LT3755

LT3755IMSE-2-PBF LINEAR_DIMENSIONS | Alldatasheet

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40VIN, 75VOUT LED Controllers The L T®3755, L T3755-1 and L T3755-2 are DC/DC control- lers designed to operate as a constant-current source for driving high current LEDs. They drive a low side external N-channel power MOSFET from an internal regulated 7.15V supply. The fixed frequency, current mode architecture results in stable operation over a wide range of supply and output voltages. A ground referenced voltage FB pin serves as the input for several LED protection features, and also makes it possible for the converter to operate as a constant-voltage source. A frequency adjust pin allows the user to program the frequency from 100kHz to 1MHz to optimize efficiency, performance or external component size. The L T3755/L T3755-1/L T3755-2 sense output current at the high side of the LED string. High side current sensing is the most flexible scheme for driving LEDs, allowing boost, buck mode or buck-boost mode configuration. The PWM input provides LED dimming ratios of up to 3000:1, and the CTRL input provides additional analog dimming capability. L, L T , L TC, L TM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. T rue 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. 50W White Automotive LED Headlamp Driver n 3000:1 T rue Color PWM™ Dimming n Wide Input Voltage Range: 4.5V to 40V n Output Voltage Up to 75V n Constant-Current and Constant-Voltage Regulation n 100mV High Side Current Sense n Drives LEDs in Boost, Buck Mode, Buck-Boost Mode, SEPIC or Flyback Topology n Adjustable Frequency: 100kHz to 1MHz n Open LED Protection n Programmable Undervoltage Lockout with Hysteresis n Improved Open LED Status Pin (L T3755-2) n Frequency Synchronization (L T3755-1) n PWM Disconnect Switch Driver n CTRL Pin Provides Analog Dimming n Low Shutdown Current: <1µA n Programmable Soft-Start n Thermally Enhanced 16-Lead QFN (3mm × 3mm) and MSOP Packages n High Power LED n Battery Chargers n Accurate Current Limited Voltage Regulators VIN L T3755-2 22µH GNDVC INTVCC SHDN/UVLO FB VREF ISP 332k 100k INTVCC 1M4.7µF 0.001µF 0.01µF VIN 8V TO 40V 185k 10k 28.7k 375kHz 4.7µF 40.2k CTRL 0.015/uni03A9 0.1/uni03A9 23.7k 4.7µF 50W LED STRING

37551 TA01a

VIN (V) 0 10 EFFICIENCY (%) 100 20 30 40

37551 TA01b

(Note 1) orDer inForMaTion 16 15 14 13 5 6 7 8 TOP VIEW GND UD PACKAGE 16-LEAD (3mm s 3mm) PLASTIC QFN 1 V REF PWM SYNC OR OPENLED SS FB PWMOUT GATE SENSE CTRL VC ISP ISN RT SHDN/UVLO INTV CC VIN TJMAX = 125°C, θJA = 68°C/W , θJC = 4.2°C/W EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB PWMOUT FB ISN ISP VC CTRL V REF PWM GATE SENSE V IN INTVCC SHDN/UVLO RT SS SYNC OR OPENLED TOP VIEW MSE PACKAGE 16-LEAD PLASTIC MSOP GND TJMAX = 125°C (E, I GRADES), TJMAX = 150°C (H GRADE), θJA = 43°C/W , θJC = 4°C/W EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB pin conFiguraTion Operating Junction Temperature Range (Notes 2, 5) LT3755E/L LT3755H Lead Temperature (Soldering, 10 sec) LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE L T3755EUD#PBF L T3755EUD#TRPBF LDGC 16-Lead (3mm × 3mm) Plastic QFN –40°C to 125°C L T3755IUD#PBF L T3755IUD#TRPBF LDGC 16-Lead (3mm × 3mm) Plastic QFN –40°C to 125°C L T3755EUD-1#PBF L T3755EUD-1#TRPBF LDMS 16-Lead (3mm × 3mm) Plastic QFN –40°C to 125°C L T3755IUD-1#PBF L T3755IUD-1#TRPBF LDMS 16-Lead (3mm × 3mm) Plastic QFN –40°C to 125°C L T3755EUD-2#PBF L T3755EUD-2#TRPBF LFJZ 16-Lead (3mm × 3mm) Plastic QFN –40°C to 125°C L T3755IUD-2#PBF L T3755IUD-2#TRPBF LFJZ 16-Lead (3mm × 3mm) Plastic QFN –40°C to 125°C L T3755EMSE#PBF L T3755EMSE#TRPBF 3755 16-Lead Plastic MSOP –40°C to 125°C L T3755IMSE#PBF L T3755IMSE#TRPBF 3755 16-Lead Plastic MSOP –40°C to 125°C L T3755EMSE-1#PBF L T3755EMSE-1#TRPBF 37551 16-Lead Plastic MSOP –40°C to 125°C L T3755IMSE-1#PBF L T3755IMSE-1#TRPBF 37551 16-Lead Plastic MSOP –40°C to 125°C L T3755EMSE-2#PBF L T3755EMSE-2#TRPBF 37552 16-Lead Plastic MSOP –40°C to 125°C L T3755IMSE-2#PBF L T3755IMSE-2#TRPBF 37552 16-Lead Plastic MSOP –40°C to 125°C L T3755HMSE-2#PBF L T3755HMSE-2#TRPBF 37552 16-Lead Plastic MSOP –40°C to 150°C Consult L TC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container . Consult L TC Marketing for information on non-standard lead based finish parts. 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/

elecTrical characTerisTics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 24V , SHDN/UVLO = 24V , CTRL = 2V , PWM = 5V , unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX UNITS VIN Minimum Operating Voltage VIN Tied to INTVCC l 4.5 V VIN Shutdown IQ SHDN/UVLO = 0V , PWM = 0V SHDN/UVLO = 1.15V , PWM = 0V 0.1 1 µA µA V IN Operating IQ (Not Switching) PWM = 0V 1.4 1.7 mA VREF Voltage 100µA ≤ IVREF ≤ 0µA l 1.965 2.00 2.045 V VREF Line Regulation 4.5V ≤ VIN ≤ 40V 0.006 %/V SENSE Current Limit Threshold l 98 108 118 mV SENSE Input Bias Current Current Out of Pin 40 µA SS Pull-Up Current Current Out of Pin 8 10 13 µA Error Amplifier ISP/ISN Full-Scale Current Sense Threshold FB = 0V , ISP = 48V l 96 100 103 mV ISP/ISN Full-Scale Current Sense Threshold at CTRL = 0V CTRL = 0V , FB = 0V , ISP = 48V –12 –9.5 –7 mV CTRL Pin Range for Current Sense Threshold Adjustment l 0 1.1 V CTRL Input Bias Current Current Out of Pin 50 100 nA LED Current Sense Amplifier Input Common Mode Range (VISN) l 2.9 75 V ISP/ISN Short-Circuit Threshold ISN = 0V 115 150 200 mV ISP/ISN Short-Circuit Fault Sensing Common Mode Range (VISN) l 0 3 V ISP/ISN Input Bias Current (Combined) PWM = 5V (Active), ISP = ISN = 48V PWM = 0V (Standby), ISP = ISN = 48V 0.1 µA µA LED Current Sense Amplifier g m V(ISP – ISN) = 100mV 120 µS VC Output Impedance 1V < VC < 2V 15000 kΩ VC Standby Input Bias Current PWM = 0V –20 20 nA FB Regulation Voltage (VFB) ISP = ISN l 1.232 1.220 1.250 1.250 1.265 1.270 V V FB Amplifier g m FB = VFB, ISP = ISN 480 µS FB Pin Input Bias Current Current Out of Pin 40 100 nA FB Open LED Threshold OPENLED Falling (L T3755 and L T3755-2) V FB – 65mV VFB – 50mV VFB – 40mV V FB Overvoltage Threshold PWMOUT Falling VFB + 50mV VFB + 60mV VFB + 75mV V VC Current Mode Gain – (∆VVC/∆VSENSE) 4 V/V Oscillator Switching Frequency RT = 100k RT = 10k l 90 925 100 1000 125 1050 kHz kHz Minimum Off-Time 170 ns

elecTrical characTerisTics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 24V , SHDN/UVLO = 24V , CTRL = 2V , PWM = 5V , unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX UNITS Linear Regulator INTVCC Regulation Voltage 7 7.15 7.3 V Dropout (VIN – INTVCC) IINTVCC = –10mA, VIN = 7V 350 mV INTVCC Undervoltage Lockout 3.9 4.1 4.3 V INTVCC Current Limit 29 34 40 mA INTVCC Current in Shutdown SHDN/UVLO = 0V , INTVCC = 7V 8 12 µA Logic Inputs/Outputs PWM Input High Voltage l 1.5 V PWM Input Low Voltage l 0.4 V PWM Pin Resistance to GND 45 60 kΩ PWMOUT Output Low (VOL) 0 50 mV PWMOUT Output High (VOH) INTVCC – 0.05 V SHDN/UVLO Threshold Voltage Falling E-, I-Grades H-Grade l l 1.185 1.175 1.220 1.245 1.245 V V SHDN/UVLO Rising Hysteresis 20 mV SHDN/UVLO Input Low Voltage I VIN Drops Below 1µA 0.4 V SHDN/UVLO Pin Bias Current Low SHDN/UVLO = 1.15V 1.7 2.05 2.5 µA SHDN/UVLO Pin Bias Current High SHDN/UVLO = 1.30V 10 100 nA OPENLED Output Low (VOL) IOPENLED = 0.5mA (L T3755 and L T3755-2) 200 mV SYNC Pin Resistance to GND L T3755-1 Only 30 kΩ SYNC Input High L T3755-1 Only 1.5 V SYNC Input Low L T3755-1 Only 0.4 V Gate Driver tr GATE Driver Output Rise Time CL = 3300pF 35 ns tf GATE Driver Output Fall Time CL = 3300pF 35 ns GATE Output Low (VOL) 0.05 V GATE Output High (VOH) INTVCC – 0.05 V 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 L T3755E, L T3755E-1 and L T3755E-2 are guaranteed to meet performance specifications from 0°C to 125°C junction temperature. Specifications over the –40°C to 125°C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The L T3755I, L T3755I-1 and L T3755I-2 are guaranteed to meet performance specifications over the –40°C to 125°C operating junction temperature range. The L T3755H-2 is guaranteed to meet performance specifications over the full –40°C to 150°C operating junction temperature range. High junction temperatures degrade operating lifetimes. Operating lifetime is derated at junction temperatures greater than 125°C. Note 3: For V IN below 6V , the SHDN/UVLO pin must not exceed VIN for proper operation. Note 4: GATE and PWMOUT pins are driven either to GND or INTVCC by internal switches. Do not connect these pins externally to a power supply. Note 5: The L T3755 includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed the maximum operating junction temperature when overtemperature protection is active. Continuous operating above the specified maximum operating junction temperature may impair device reliability.

Typical perForMance characTerisTics FB Regulation Voltage vs Temperature VREF Voltage vs Temperature VREF Voltage vs VIN Switching Frequency vs RT Switching Frequency vs Temperature SHDN/UVLO Hysteresis Current vs Temperature V (ISP – ISN) Threshold vs VCTRL V(ISP – ISN) Threshold vs VISP V(ISP – ISN) Threshold vs Temperature RT (k) SWITCHING FREQUENCY (kHz)

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VCTRL (V) –20 V(ISP – ISN) THRESHOLD (mV) 100 0.5 1 1.5 120

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ISP VOL TAGE (V) V(ISP – ISN) THRESHOLD (mV) 101 103 100 102

37551 G02

VCTRL = 2V 20 40 8060 101 103 100 102

37551 G03

V(ISP – ISN) THRESHOLD (mV) TEMPERATURE (°C) –50 0 50 75–25 25 100 150 125 VCTRL = 2V

37551 G04

VFB (V) TEMPERATURE (°C) –50 0 50 75–25 25 100 150 125 1.20 1.22 1.24 1.26 1.28 1.21 1.23 1.25 1.27

37551 G05

VREF (V) TEMPERATURE (°C) –50 0 50 75–25 25 100 150 125 1.96 1.98 2.00 2.02 2.04 1.97 1.99 2.01 2.03 VIN (V)

1.96 VREF (V)

1.98 2.00 2.02 10 20 4030 2.04 1.97 1.99 2.01 2.03

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37551 G08

SWITCHING FREQUENCY (kHz) TEMPERATURE (°C) –50 0 50 75–25 25 100 150 125 300 350 400 450 500 RT = 26.7k

37551 G09

TEMPERATURE (°C) –50 0 50 75–25 25 100 150 125 1.6 ISHDN/UVLO (µA) 2.0 2.4 1.8 2.2 TA = 25°C, unless otherwise noted.

Typical perForMance characTerisTics INTVCC Voltage vs VIN INTVCC Current Limit vs Temperature INTVCC Voltage vs Temperature Quiescent Current vs VIN SENSE Current Limit Threshold vs Temperature SHDN/UVLO Threshold vs Temperature VIN (V) VIN CURRENT (mA) 1.0 2.0 0.5 1.5

37551 G10

PWM = 0V

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SENSE THRESHOLD (mV) TEMPERATURE (°C) 100 110 105 –50 0 50 75–25 25 100 150 125

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SHDN/UVLO VOLTAGE (V) TEMPERATURE (°C) 1.18 1.22 1.28 1.20 1.24 1.26 SHDN/UVLO RISING SHDN/UVLO FALLING –50 0 50 75–25 25 100 150 125 0 10 20 4030 VIN (V) VINTVCC (V)

37551 G13 37551 G14

INTVCC CURRENT LIMIT (mA) TEMPERATURE (°C) –50 0 50 75–25 25 100 150 125

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INTVCC (V) TEMPERATURE (°C) –50 0 50 75–25 25 100 150 125 7.0 7.2 7.4 7.1 7.3 TA = 25°C, unless otherwise noted. SENSE Current Limit Threshold vs Duty Cycle V (ISP-ISN) Threshold vs FB Voltage Gate Rise/Fall Time vs Capacitance DUTY CYCLE (%) SENSE THRESHOLD (mV) 105 115 100 110

37551 G16

FB VOL TAGE (V)

37551 G17

V(ISP – ISN) THRESHOLD (mV) VCTRL = 2V CAPACITANCE (nF)

37551 G18

TIME (ns) GATE RISE TIME GATE FALL TIME 10% TO 90%

PWMOUT (Pin 1/Pin 11): Buffered Version of PWM Signal for Driving LED Load Disconnect NMOS or Level Shift. This pin also serves in a protection function for the FB overvoltage condition—will toggle if the FB input is greater than the FB regulation voltage (V FB) plus 60mV (typical). The PWMOUT pin is driven from INTVCC. Use of a FET with gate cut-off voltage higher than 1V is recommended. FB (Pin 2/Pin 12): Voltage Loop Feedback Pin. FB is intended for constant-voltage regulation or for LED protec- tion/open LED detection. The internal transconductance amplifier with output VC will regulate FB to 1.25V (nominal) through the DC/DC converter . If the FB input is regulating the loop, the OPENLED pull-down is asserted. This ac- tion may signal an open LED fault. If FB is driven above the FB threshold (by an external power supply spike, for example), the OPENLED pull-down will be de-asserted and the PWMOUT pin will be driven low to protect the LEDs from an overcurrent event. Do not leave the FB pin open. If not used, connect to GND. I SN (Pin 3/Pin 13): Connection Point for the Negative Terminal of the Current Feedback Resistor . If ISN is greater than 2.9V , the LED current can be programmed by I LED = 100mV/RLED when VCTRL > 1.2V or ILED = (VCTRL –100mV)/ (10 • RLED) when VCTRL ≤ 1V . Input bias current is typi- cally 25µA. Below 3V , ISN is an input to the short-circuit protection feature that forces GATE to 0V if ISP exceeds ISN by more than 150mV (typ). ISP (Pin 4/Pin 14): Connection Point for the Positive Terminal of the Current Feedback Resistor . Input bias current is dependent upon CTRL pin voltage as shown in the TPC. ISP is an input to the short-circuit protection feature when ISN is less than 3V . VC (Pin 5/Pin 15): T ransconductance Error Amplifier Output Pin Used to Stabilize the Voltage Loop with an RC Network. This pin is high impedance when PWM is low, a feature that stores the demand current state variable for the next PWM high transition. Connect a capacitor between this pin and GND; a resistor in series with the capacitor is recommended for fast transient response. (MSOP/QFN) Typical perForMance characTerisTics TA = 25°C, unless otherwise noted. INTVCC Dropout Voltage vs Current, Temperature ISP/ISN Input Bias Current vs CTRL Voltage CTRL (V)

37551 G19

0 0.5 1 1.5 2 INPUT BIAS CURRENT (µA) ISP ISN LDO CURRENT (mA)

37551 G20

–2.5 –1.5 –2.0 –1.0 –0.5LDO DROPOUT (V) TA = –45°C VIN = 7V TA = 125°C TA = 150°C TA = 25°C

CTRL (Pin 6/Pin 16): Current Sense Threshold Adjustment Pin. Regulating threshold V(ISP – ISN) is 1/10th VCTRL plus an offset for 0V < VCTRL < 1V . For VCTRL > 1.2V the current sense threshold is constant at the full-scale value of 100mV . For 1V < VCTRL < 1.2V , the dependence of current sense threshold upon VCTRL transitions from a linear function to a constant value, reaching 98% of full-scale value by V CTRL = 1.1V . Do not leave this pin open. VREF (Pin 7/Pin 1): Voltage Reference Output Pin, Typically 2V . This pin drives a resistor divider for the CTRL pin, either for analog dimming or for temperature limit/compensation of LED load. Can supply up to 100μA. PWM (Pin 8/Pin 2): A signal low turns off switcher , idles oscillator and disconnects VC pin from all internal loads. PWMOUT pin follows PWM pin. PWM has an internal pull-down resistor . If not used, connect to INTV CC. OPENLED (Pin 9/Pin 3, L T3755 and L T3755-2): An open- collector pull-down on OPENLED asserts if the FB input is greater than the FB regulation threshold minus 50mV (typical). To function, the pin requires an external pull-up current less than 1mA. When the PWM input is low and the DC/DC converter is idle, the OPENLED condition is latched to the last valid state when the PWM input was high. When PWM input goes high again, the OPENLED pin will be updated. This pin may be used to report an open LED fault. SYNC (Pin 9/Pin 3, L T3755-1 Only): The SYNC pin is used to synchronize the internal oscillator to an external logic level signal. The R T resistor should be chosen to program an internal switching frequency 20% slower than the SYNC pulse frequency. Gate turn-on occurs a fixed delay after the rising edge of SYNC. For best PWM performance, the PWM rising edge should occur at least 200ns before the SYNC rising edge. Use a 50% duty cycle waveform to drive this pin. This pin replaces OPENLED on L T3755-1 option parts. If not used, tie this pin to GND. SS (Pin 10/Pin 4): Soft-Start Pin. This pin modulates oscillator frequency and compensation pin voltage (VC) clamp. The soft-start interval is set with an external capaci- tor . The pin has a 10µA (typical) pull-up current source to an internal 2.5V rail. The soft-start pin is reset to GND by an undervoltage condition (detected by SHDN/UVLO pin) or thermal limit. RT (Pin 11/Pin 5): Switching Frequency Adjustment Pin. Set the frequency using a resistor to GND (for resistor values, see the Typical Performance curve or Table 1). Do not leave the RT pin open. SHDN/UVLO (Pin 12/Pin 6): Shutdown and Undervoltage Detect Pin. An accurate 1.22V falling threshold with ex- ternally programmable hysteresis detects when power is OK to enable switching. Rising hysteresis is generated by the external resistor divider and an accurate internal 2.1µA pull-down current. Above the threshold (but below 6V), SHDN/UVLO input bias current is sub-µA. Below the falling threshold, a 2.1µA pull-down current is enabled so the user can define the hysteresis with the external resis- tor selection. An undervoltage condition resets soft-start. Tie to 0.4V , or less, to disable the device and reduce V IN quiescent current below 1µA. INTVCC (Pin 13/Pin 7): Regulated Supply for Internal Loads, GATE Driver and PWMOUT Driver . Supplied from VIN and regulates to 7.15V (typical). INTV CC must be bypassed with a 4.7µF capacitor placed close to the pin. Connect INTV CC directly to VIN if VIN is always less than or equal to 8V . VIN (Pin 14/Pin 8): Input Supply Pin. Must be locally bypassed with a 0.22µF (or larger) capacitor placed close to the IC. SENSE (Pin 15/Pin 9): The current sense input for the control loop. Kelvin connect this pin to the positive ter- minal of the switch current sense resistor , RSENSE, in the source of the NFET . The negative terminal of the current sense resistor should be Kelvin connected to the GND plane of the IC. GATE (Pin 16/Pin 10): N-channel FET Gate Driver Output. Switches between INTV CC and GND. Driven to GND during shutdown, fault or idle states. Exposed Pad (Pin 17/Pin 17): Ground. This pin also serves as current sense input for control loop, sensing negative terminal of current sense resistor . Solder the Exposed Pad directly to ground plane. pin FuncTions (MSOP/QFN)

– + ++– ++– FREQ PROG 1.25V SSCLAMP 1.1V CTRL VREF SHDN/UVLO ISP ISN 150mV 50k 170k 140µA 2.1µA CTRL BUFFER gm EAMP PWM COMPARATOR DRIVER ISENSE gm A10 OVFB COMPARATOR 1.25V FB SHORT-CIRCUIT DETECT OPTION FOR L T3755 AND L T3755-2 OPTION FOR L T3755-1 SCILMB SCILMB PWMOUT PWM 1.25V VIN INTVCC VC A2 R Q S RAMP GENERATOR 100KHz TO 1MHz OSCILLATOR A8 7.15V LDO GATE SENSE 37551 BD OPENLED GND 1.2V FB 1.22V 1.3V RT SYNCSS SHDN 10µA 10µA AT FB = 1.25V VC TLIM 165°C FAUL T LOGIC 10µA 10µA AT A1+ = A1–

The L T3755 is a constant-frequency, current mode control- ler with a low side NMOS gate driver . The GATE pin and PWMOUT pin drivers and other chip loads are powered from INTV CC, which is an internally regulated supply. In the discussion that follows it will be helpful to refer to the Block Diagram of the IC. In normal operation with the PWM pin low, the GATE and PWMOUT pins are driven to GND, the VC pin is high impedance to store the previous switching state on the external compensation capacitor , and the ISP and ISN pin bias currents are reduced to leakage levels. When the PWM pin transitions high, the PWMOUT pin transitions high after a short delay. At the same time, the internal oscillator wakes up and gener- ates a pulse to set the PWM latch, turning on the external power MOSFET switch (GATE goes high). A voltage input proportional to the switch current, sensed by an external current sense resistor between the SENSE and GND input pins, is added to a stabilizing slope compensation ramp and the resulting “switch current sense” signal is fed into the positive terminal of the PWM comparator . The current in the external inductor increases steadily during the time the switch is on. When the switch current sense voltage exceeds the output of the error amplifier , labeled “VC”, the latch is reset and the switch is turned off. During the switch-off phase, the inductor current decreases. At the completion of each oscillator cycle, internal signals such as slope compensation return to their starting points and a new cycle begins with the set pulse from the oscillator . Through this repetitive action, the PWM control algorithm establishes a switch duty cycle to regulate a current or voltage in the load. The VC signal is integrated over many switching cycles and is an amplified version of the differ- ence between the LED current sense voltage, measured between ISP and ISN, and the target difference voltage set by the CTRL pin. In this manner , the error amplifier sets the correct peak switch current level to keep the LED current in regulation. If the error amplifier output increases, more current is demanded in the switch; if it decreases, less current is demanded. The switch current is monitored during the on-phase and the voltage across the SENSE pin is not allowed to exceed the current limit threshold of 108mV (typical). If the SENSE pin exceeds the current limit threshold, the SR latch is reset regardless of the output state of the PWM comparator . Likewise, at an ISP/ISN common mode voltage less than 3V , the dif- ference between ISP and ISN is monitored to determine if the output is in a short-circuit condition. If the difference between ISP and ISN is greater than 150mV (typical), the SR latch will be reset regardless of the PWM comparator . These functions are intended to protect the power switch as well as various external components in the power path of the DC/DC converter . In voltage feedback mode, the operation is similar to that described above, except the voltage at the VC pin is set by the amplified difference of the internal reference of 1.25V (nominal) and the FB pin. If FB is lower than the reference voltage, the switch current will increase; if FB is higher than the reference voltage, the switch demand current will decrease. The LED current sense feedback interacts with the FB voltage feedback so that FB will not exceed the internal reference and the voltage between ISP and ISN will not exceed the threshold set by the CTRL pin. For accurate current or voltage regulation, it is necessary to be sure that under normal operating conditions the appropriate loop is dominant. To deactivate the voltage loop entirely, FB can be connected to GND. To deactivate the LED current loop entirely, the ISP and ISN should be tied together and the CTRL input tied to V REF . T wo LED specific functions featured on the L T3755 are controlled by the voltage feedback pin. First, when the FB pin exceeds a voltage 50mV lower (–4%) than the FB regulation voltage, the pull-down driver on the OPENLED pin is activated (L T3755 and L T3755-2 only). This func- tion provides a status indicator that the load may be disconnected and the constant-voltage feedback loop is taking control of the switching regulator . When the FB pin exceeds the FB regulation voltage by 60mV (5% typical), the PWMOUT pin is driven low, ignoring the state of the PWM input. In the case where the PWMOUT pin drives a disconnect NFET , this action isolates the LED load from GND preventing excessive current from damaging the LEDs. If the FB input exceeds both the open LED and the overvoltage thresholds, then an externally driven overvoltage event has caused the FB pin to be too high and the OPENLED pull-down will be de-asserted. The L T3755-2 will re-assert the OPENLED signal when FB falls below the overvoltage threshold and remains above the OPENLED threshold. The L T3755 is prevented from re-asserting OPENLED until FB drops below both thresholds. operaTion

the L T3755 from excessive on-chip power dissipation. current is to allow the user to program the rising hysteresis.

37551 F01

Figure 1. Resistor Connection to Set above by an increasing amount as CTRL voltage increases. is ~98% of the equation’s estimate.

average value and the user-programmed value.

37551 F02

Figure 2. Feedback Resistor Connection for

37551 F03

Figure 3. Feedback Resistor Connection for the ISN voltage range where these two functions overlap.

minimizes recovery time when the PWM signal goes high. node from discharging during the PWM signal low phase. of operating frequency and external component selection. may not allow sufficiently high or low duty cycle operation. RT pin to GND is required—do not leave this pin open. Table 1. Switching Frequency vs RT Value GATE rise/fall times and SW node rise/fall times. Figure 4. Typical Minimum On and Off

37551 F04

The L T3755 is rated to a maximum input voltage of 40V .

when operating at high ambient temperatures. The ma- jority of the power dissipation in the IC comes from the supply current needed to drive the gate capacitance of the external power MOSFET . This gate drive current can be calculated as: I GATE = fSW • QG A low QG power MOSFET should always be used when op- erating at high input voltages, and the switching frequency should also be chosen carefully to ensure that the IC does not exceed a safe junction temperature. The internal junc- tion temperature of the IC can be estimated by: T J = TA + [VIN (IQ + fSW • QG) • θJA] where TA is the ambient temperature, IQ is the quiescent current of the part (maximum 1.7mA) and θJA is the package thermal impedance (68°C/W for the 3mm × 3mm QFN package). For example, an application with T A(MAX) = 85°C, VIN(MAX) = 40V , fSW = 400kHz, and having a FET with QG = 20nC, the maximum IC junction temperature will be approximately: T = 111°C The Exposed Pad on the bottom of the package must be soldered to a ground plane. This ground should then be connected to an internal copper ground plane with thermal vias placed directly under the package to spread out the heat dissipated by the IC. If L T3755 junction temperature reaches 165°C, the GATE and PWMOUT pins will be driven to GND and the soft- start (SS) pin will be discharged to GND. Switching will be enabled after device temperature is reduced 10°C. This function is intended to protect the device during momentary thermal overload conditions. Frequency Synchronization (L T3755-1 Only) The L T3755-1 switching frequency can be synchronized to an external clock using the SYNC pin. For proper operation, the R T resistor should be chosen for a switching frequency 20% lower than the external clock frequency. The SYNC pin is disabled during the soft-start period. Observation of the following guidelines about the SYNC waveform will ensure proper operation of this feature. Driving SYNC with a 50% duty cycle waveform is always a good choice, otherwise, maintain the duty cycle between 20% and 60%. When using both PWM and SYNC features, the PWM signal rising edge should occur at least 200ns before the SYNC rising edge (V IH) for optimal PWM performance. If the SYNC pin is not used, it should be connected to GND. Open LED Detection (L T3755 and L T3755-2) The L T3755 and L T3755-2 provide an open-drain status pin, OPENLED, that pulls low when the FB pin is within ~50mV of its 1.25V regulated voltage. If the open LED clamp voltage is programmed correctly using the FB pin, then the FB pin should never exceed 1.1V when LEDs are connected, therefore, the only way for the FB pin to be within 50mV of the regulation voltage is for an open LED event to have occurred. The key difference between the L T3755 and L T3755-2 is the behavior of the OPENLED pin when the FB pin crosses and re-crosses the FB overvoltage threshold (1.31V typ). The L T3755-2 asserts/de-asserts OPENLED freely when crossing the 1.31V threshold. The L T3755, by comparison, de-asserts OPENLED when FB exceeds 1.31V and is prevented from re-asserting OPENLED until the FB pin falls below the 1.2V (typ) open LED threshold and clears the fault. The L T3755-2 has the more general purpose behavior and is recommended for applications using OPENLED. Input Capacitor Selection The input capacitor supplies the transient input current for the power inductor of the converter and must be placed and sized according to the transient current requirements. The switching frequency, output current and tolerable input voltage ripple are key inputs to estimating the capacitor value. An X7R type ceramic capacitor is usually the best choice since it has the least variation with temperature and DC bias. Typically, boost and SEPIC converters require a lower value capacitor than a buck mode converter . As- suming that a 100mV input voltage ripple is acceptable, the required capacitor value for a boost converter can be estimated as follows: CIN(µF) = ILED (A) •VOUT VIN

• tSW (µs) • µF A • µs

400kHz buck mode converter with a 1A load. where D is the switch duty cycle. Table 2. Recommended Ceramic Capacitor Manufacturers Table 3. MOSFET Manufacturers

recommended component vendors. Table 4. Schottky Rectifier Manufacturers Table 5 provides some recommended inductor vendors. Table 5. Inductor Manufacturers resistor and capacitor determine the loop stability.

The inductor and output capacitor are chosen based on performance, size and cost. The compensation resistor and capacitor at VC are selected to optimize control loop response and stability. For typical LED applications, a 2.2nF compensation capacitor at VC is adequate, and a series resistor should always be used to increase the slew rate on the VC pin to maintain tighter regulation of LED current during fast transients on the input supply to the converter . Board Layout The high speed operation of the L T3755 demands careful attention to board layout and component placement. The exposed pad of the package is the only GND terminal of the IC and is also important for thermal management of the IC. It is crucial to achieve a good electrical and thermal contact between the exposed pad and the ground plane of the board. To reduce electromagnetic interference (EMI), it is important to minimize the area of the high dV/dt switching node between the inductor , switch drain and anode of the Schottky rectifier . Use a ground plane under the switching node to eliminate interplane coupling to sensitive signals. The lengths of the high dI/dt traces: 1) from the switch node through the switch and sense resistor to GND, and 2) from the switch node through the Schottky rectifier and filter capacitor to GND should be minimized. The ground points of these two switching current traces should come to a common point then connect to the ground plane under the L T3755. Likewise, the ground terminal of the bypass capacitor for the INTV CC regulator should be placed near the GND of the switching path. Typically this requirement will result in the external switch being closest to the IC, along with the INTV CC bypass capacitor . The ground for the compensation network and other DC control signals should be star connected to the underside of the IC. Do not extensively route high impedance signals such as FB and VC, as they may pick up switching noise. In particular , avoid routing FB and PWMOUT in parallel for more than a few millimeters on the board. Minimize resistance in series with the SENSE input to avoid changes (most likely reduction) to the switch current limit threshold. 20W SEPIC LED Driver Efficiency vs VIN VIN L T3755-2 L1A 22µH GNDVC INTVCC SHDN/UVLO FB VREF ISP 100k INTVCC 4.7µF 50V 0.001µF 0.01µF L1: WÜRTH ELEKTRONIK 744870220 M1: VISHAY SILICONIX SI7454DP D1: DIODES INC. - PDS5100 M2: VISHAY SILICONIX SI2318DS V IN 8V TO 40V 187k 25k L1B 30k 28.7k 375kHz 4.7µF 10V CTRL 0.015/uni03A9 0.1/uni03A9 511k 100V 2.2µF 50V 4.7µF 50V 20W LED STRING

37551 TA04a

VIN (V) EFFICIENCY (%) 100 10 20 30

37551 TA04b

VOUT = 18V ILED = 1A

37551 F05

Figure 5. Boost Converter Suggested Layout

22µH D1 GNDVC INTVCC SHDN/UVLO FB VREF ISP 16.9k INTVCC 100k CIN 4.7µF CC 0.001µF CSS 0.01µF VIN 8V TO 40V 187k RC 10k RT 28.7k 375kHz C VCC 4.7µF SEE SUGGESTED LAYOUT , FIGURE 5 100k NTC RT1 CTRL R SENSE 0.015/uni03A9 RLED 0.1/uni03A9 23.7k C OUT 4.7µF 50W LED STRING

37551 TA02a

L1: COIL TRONICS DR127-220 M1: VISHAY SILICONIX SI7850DP D1: DIODES INC. PDS5100 M2: VISHAY SILICONIX SI2308DS RT1: MURATA NCP18WM1045 50W White LED Headlamp Driver Efficiency vs Load LOAD (A) 0.0 EFFICIENCY (%) 100 0.2 0.4 0.6 0.8

37551 TA02c

1.0 VIN = 12V Waveforms for 50W LED Driver with PWM Disconnect NFET IL1 2A/DIV ILED 500mA/DIV 50µs/DIV VOUT = 50V 10V/DIV PWM 0V TO 5V

37551 TA02b

VIN = 12V TEMPERATURE (°C) V(ISP-ISN) THRESHOLD (mV) 45 65 10585 120 100 125

37551 TA02d

V(ISP-ISN) Threshold vs Temperature for NTC Resistor Divider Typical applicaTions

Buck Mode 1.4A LED Driver VIN L T3755-2 33µH GND 200k 200k VC INTVCC SHDN/UVLO FB VREF ISP1MC1 1µF 0.1µF VIN 15V TO 40V 0.001µF 0.068/uni03A9 VIN L1: COIL TRONICS DR125-330 M1: VISHAY SILICONIX SI7850DP D1: ON SEMICONDUCTOR MBRS360 M2: ZETEX ZXMN4A06G M3: ZETEX ZXM62P03E6 Q1: ZETEX FMMT558 107k 28.7k 375kHz C2 4.7µF CTRL 0.033/uni03A9 1.5k 110k 20k 47k 4.7µF

37551 TA03a

1.4A 4.7µF

3 LUXEON K2

• WHITE 100k INTVCC Efficiency vs VIN VIN (V) EFFICIENCY (%) 100 20 25 30 35

37551 TA03c

2µs/DIV

37551 TA03b

VIN = 24V VLED = 10V 1000:1 PWM Dimming at 120Hz with Buck Mode

Buck Mode 500mA LED Driver for 20kHz PWM Dimming VIN L T3755-2 3.3µH GND VC FB SHDN/UVLO VREF ISP1M 1M 6V 0.1µF VIN 22V TO 36V 470pF 2200pF 0.2/uni03A9 L1: TOKO 962BS_3R3M M1: VISHAY SILICONIX SI7850DP M2: VISHAY SILICONIX SI2306DS D1: DIODES, INC SBM540 68.1k CTRL 0.033/uni03A9 22k 2.2µF 2s 50V 4.7µF 2s 25V 0.22µF OPENLED 13k 800kHz 4.7µF Efficiency vs VIN PWM ILED 500mA/DIV GATE INDUCTOR CURRENT 1A/DIV 500ns/DIV

37551 TA06b

Minimum PWM Pulse Switching Waveform

37551 TA06c

EFFICIENCY (%) VIN (V) 15 25 3020 35 40 100 VLED = 16V ILED = 0.5A

21W Buck-Boost Mode with 250:1 PWM Dimming and Open LED Protection VIN L T3755-2 GND SHDN/UVLO VREF INTVCC 499k 100k 0.1/uni03A9 21.5V 50V 2.2µF V IN 8V TO 36V 15µH M1: VISHAY SILICONIX SI7850DP M2: VISHAY SILICONIX SI2319DS Q1: ZETEX FMMT558 Q2: MMBTA42 D1: DIODES INC. PDS560 L1: SUMIDA CDRH127/LD-150 93.1k 392k 1.5k 20.0k 100k 549k 75.0k 0.01µF 4.7µF CTRL VC 4.7k 4700pF

37551 TA07aOPENLED

2.2µF M2D1 28.7k 375kHz 0.02/uni03A9 VIN Buck-Boost Mode LED Current vs Low Input VoltageBuck-Boost Mode Efficiency vs Input Voltage

37551 TA07b

EFFICIENCY (%) INPUT VOL TAGE (V) 0 15 10 25 305 20 35 40 100

37551 TA07c

LED CURRENT (A) INPUT VOL TAGE (V) 8 11 10 13 149 12 15 16 0.8 0.95 1.10 0.85 0.9 1.05

16-Lead Plastic MSOP , Exposed Die Pad (Reference L TC DWG # 05-08-1667 Rev A) MSOP (MSE16) 0608 REV A 0.53 p 0.152 (.021 p .006) SEATING PLANE 0.18 (.007) 1.10 (.043) MAX 0.17 –/uni00A00.27 (.007 – .011) TYP 0.86 (.034) REF 0.50 (.0197) BSC 16151413121110 1 2 3 4 5 6 7 8 1 8 NOTE: 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX 0.254 (.010) 0o – 6o TYP DETAIL “A” DETAIL “A” GAUGE PLANE 5.23 (.206) MIN 3.20 – 3.45 (.126 – .136) 0.889 p 0.127 (.035 p .005) RECOMMENDED SOLDER PAD LAYOUT 0.305 p 0.038 (.0120 p .0015) TYP 0.50 (.0197) BSC BOTTOM VIEW OF EXPOSED PAD OPTION 2.845 p 0.102 (.112 p .004) 2.845 p 0.102 (.112 p .004) 4.039 p 0.102 (.159 p .004) (NOTE 3) 1.651 p 0.102 (.065 p .004) 1.651 p 0.102 (.065 p .004) 0.1016 p 0.0508 (.004 p .002) 3.00 p 0.102 (.118 p .004) (NOTE 4) 0.280 p 0.076 (.011 p .003) REF 4.90 p 0.152 (.193 p .006) 16-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1667 Rev A) DETAIL “B” DETAIL “B” CORNER TAIL IS PART OF THE LEADFRAME FEATURE. FOR REFERENCE ONL Y NO MEASUREMENT PURPOSE

0.12 REF

0.35 REF

16-Lead Plastic QFN (3mm × 3mm) (Reference L TC DWG # 05-08-1691) 3.00 ± 0.10 (4 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 1.45 ± 0.05 (4 SIDES) NOTE: 1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (WEED-2) 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 PIN 1 TOP MARK (NOTE 6) 0.40 ± 0.10 BOTTOM VIEW—EXPOSED PAD 1.45 ± 0.10 (4-SIDES) 0.75 ± 0.05 R = 0.115 TYP 0.25 ± 0.05 PIN 1 NOTCH R = 0.20 TYP OR 0.25 × 45° CHAMFER 15 16

0.50 BSC

0.200 REF

2.10 ± 0.05 3.50 ± 0.05 0.70 ±0.05 0.00 – 0.05 (UD16) QFN 0904 0.25 ±0.05

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 representa- tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.

revision hisTory

REV DATE DESCRIPTION PAGE NUMBER D 03/10 Revised Entire Data Sheet to Include H-Grade 1-26 (Revision history begins at Rev D)

Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear .com  LINEAR TECHNOLOGY CORPORATION 2008 LT 0310 REV D • PRINTED IN USA relaTeD parTs Typical applicaTion Buck-Boost LED Driver for Automotive PART NUMBER DESCRIPTION COMMENTS L T3474 36V, 1A (ILED), 2MHz, Step-Down LED Driver V IN: 4V to 36V, VOUT(MAX) = 13.5V, T rue Color PWM Dimming = 400:1, ISD < 1µA, TSSOP16E Package L T3475 Dual 1.5A (ILED), 36V, 2MHz Step-Down LED Driver V IN: 4V to 36V, VOUT(MAX) = 13.5V, T rue Color PWM Dimming = 3000:1, ISD < 1µA, TSSOP20E Package L T3476 Quad Output 1.5A, 36V, 2MHz High Current LED Driver with 1000:1 Dimming VIN: 2.8V to 16V, VOUT(MAX) = 36V, T rue Color PWM Dimming = 1000:1, ISD < 10µA, 5mm × 7mm QFN Package L T3477 3A, 42V, 3MHz Boost, Buck-Boost, Buck LED Driver V IN: 2.5V to 25V, VOUT(MAX) = 40V, Dimming = Analog/PWM, ISD < 1µA, QFN and TSSOP20E Packages L T3478/L T3478-1 4.5A, 42V, 2.5MHz High Current LED Driver with 3000:1 Dimming VIN: 2.8V to 36V, VOUT(MAX) = 42V, T rue Color PWM Dimming = 3000:1, ISD < 3µA, TSSOP16E Package L T3486 Dual 1.3A, 2MHz High Current LED Driver VIN: 2.5V to 24V, VOUT(MAX) = 36V, T rue Color PWM Dimming = 1000:1, ISD < 1µA, 5mm × 3mm DFN and TSSOP16E Packages L T3496 T riple 0.75A, 2.1MHz, 45V LED Driver VIN: 3V to 30V, VOUT(MAX) = 45V, Dimming = 3000:1, ISD < 1µA, 4mm × 5mm QFN and TSSOP16E Packages L T3517 1.3A, 2.5MHz, 45V LED Driver VIN: 3V to 30V, VOUT(MAX) = 45V, Dimming = 3000:1, ISD < 1µA, 4mm × 4mm QFN and TSSOP16E Packages L T3518 2.3A, 2.5MHz, 45V LED Driver VIN: 3V to 30V, VOUT(MAX) = 45V, Dimming = 3000:1, ISD < 1µA, 4mm × 4mm QFN and TSSOP16E Packages L T3756/L T3756-1/ L T3756-2 100V IN, 100VOUT LED Controller VIN: 6V to 100V, VOUT(MAX) = 100V, T rue Color PWM Dimming = 3000:1, ISD < 1µA, 3mm × 3mm QFN-16 and MS16E Packages L TC®3783 High Current LED Controller VIN: 3V to 36V, VOUT(MAX) = Ext FET, T rue Color PWM Dimming = 3000:1, ISD < 20µA, 5mm × 4mm QFN10 and TSSOP16E Packages Efficiency vs VIN VIN L T3755-2 L TC4440-5 GNDVC INTVCC SHDN/UVLO FB VREF ISP 383k 100k INTVCC INTVCC INTVCC 4.7µF 0.01µF 0.01µF VIN 6V TO 36V 330k 10k 28.7k 375kHz 4.7µF 47k CTRL 0.025/uni03A9 0.1/uni03A9 40k 4.7µF

37551 TA05a

V CC GND TG TS BOOST M1, M2: VISHAY SILICONIX SI7850DP D1, D2: DIODES, INC SBM540 0.22µF 22µF VIN (V) EFFICIENCY (%) 10 20 30

37551 TA05b