RS (mΩ) POWER DISSIPATION (W) 1 50 0.05 VENDOR WEBSITE 5 10 0.25 Coilcraft www.coilcraft.com 10 5 0.5 Sumida www.sumida.com 25 2 1.25 Table 2. Recommended Inductor Manufacturers Vishay www.vishay.com Wurth Electronics www.we-online.com NEC-Tokin www.nec-tokin.com 30 25 LED CURRENT (A) 1.4 20 1.2 15 10 5 0 0 2 4 6 8 10 12 14 16 18 20 RS (mΩ) 3743 F02 Figure 2. RS Value Selection for LED Current POWER DISSIPATION (W) IO = 1.0 0.8 0.6 0.4 0.2 0 0 2 4 6 8 10 12 RS (mΩ) 14 16 18 20 3743 F03 Figure 3. Power Dissipation in RS 3743fe For more information www.linear.com/LT3743 13 LT3743 APPLICATIONS INFORMATION Switching MOSFET Selection When selecting switching MOSFETs, the following parameters are critical in determining the best devices for a given application: total gate charge (QG), on-resistance (RDS(ON)), gate to drain charge (QGD), gate-to-source charge (QGS), gate resistance (RG), breakdown voltages (maximum VGS and VDS) and drain current (maximum ID). The following guidelines provide information to make the selection process easier. Both of the switching MOSFETs need to have their maximum rated drain currents greater than the maximum inductor current. The following equation calculates the peak inductor current: 2 VIN • ( VF +RDIO ) – ( VF +RDIO ) IMAX =IO + 2 • fS •L • VIN where VIN is the input voltage, L is the inductance value, VF is the LED forward voltage drop, RD is the dynamic series resistance of the LED, IO is the regulated output current and fS is the switching frequency. During MOSFET selection, notice that the maximum drain current is temperature dependant. Most data sheets include a table or graph of the maximum rated drain current vs temperature. The maximum VDS should be selected to be higher than the maximum input supply voltage (including transient) for both MOSFETs. The signals driving the gates of the switching MOSFETs have a maximum voltage of 5V with respect to the source. During start-up and recovery conditions, the gate drive signals may be as low as 3V. To ensure that the LT3743 recovers properly, the maximum threshold should be less than 2V. For a robust design, select the maximum VGS greater than 7V. Power losses in the switching MOSFETs are related to the on-resistance, RDS(ON); the transitional loss related to the gate resistance, RG; gate-to-drain capacitance, QGD and gate-to-source capacitance, QGS. Power loss to the on-resistance is an Ohmic loss, I2 RDS(ON), and usually dominates for input voltages less than ~15V. Power losses to the gate capacitance dominate for voltages greater than ~12V. When operating at higher input voltages, efficiency 14 can be optimized by selecting a high side MOSFET with higher RDS(ON) and lower CGD. The power loss in the high side MOSFET can be approximated by: PLOSS = (ohmic loss) + (transition loss) PLOSS ( VF +RDIO ) •I2 R VIN O DS(ON) • T + VIN •IOUT • ((QGD +QGS ) • (2 •RG +RPU +RPD )) • fS 5V where ρT is a temperature-dependant term of the MOSFET’s on-resistance. Using 70°C as the maximum ambient operating temperature, ρT is roughly equal to 1.3. RPD and RPU are the LT3743 high side gate driver output impedance, 1.3Ω and 2.3Ω respectively. A good approach to MOSFET sizing is to select a high side MOSFET, then select the low side MOSFET. The tradeoff between RDS(ON), QG, QGD and QGS for the high side MOSFET is shown in the following example. VO is equal to 4V. Comparing two N-channel MOSFETs, with a rated VDS of 40V and in the same package, but with 8× different RDS(ON) and 4.5× different QG and QGD: M1: RDS(ON) = 2.3mΩ, QG = 45.5nC, QGS = 13.8nC, QGD = 14.4nC , RG = 1Ω M2: RDS(ON) = 18mΩ, QG = 10nC, QGS = 4.5nC, QGD = 3.1nC , RG = 3.5Ω Power loss for both MOSFETs is shown in Figure 4. Observe that while the RDS(ON) of M1 is eight times lower, the power loss at low input voltages is equal, but four times higher at high input voltages than the power loss for M2. Another power loss related to switching MOSFET selection is the power lost to driving the gates. The total gate charge, QG, must be charged and discharged each switching cycle. The power is lost to the internal LDO within the LT3743. The power lost to the charging of the gates is: PLOSS_LDO ≈ (VIN – 5V) • (QGLG + QGHG) • fS where QGLG is the low side gate charge and QGHG is the high side gate charge. 3743fe For more information www.linear.com/LT3743 LT3743 APPLICATIONS INFORMATION 7 2.5 MOSFET POWER LOSS (W) MOSFET POWER LOSS (W) 6 5 TOTAL 4 TRANSITIONAL 3 2 1 0 2.0 1.5 TOTAL 1.0 TRANSITIONAL 0.5 OHMIC OHMIC 0 10 20 30 0 40 0 INPUT VOLTAGE (V) 20 10 30 40 INPUT VOLTAGE (V) 3743 F04a 3743 F04b Figure 4a. Power Loss Example for M1 Figure 4b. Power Loss Example for M2 Figure 4 Whenever possible, utilize a switching MOSFET that minimizes the total gate charge to limit the internal power dissipation of the LT3743. Table 3. Recommended Switching FETs VIN VOUT ID (V) (V) (A) TOP FET 8 4 24 4 24 2-4 20 RJK0365DPA 12 2-4 10 FDMS8680 36 4 20 Si7884BDP 24 4 40 PSMN4R030YL BOTTOM FET MANUFACTURER 5-10 RJK0365DPA RJK0330DPB Renesas 5 RJK0368DPA RJK0332DPB www.renesas.com RJK0346DPA FDMS8672AS Fairchild www.fairchildsemi. com SiR470DP Vishay www.vishay.com RJK0346DPA NXP/Philips www.nxp.com Input Capacitor Selection The input capacitor should be sized at 4µF for every 1A of output current and placed very close to the high side MOSFET. A small 1µF ceramic capacitor should be placed near the VIN and ground pins of the LT3743 for optimal noise immunity. The input capacitor should have a ripple current rating equal to half of the maximum output current. It is recommended that several low ESR ceramic capacitors be used as the input capacitance. Use only type X5R or X7R capacitors as they maintain their capacitance over a wide range of operating voltages and temperatures. Output Capacitor Selection The output capacitors need to have very low ESR (equivalent series resistance) to allow the LED current to ramp quickly. A minimum of 50µF/A of load current should be used in most designs. The capacitors also need to be surge rated to the maximum output current. To achieve the lowest possible ESR, several low ESR capacitors should be used in parallel. Many applications benefit from the use of high density POSCAP capacitors, which are easily destroyed when exposed to overvoltage conditions. To prevent this, select POSCAP capacitors that have a voltage rating that is at least 50% higher than the regulated voltage CBOOT Capacitor Selection The CBOOT capacitor must be sized less than 220nF and more than 50nF to ensure proper operation of the LT3743. Use 220nF for high current switching MOSFETs with high gate charge. 3743fe For more information www.linear.com/LT3743 15 LT3743 APPLICATIONS INFORMATION VCC_INT Capacitor Selection 60 The bypass capacitor for the VCC_INT pin should be larger than 5µF for stability and has no ESR requirement. It is recommended that the ESR be lower than 50mΩ to reduce noise within the LT3743. For driving MOSFETs with gate charges larger than 10nC, use 0.5µF/nC of total gate charge. VSENSE+ – VSENSE– (mV) 50 LED Current Dimming The LT3743 provides the capability of traditional zero to full current PWM dimming as well as PWM dimming between two regulated LED current states. When the PWM signal is low, no switching occurs and the output capacitors are disconnected from ground. When PWM is high and CTRL_SEL is low, the inductor current is regulated to the low current state. In this state, the PWMGL signal is high, connecting the output capacitor for the low regulated current state. When PWM and CTRL_SEL are both high, the inductor current is regulated to the high current state. In this state, the PWMGH signal is high, connecting the output capacitor for the high regulated current state. The transition time between each of the regulated inductor currents is determined by the inductor size, VIN and VO. Due to the use of the switched output capacitors, the LED current will begin to flow within 130ns of the transition on the CTRL_SEL pin. Figure 8 shows the LED and inductor current waveforms with the various states of the control signals. To adjust the regulated LED current for the two control states, an analog voltage is applied to the CTRL_L and CTRL_H pins. Figure 6 shows the regulated voltage across the sense resistor for control voltages up to 2V. Figure 7 shows the CTRL_L voltage created by a voltage divider from VREF to ground. When sizing the resistor divider, please be aware that the VREF pin is current limited to 500µA. Above 1.5V, the control voltage has no effect on the regulated LED current. For the widest dimming range, use the highest switching frequency possible and lowest PWM frequency. For configuration with the maximum PWM range, please contact factory for optimized component selection. 16 40 30 20 10 0 0 0.5 1.0 VCTRL (V) 1.5 2.0 3743 F06 Figure 6. LED Current vs CTRL Voltage VREF LT3743 R2 CTRL_L R1 3743 F07 Figure 7. Analog Control of LED Current tPWM tON(PWM) PWM CTRL_SEL INDUCTOR CURRENT PWMGH PWMGL ICTRL_H LED CURRENT ICTRL_L 3743 F08 Figure 8. LED Current vs CTRL Voltage 3743fe For more information www.linear.com/LT3743 LT3743 APPLICATIONS INFORMATION MOSFET Selection for the Switched Output Capacitors The MOSFETs used for the switched-output capacitor need to also handle the maximum regulated current while the capacitor is charged. The output drivers on the PWMGH and PWMGL pins have a pull-up impedance of 3.2Ω and a pull-down impedance of 1.75Ω. This provides adequate gate drive for the PWM MOSFETs without the need for an additional gate driver. If the LED forward resistance and the difference between the two regulated currents is large enough, then two MOSFETs are required to prevent the body diode of the MOSFET from conducting and discharging the capacitor for the high current state. Figure 9 shows the output capacitor for the high current regulation state discharged with the body diode when a single MOSFET is used. Figure 10 shows the application circuit with a drain-to-drain configuration for the high current output capacitor. In this configuration, the body diode of the upper MOSFET blocks conduction and prevents discharge of the high current output capacitor. If the voltage between the low state and the high state is very large (greater than the threshold of the MOSFET) then the capacitor may once again be discharged. To account for this, choose a MOSFET that has a threshold greater than the voltage difference. If the voltage difference exceeds 1.5V, use the circuit shown in Figure 11. The circuit shown will keep the capacitor from discharging to a voltage difference of approximately 2V + VTH. ICTRL_L = 1A ICTRL_H = 20A VF = 3V RD = 200mΩ PWMGH LT3743 PWMGL VCC_INT 3.01k ICTRL_L = 1A ICTRL_H = 20A 2k 3743 F11 3.8V 3.04V VF = 3V RD = 40mΩ Figure 11. Application for Large Differences in Regulated Currents 0V PWMGH OFF LT3743 Board and Interconnect Inductance 5V PWMGL 2V ON 3743 F09 Figure 9. Body Diode of High Current FET Discharges the Output Capacitor The board and interconnect inductance from the output capacitors to the load also determine the rate of change in load (LED) current. The rate of change in load current will be: ICTRL_L = 1A ICTRL_H = 20A 3.8V 3.04V VF = 3V RD = 40mΩ PWMGH LT3743 PWMGL 3743 F10 Figure 10. With a Drain-to-Drain Configuration, the Body Diode of the Top FET Blocks the Current Path That Would Discharge the High Current Output Capacitor dIL VHIGH – VLOW = dt LBOARD where dIL/dt is the rate of change in the load current, VHIGH is the output voltage when the inductor is regulated at the high level, and VLOW is the output voltage when the inductor is regulated at the low state. When measuring the LED current do not use a current probe. The core material used in most probes adds inductance and slows the rise time of the LED current. Instead, when measuring the current, use a sense resistor. For more information www.linear.com/LT3743 3743fe 17 LT3743 APPLICATIONS INFORMATION Voltage Regulation and Overvoltage Protection The LT3743 uses the FB pin to regulate the output to a maximum voltage and to provide a high speed overvoltage lockout to avoid high voltage conditions that may damage expensive high current LEDs. The regulated output voltage is programmed using a resistor divider from the output and ground (Figure 12). When the output voltage exceeds 130% of the regulated voltage level (1.3V at the FB pin), the internal open-LED flag is set, terminating switching and forcing both PWMGL and PWMGH signals high. The regulated output voltage must be greater than 2V and is set by the equation: R2 VOUT =1V 1+ R1 fS ≤ (163°C – TA ) (35°C/W ) • ( VIN – 5V ) • (QGHG +QGLG ) fS ≤ 60mA (QGLG +QGHG ) Since the regulated output current flowing into the LED may be very large, the switching frequency needs to be carefully considered. Higher switching frequencies will reduce the large size of high saturation current inductors, while reducing efficiency and increasing power loss within the LT3743. VOUT LT3743 The internal power consumption of the LT3743 is determined by the switching frequency, VIN, and the gate charge, QG of the external switching MOSFETs selected. The 4mm × 5mm QFN package has a θJA of 35°C/W. The following equation calculates the maximum switching frequency to avoid current limit and thermal shutdown at a given ambient operating temperature, TA: R2 FB Table 4. Switching Frequency R1 SWITCHING FREQUENCY (MHz) RT (kΩ) 3743 F12 Figure 12. Output Voltage Regulation and Overvoltage Protection Feedback Connections 1 40.2 0.750 53.6 0.5 82.5 0.3 143 0.2 221 Soft-Start Programming Switching Frequency The LT3743 has an operational switching frequency range between 200kHz and 1MHz. This frequency is programmed with an external resistor from the RT pin to ground. Do not leave this pin open under any condition. The RT pin is also current limited to 60µA. See Table 4 and Figure 13 for resistor values and the corresponding switching frequencies. 18 1.2 1.0 FREQUENCY (MHz) Unlike conventional voltage regulators, the LT3743 utilizes the soft-start function to control the regulated inductor current. The charging current is 5.5µA and reduces the regulated current for both the high and low regulated current states. The SS pin is latched in a discharge state until the first PWM pulse and is reset by UVLO and thermal shutdown. 0.8 0.6 0.4 0.2 0 0 50 100 150 200 250 300 350 400 450 500 RT (kΩ) 3743 F13 Figure 13. Frequency vs RT Resistance 3743fe For more information www.linear.com/LT3743 LT3743 APPLICATIONS INFORMATION Thermal Shutdown The internal thermal shutdown within the LT3743 engages at 163°C and terminates switching, resets soft-start and shuts down the PWMGL and PWMGH drivers. When the part has cooled to 155°C, the internal reset is cleared and softstart is allowed to charge once the PWM signal is asserted. The EN/UVLO pin as an absolute maximum voltage of 6V. To accommodate the largest range of applications, there is an internal Zener diode that clamps this pin. For applications where the supply range is greater than 4:1, size R2 greater than 375k. VIN Switching Frequency Synchronization The nominal switching frequency of the LT3743 is determined by the resistor from the RT pin to ground and may be set from 200kHz to 1MHz. The internal oscillator may also be synchronized to an external clock through the SYNC pin. The external clock applied to the SYNC pin must have a logic low below 0.3V and a logic high higher than 1.25V. The input frequency must be 20% higher than the frequency determined by the resistor at the RT pin. Input signals outside of these specified parameters will cause erratic switching behavior and subharmonic oscillations. The synchronization range is 240kHz to 1.2MHz. Synchronization is tested at 500kHz with a 200k RT resistor. Operation under other conditions is guaranteed by design. When synchronizing to an external clock, please be aware that there will be a fixed delay from the input clock edge to the edge of switch. The SYNC pin must be grounded if the synchronization to an external clock is not required. When SYNC is grounded, the switching frequency is determined by the resistor at the RT pin. Shutdown and UVLO The LT3743 has an internal UVLO that terminates switching, resets all synchronous logic, and discharges the soft-start capacitor for input voltages below 4.2V. The LT3743 also has a precision shutdown at 1.55V on the EN/UVLO pin. Partial shutdown occurs at 1.55V and full shutdown is guaranteed below 0.5V with <1µA IQ in the full shutdown state. Below 1.55V, an internal current source provides 5.5µA of pull-down current to allow for programmable UVLO hysteresis. The following equations determine the voltage divider resistors for programming the UVLO voltage and hysteresis as configured in Figure 14. R2 = VHYST VUVLO – 5.5µA 66µA 1.55V •R2 R1= VUVLO – 1.55V VIN LT3743 R2 EN/UVLO R1 3743 F14 Figure 14. UVLO Configuration LED Current Derating Using the CTRL_T Pin The LT3743 is designed specifically for driving high current LEDs. Most high current LEDs require derating the maximum current based on operating temperature to prevent damage to the LED. In addition, many applications have thermal limitations that will require the regulated current to be reduced based on LED and/or board temperature. To achieve this, the LT3743 uses the CTRL_T pin to reduce the effective regulated current in the LED for both the high and low control currents. While CTRL_H and CTRL_L program the regulated current in the LED, CTRL_T can be configured to reduce this regulated current based on the analog voltage at the CTRL_T pin. The LED/board temperature derating is programmed using a resistor divider with a temperature dependant resistance (Figure 15). When the board/LED temperature rises, the CTRL_T voltage will decrease. To reduce the regulated current, the CTRL_T voltage must be lower than voltage at the CTRL_L and CTRL_H pins. RV RV VREF R2 LT3743 RNTC RNTC RX RNTC RNTC RX CTRL_T R1 (OPTION A TO D) 3743 F15 A B C D Figure 15. LED Current Derating vs Temperature Using NTC Resistor 3743fe For more information www.linear.com/LT3743 19 LT3743 APPLICATIONS INFORMATION Average Current Mode Control Compensation The use of average current mode control allows for precise regulation of the inductor and LED currents. Figure 16 shows the average current mode control loop used in the LT3743, where the regulation current is programmed by a current source and a 3k resistor. To design the compensation network, the maximum compensation resistor needs to be calculated. In current mode controllers, the ratio of the sensed inductor current ramp to the slope compensation ramp determines the stability of the current regulation loop above 50% duty cycle. In the same way, average current mode controllers require the slope of the error voltage to not exceed the PWM ramp slope during the switch off-time. Since the closed-loop gain at the switching frequency produces the error signal slope, the output impedance of the error amplifier will be the compensation resistor, RC. MODULATOR VCTRL • 11µA/V 3k L RS LOAD + gm ERROR AMP – RC 3743 F16 CC Figure 16. LT3743 Average Current Mode Control Scheme Use the following equations as a good starting point for compensation component sizing: RC = fS •L •1000V 0.002 [Ω], CC = [F] VO •RS fS where fS is the switching frequency, L is the inductance value, VIN is the input voltage and RS is the sense resistor. For most LED applications, a 4.7nF compensation capacitor is adequate and provides excellent phase margin with optimized bandwidth. Please refer to Table 6 for recommended compensation values. For applications where the load is not an LED, please call the factory for additional compensation assistance. Board Layout Considerations Average current mode control is relatively immune to the switching noise associated with other types of control schemes. Placing the sense resistor as close as possible to the SENSE+ and SENSE– pins avoids noise issues and ensures the fastest LED current transition time. For currents exceeding 5A, use 10Ω resistors in-series with SENSE+ and SENSE–, with a 33nF capacitor placed as close as possible to the SENSE+ and SENSE– pins. Utilizing a good ground plane underneath the switching components will minimize interplane noise coupling. To dissipate the heat from the switching components, increase the area of the switching node as much as possible without negatively affecting the radiated noise. The interconnect inductance and resistance between the output capacitors and the LED load directly impacts the rise time of the load current. To reduce the inductance and resistance, make the traces as wide as physically possible and minimize the trace length. Table 6. Recommended Compensation Values VIN (V) VO (V) IL (A) fSW (MHz) L (µH) RS (mΩ) RC (kΩ) CC (nF) 12 4 5 0.5 1.5 5 47.5 4.7 12 4 10 0.5 1.5 5 47.5 4.7 12 5 20 0.25 1.8 2.5 38.3 8.2 24 4 2 0.5 1.0 2.5 52.3 4.7 24 4 20 0.5 1.0 2.5 52.3 4.7 20 3743fe For more information www.linear.com/LT3743 LT3743 TYPICAL APPLICATIONS 12V, 20A LED Driver EN/UVLO PWM CTRL_SEL 82.5k VIN EN/UVLO PWM CTRL_SEL RT SYNC 1µF HG 100nF CBOOT VREF LT3743 2.2nF LG 22µF VIN 12V M1 L1 1.0µH SW VCC_INT CTRL_H 50k 10Ω M2 OUTPUT 20A MAXIMUM 2.5mΩ 10Ω C1 330µF ×3 33nF GND RNTC 10k 10nF M3 SENSE+ SENSE– PWMGH CTRL_T SS 1µF D1 C3 330µF ×3 M4 PWMGL 60.4k FB VCL VCH 34k 34k 4.7nF 4.7nF C2 330µF ×3 D1: LUMINUS PT120 L1: IHLP4040DZER1R0M01 M1: RJK0365DPA M2: RJK0346DPA M3, M4: Si7236DP C1, C2, C3: PTPR330M9L (THREE IN PARALLEL) 10k 3743 TA02 Efficiency (Stepping from 2A to 20A) 94 VIN = 12V GREEN LED 92 90 EFFIENCY (%) RHOT 499Ω 50k CTRL_L 220µF 88 86 84 82 80 0 20 40 60 100 80 CTRL_SEL DIMMING DUTY CYCLE (%) 3743 TA02b 3743fe For more information www.linear.com/LT3743 21 LT3743 TYPICAL APPLICATIONS 6V to 36V, 2A LED Driver With Shunted Output EN/UVLO INTVCC CTRL_SEL 82.5k VIN EN/UVLO PWM CTRL_SEL RT SYNC 1µF HG 100nF CBOOT VREF LT3743 2.2nF RHOT 499Ω CTRL_L CONTROL INPUT SW VCC_INT LG CTRL_H 8.2µF VIN 6V TO 36V Shunted Output with CTRL_H Equal to CTRL_L M1 L1 10µH OUTPUT 2A MAXIMUM 25mΩ 2.2µF 22µF CTRL_SEL 5V/DIV IL D1 2A/DIV M2 ILED 1A/DIV GND CTRL_T RNTC 10k SENSE+ SENSE– SW 2V/DIV PWMGH 10nF SS PWMGL M3 20µs/DIV 3743 TA03b 40.2k FB VCL VCH 34k 34k 4.7nF 10k D1: LUMINUS CBT-40 L1: MSS1048-103MLB M1, M2: Si7848BDP M3: Si2312BDS 4.7nF 3743 TA03 6V to 36V, 2A LED Driver With Current Limited Shunted Output EN/UVLO INTVCC CTRL_SEL 82.5k VIN EN/UVLO PWM CTRL_SEL RT SYNC 1µF HG 100nF CBOOT 2.2nF VREF LT3743 CONTROL INPUT RHOT 499Ω CTRL_H CTRL_L CTRL_T RNTC 10k 10nF SS 8.2µF VIN 6V TO 36V Shunted Output with CTRL_L at GND M1 L1 10µH SW 25mΩ OUTPUT 2A MAXIMUM CTRL_SEL 5V/DIV 2.2µF VCC_INT LG 22µF D1 M2 IL 2A/DIV GND ILED 1A/DIV SENSE+ SENSE– PWMGH SW 10V/DIV 20µs/DIV M3 PWMGL 3743 TA04b 40.2k FB VCL 34k 4.7nF 22 VCH 34k 4.7nF D1: LUMINUS CBT-40 L1: IHLP4040DZE10R0M01 M1, M2: Si7848BDP M3: Si2312BDS 10k 3743 TA04 3743fe For more information www.linear.com/LT3743 LT3743 TYPICAL APPLICATIONS 6V to 30V, 20A LED Driver with Switched Cathode EN/UVLO PWM VCC_INT 82.5k 1µF HG 150nF CBOOT VREF LT3743 2.2nF CTRL_L RHOT CONTROL 499Ω INPUT 10nF 82µF M1 L1 1.1µH 2.5mΩ SW VCC_INT LG CTRL_H 22µF 10Ω M2 CTRL_T SENSE+ SENSE– PWMGL SS PWMGH OUTPUT 20A MAXIMUM 10Ω C1 330µF ×3 D1 33nF GND RNTC 10k VIN 6V TO 30V VIN EN/UVLO PWM CTRL_SEL RT SYNC M3 60.4k FB VCL VCH 34k 4.7nF 10k D1: LUMINUS PT121 L1: MVR1261C-112ML 3743 TA05 M1: RJK0365DPA M2: RJK0328DPB M3: SiR496DP C1: PTPR330M9L (THREE IN PARALLEL) Switched Cathode PWM Dimming (100:1) 0A to 20A 0A to 20A Efficiency 100 90 PWM 5V/DIV EFFICIENCY (%) 80 ILED 10A/DIV SW 10V/DIV 10µs/DIV 3743 TA05b 70 60 50 40 30 20 VIN = 12V GREEN LED 10 0 0 20 60 80 40 PWM DIMMING DUTY CYCLE (%) 100 3743 TA05c 3743fe For more information www.linear.com/LT3743 23 LT3743 TYPICAL APPLICATIONS 24V, 20A 3-LED Driver 82.5k VIN EN/UVLO PWM CTRL_SEL RT SYNC 1µF HG 100nF CBOOT VREF LT3743 2.2nF 20k CTRL_H 60.4k RHOT 499Ω SW VCC_INT LG GND CTRL_L 82µF RNTC 10k 10nF 95 L1 1.2µH 470µF 20µF M2 10Ω 10Ω 33nF RED LEDs 85 80 75 70 65 60 M3 PWMGL 316k FB VIN = 24V 3 RED LEDs 0 20 60 40 DUTY CYCLE (%) 80 100 3743 TA07b VCH 20k 24.3k 4.7nF 24 90 OUTPUT 20A MAXIMUM 2.5mΩ SENSE+ SENSE– SS VCL 100 M1 PWMGH CTRL_T Efficiency VIN 24V EFFICIENCY (%) EN/UVLO PWM VCC_INT 3743 TA07 L1: 1HLP5050FDER1R2M01 M1: Si7848BDP M2, M3: RJK0330DPB 3743fe For more information www.linear.com/LT3743 LT3743 PACKAGE DESCRIPTION Please refer to http://www.linear.com/product/LT3743#packaging for the most recent package drawings. UFD Package 28-Lead Plastic QFN (4mm × 5mm) (Reference LTC DWG # 05-08-1712 Rev B) 0.70 ±0.05 4.50 ± 0.05 3.10 ± 0.05 2.50 REF 2.65 ± 0.05 3.65 ± 0.05 PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC 3.50 REF 4.10 ± 0.05 5.50 ± 0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 4.00 ± 0.10 (2 SIDES) 0.75 ± 0.05 R = 0.05 TYP PIN 1 NOTCH R = 0.20 OR 0.35 × 45° CHAMFER 2.50 REF R = 0.115 TYP 27 28 0.40 ± 0.10 PIN 1 TOP MARK (NOTE 6) 1 2 5.00 ± 0.10 (2 SIDES) 3.50 REF 3.65 ± 0.10 2.65 ± 0.10 (UFD28) QFN 0506 REV B 0.200 REF 0.00 – 0.05 0.25 ± 0.05 0.50 BSC BOTTOM VIEW—EXPOSED PAD NOTE: 1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WXXX-X). 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 3743fe For more information www.linear.com/LT3743 25 LT3743 PACKAGE DESCRIPTION Please refer to http://www.linear.com/product/LT3743#packaging for the most recent package drawings. FE Package 28-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663 Rev I) Exposed Pad Variation EB 9.60 – 9.80* (.378 – .386) 4.75 (.187) 4.75 (.187) 28 27 26 2524 23 22 21 20 1918 17 16 15 6.60 ±0.10 4.50 ±0.10 2.74 (.108) SEE NOTE 4 0.45 ±0.05 EXPOSED PAD HEAT SINK ON BOTTOM OF PACKAGE 6.40 2.74 (.252) (.108) BSC 1.05 ±0.10 0.65 BSC RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.50* (.169 – .177) 0.09 – 0.20 (.0035 – .0079) 0.25 REF 0.50 – 0.75 (.020 – .030) NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS 2. DIMENSIONS ARE IN MILLIMETERS (INCHES) 3. DRAWING NOT TO SCALE 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1.20 (.047) MAX 0° – 8° 0.65 (.0256) BSC 0.195 – 0.30 (.0077 – .0118) TYP 0.05 – 0.15 (.002 – .006) FE28 (EB) TSSOP REV I 0211 4. RECOMMENDED MINIMUM PCB METAL SIZE FOR EXPOSED PAD ATTACHMENT *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.150mm (.006") PER SIDE 3743fe For more information www.linear.com/LT3743 LT3743 REVISION HISTORY REV DATE DESCRIPTION PAGE NUMBER A 2/10 Revised Features and Typical Application 1 Updated Electrical Characteristics values 3, 4 Revised values on curves G32 and G33 in the Typical Performance Characteristics section 8 Revised the Block Diagram 11 Changed value in equation and made minor text edit in the Inductor Selection section 13 Revised Table 4 values 18 Added text to Average Current Mode Control Compensation and Board Layout Considerations sections in the Applications Information section 20 Revised all Typical Applications drawings B 8/10 3, 4 Revised Pin Functions 9, 10 Revised Block Diagram 11 Changed soft-start current in Operation section 12 Revised units for M1 and M2 equations 14 Removed 0.1MHz switching frequency from Table 4 18 Added text to Switching Frequency Synchronization, Shutdown and UVLO sections in the Applications Information section 19 Corrected M2 and M3 part numbers on Typical Applications drawings C 9/11 D 11/12 E 10/15 21 to 25, 28 Updated Electrical Characteristics values and conditions Revised Feedback Regulation Voltage listing in Electrical Characteristics section Clarified VCL and VCH pins 24, 28 3 1, 2, 9, 11, 21-24 Clarified Regulated Current vs VFB graph 6 Clarified Minimum Off-Time graph 7 Added patent number 1 Revised UVLO Hysteresis Equation 19 Corrected typo in Block Diagram 11 3743fe Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. For more information www.linear.com/LT3743 27 LT3743 TYPICAL APPLICATION 12V, 40A Pulsed LED Driver EN/UVLO PWM CTRL_SEL EN/UVLO PWM CTRL_SEL 150k RT SYNC VIN 4.7µF ×8 HG 1µF RHOT 499Ω 50k 1µF CTRL_L 220nF L1 1µH CTRL_H 50k 1µF SW VCC_INT 22µF LG CTRL_T RNTC 10k M2 ×2 GND SENSE+ 220µF M1 ×2 CBOOT VREF LT3743 220µF OUTPUT 40A MAXIMUM 1.25mΩ 10Ω 10Ω 33nF SENSE– C1 330µF ×3 SS 1µF CTRL_SEL 5V/DIV D1 ILED 20A/DIV C3 330µF ×3 SW 10V/DIV M3 M4 PWMGH 1µF VIN = 12V 4A to 40A LED Current Step VIN 12V 20µs/DIV 3743 TA08b PWMGL 140k FB VCL 51k 5.6nF 1nF VCH 51k 5.6nF C2 330µF ×3 20k 3743 TA08 L1: 1HLP5050FDER1R0M01 M1, M2: RJK0330DPB M3, M4: Si7234DP C1, C2, C3: PTPR33OM9L RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT3755/LT3755-1 High Side 40V, 1MHz LED Controller with True Color 3000:1 PWM Dimming VIN: 4.5V to 40V, VOUT(MAX) = 60V, Dimming = 3000:1 True Color PWM™, ISD < 1µA, 3mm × 3mm QFN16, MSOP16E LT3756/LT3756-1 High Side 100V, 1MHz LED Controller with True Color 3000:1 PWM Dimming VIN: 6V to 100V, VOUT(MAX) = 100V, Dimming = 3000:1 True Color PWM, ISD < 1µA, 3mm × 3mm QFN16, MSOP16E LTC3783 High Side 36V, 1MHz LED Controller with True Color 3000:1 PWM Dimming VIN: 3V to 36V, VOUT(MAX) = 40V, Dimming = 3000:1 True Color PWM, ISD < 20µA, 4mm × 5mm DFN16, TSSOP16E LT3517 1.3A, 2.5MHz High Current LED Driver with 3000:1 Dimming VIN: 3V to 30V, Dimming = 3000:1 True Color PWM, ISD < 1µA, 4mm × 4mm QFN16 LT3518 2.3A, 2.5MHz High Current LED Driver with 3000:1 Dimming VIN: 3V to 30V, Dimming = 3000:1 True Color PWM, ISD < 1µA, 4mm × 4mm QFN16 LT3496 Triple Output 750mA, 2.1MHz High Current LED Driver with 3000:1 Dimming VIN: 3V to 30V, VOUT(MAX) = 40V, Dimming = 3000:1 True Color PWM, ISD < 1µA, 4mm × 5mm QFN28 LT3474/LT3474-1 36V, 1A (ILED), 2MHz Step-Down LED Driver VIN: 4V to 36V, VOUT(MAX) = 13.5V, Dimming = 400:1 True Color PWM, ISD < 1µA, TSSOP16E LT3475/LT3475-1 Dual 1.5A (ILED), 36V Step-Down LED Driver VIN: 4V to 36V, VOUT(MAX) = 13.5V, Dimming = 3000:1 True Color PWM, ISD < 1µA, TSSOP20E LT3476 Quad Output 1.5A, 2MHz High Current LED Driver with 1000:1 Dimming VIN: 2.8V to 16V, VOUT(MAX) = 36V, Dimming = 1000:1 True Color PWM, ISD < 10µA, 5mm × 7mm QFN10 LT3478/LT3478-1 4.5A, 2MHz High Current LED Driver with 3000:1 Dimming VIN: 2.8V to 36V, VOUT(MAX) = 40V, Dimming = 1000:1 True Color PWM, ISD < 10µA, 5mm × 7mm QFN10 28 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 For more information www.linear.com/LT3743 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LT3743 3743fe LT 1015 REV E • PRINTED IN USA LINEAR TECHNOLOGY CORPORATION 2009