LT3796/LT3796-1 100V Constant-Current and Constant-Voltage Controller with Dual Current Sense Description Features n n n n n n n n n n n n n n n n 3000:1 True Color PWM™ Dimming Wide Input Voltage Range: 6V to 100V Current Monitoring Up to 100V High Side PMOS Disconnect and PWM Switch Driver Constant-Current and Constant-Voltage Regulation Dual Current Sense Amplifiers with Reporting C/10 Detection for Battery and SuperCap Charging Linear Current Sense Threshold Programming Short-Circuit Protection Adjustable Frequency: 100kHz to 1MHz Frequency Synchronization (LT3796) Independent Top Gate Enable Pin (LT3796-1) Programmable Open LED Protection with VMODE Flag Programmable Undervoltage Lockout with Hysteresis Soft-Start with Programmable Fault Restart Timer Available in 28-Lead TSSOP Package The LT®3796/LT3796-1 are DC/DC controllers designed to regulate a constant-current or constant-voltage and are ideal for driving LEDs. The fixed frequency and current mode architecture result in stable operation over a wide range of supply and output voltages. Two ground referred voltage FB pins serve as the input for several LED protection features, and also allow the converter to operate as a constant-voltage source. The LT3796/LT3796-1 feature a programmable threshold output current sense amplifier with rail-to-rail common mode range. A separate high side amplifier is gain configurable with two resistors and can be used to regulate a second current or a voltage in combination with one of the FB pins. The PWM input provides LED dimming ratios of up to 3000:1. The LT3796-1 is optimized for a second output current regulation loop that can be enabled/disabled with a PMOS switch, either to drive a second LED string or to extend analog dimming range. Applications High Power LED, High Voltage LED, Dual String Battery and SuperCap Chargers n Accurate Current Limited Voltage Regulators n 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, 7746300. n Typical Application Boost LED Driver with Input Current Monitor 1M 2k 499k VIN VS 97.6k VREF CSOUT 0.1µF INTVCC VMODE CSN FB1 ISMON ISP 85 80 70 ISN 0 10 20 30 VIN (V) 40 50 60 37961 TA01b TG FAULT INTVCC VMODE 10nF 90 75 620mΩ 100k RT 10k 95 GND PWM VC 13.7k 15mΩ LT3796 SYNC 100k FAULT 2.2µF ×4 SENSE CSOUT PWM 1M GATE CTRL 40.2k CSP EN/UVLO 118k 10nF 100 EFFICIENCY (%) 2.2µF ×3 Efficiency vs VIN 22µH 50mΩ VIN 9V TO 60V 100V (TRANSIENT) FB2 31.6k 250kHz SS INTVCC 4.7µF 0.1µF 85V LED 400mA 37961 TA01a 3796fb For more information www.linear.com/3796 1 LT3796/LT3796-1 Absolute Maximum Ratings Pin Configuration (Note 1) VIN, VS.....................................................................100V EN/UVLO..................................................................100V ISP, ISN....................................................................100V TG, GATE...............................................................Note 2 CSP, CSN.................................................................100V VS - CSP, VS - CSN........................................ –0.3V to 4V INTVCC (Note 3)......................................8.6V, VIN + 0.3V PWM, VMODE, FAULT................................................12V FB1, FB2, SYNC (LT3796), TGEN (LT3796-1)...............8V CTRL..........................................................................15V SENSE.......................................................................0.5V ISMON, CSOUT............................................................5V VC, VREF, SS.................................................................3V RT................................................................................2V Operating Junction Temperature Range (Note 4) LT3796E/LT3796I................................... –40 to 125°C LT3796H................................................. –40 to 150°C Storage Temperature Range.......................–65 to 150°C TOP VIEW ISP 1 28 CSOUT ISN 2 27 CSP TG 3 26 CSN GND 4 25 VS ISMON 5 24 EN/UVLO FB2 6 23 VIN FB1 7 VC 8 CTRL 9 VREF 10 29 GND 22 GND 21 GND 20 INTVCC 19 GATE SS 11 18 SENSE RT 12 17 GND TGEN/SYNC* 13 PWM 14 16 VMODE 15 FAULT FE PACKAGE 28-LEAD PLASTIC TSSOP TJMAX = 150°C, θJA = 30°C/W, θJC = 10°C/W EXPOSED PAD (PIN 29) IS GND, MUST BE SOLDERED TO PCB *SYNC FOR LT3796, TGEN FOR LT3796-1 Order Information LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT3796EFE#PBF LT3796EFE#TRPBF LT3796FE 28-Lead Plastic TSSOP –40°C to 125°C LT3796IFE#PBF LT3796IFE#TRPBF LT3796FE 28-Lead Plastic TSSOP –40°C to 125°C LT3796HFE#PBF LT3796HFE#TRPBF LT3796FE 28-Lead Plastic TSSOP –40°C to 150°C LT3796EFE-1#PBF LT3796EFE-1#TRPBF LT3796FE-1 28-Lead Plastic TSSOP –40°C to 125°C LT3796IFE-1#PBF LT3796IFE-1#TRPBF LT3796FE-1 28-Lead Plastic TSSOP –40°C to 125°C LT3796HFE-1#PBF LT3796HFE-1#TRPBF LT3796FE-1 28-Lead Plastic TSSOP –40°C to 150°C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. Consult LTC 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/ 3796fb 2 For more information www.linear.com/3796 LT3796/LT3796-1 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C, VIN = 24V, EN/UVLO = 24V, CTRL = 2V, PWM = 5V, unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX UNITS VIN Minimum Operating Voltage VIN Tied to INTVCC 6 V VIN Shutdown IQ EN/UVLO = 0V, PWM = 0V EN/UVLO = 1.15V, PWM = 0V 1 12 µA µA VIN Operating IQ (Not Switching) RT = 82.5k to GND, FB1 = 1.5V 2.5 3 mA 2.015 2.06 VREF Voltage –100µA ≤ IREF ≤ 10µA VREF Pin Line Regulation 6V < VIN < 100V VREF Pin Load Regulation –100µA < IREF < 0µA SENSE Current Limit Threshold l 1.97 1.5 10 l 100 113 V m%/V m%/µA 125 mV SENSE Input Bias Current Current Out of Pin 60 µA SS Sourcing Current SS = 0V 28 µA SS Sinking Current ISP – ISN = 1V, SS = 2V 2.8 µA Error Amplifier Full Scale LED Current Sense Threshold (V(ISP-ISN)) ISP = 48V, CTRL ≥ 1.2V ISP = 0V, CTRL ≥ 1.2V l l 243 243 250 250 257 257 mV mV 9/10th LED Current Sense Threshold (V(ISP-ISN)) CTRL = 1V, ISP = 48V CTRL = 1V, ISP = 0V l l 220 220 225 225 230 230 mV mV 1/2 LED Current Sense Threshold (V(ISP-ISN)) CTRL = 0.6V, ISP = 48V CTRL = 0.6V, ISP = 0V l l 119 119 125 125 131 131 mV mV 1/10th LED Current Sense Threshold (V(ISP-ISN)) CTRL = 0.2V, ISP = 48V CTRL = 0.2V, ISP = 0V l l 16 16 25 25 32 32 mV mV ISP/ISN Current Monitor Voltage (VISMON) V(ISP-ISN) = 250mV, ISP = 48V, –50µA < IISMON < 0 µA V(ISP-ISN) = 250mV, ISP = 0V, –50µA < IISMON < 0 µA l l 0.96 0.96 1 1 1.04 1.04 V V ISP/ISN Over Current Protection Threshold (V(ISP-ISN)) ISN = 48V ISN = 0V l l 360 360 375 375 390 390 mV mV CTRL Input Bias Current Current Out of Pin, CTRL = 1.2V 50 200 nA 100 V 0.1 µA µA ISP/ISN Current Sense Amplifier Input Common Mode Range 0 ISP/ISN Input Current Bias Current (Combined) PWM = 5V (Active), ISP = 48V PWM = 0V (Standby), ISP = 48V 700 0 ISP/ISN Current Sense Amplifier gm V(ISP-ISN) = 250mV 400 VC Output Impedance µs 2000 VC Standby Input Bias Current PWM = 0V FB1, FB2 Regulation Voltage (VFB) ISP = ISN = 48V ISP = ISN = 48V –20 kΩ 20 nA 1.230 1.238 1.250 1.250 1.270 1.264 V V FB1 Amplifier gm 450 600 750 µS FB2 Amplifier gm 130 170 210 µS 100 200 nA FB1, FB2 Pin Input Bias Current FB = VFB FB1 Open LED Threshold VMODE Falling, ISP = ISN = 48V C/10 Comparator Threshold (V(ISP-ISN)) VMODE Falling, FB1 = 1.25V, ISP = 48V VMODE Falling, FB1 = 1.25V, ISN = 0V l VFB – 70mV VFB – 60mV VFB – 50mV 25 25 V mV mV FB1 Overvoltage Threshold FAULT Falling VFB + 35mV VFB + 50mV VFB + 60mV V FB2 Overvoltage Threshold TG Rising VFB + 35mV VFB + 50mV VFB + 60mV V VC Current Mode Gain (∆VVC/∆VSENSE) 4.2 V/V 3796fb For more information www.linear.com/3796 3 LT3796/LT3796-1 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C, VIN = 24V, EN/UVLO = 24V, CTRL = 2V, PWM = 5V, unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX UNITS Current Sense Amplifier (CSA) Power Supply Voltage Range (VS) l 3 100 V CSA Input Voltage Common Mode Range (VCSP and VCSN) l 2.5 100 V CSOUT Maximum Output Current l 200 µA 3 mV Input Voltage Offset (V(CSP-CSN)) VSNS = 100mV, VS = 48V (Note 5) CSP, CSN Input Bias Current RIN1 = RIN2 = 1k (Note 5) 100 nA CSP, CSN Input Current Offset RIN1 = RIN2 = 1k (Note 5) 0 nA VS Supply Current VS = 48V 80 µA 1 µs l –3 Input Step Response ( to 50% of Output Step) ∆VSENSE = 100mV Step, RIN1 = RIN2 = 1k, ROUT = 5k 0 Linear Regulator INTVCC Regulation Voltage l Dropout (VIN – INTVCC) IINTVCC = –10mA, VIN = 6V INTVCC Current Limit VIN = 100V, INTVCC = 6V VIN = 12V, INTVCC = 6V INTVCC Shutdown Bias Current if Externally Driven to 7V EN/UVLO = 0V, INTVCC = 7V 7.4 7.7 8 400 mV 20 85 mA mA 10 INTVCC Undervoltage Lockout 3.8 INTVCC Undervoltage Lockout Hysteresis V 4 µA 4.1 150 V mV Oscillator Switching Frequency RT = 82.5k RT = 19.6k RT = 6.65k Minimum Off-Time (Note 6) 190 ns Minimum On-Time (Note 6) 210 ns l l l 85 340 900 105 400 1000 125 480 1150 kHz kHz kHz LOGIC Input/Outputs PWM Input Threshold Rising l 0.96 PWM Pin Bias Current 1 1.04 10 EN/UVLO Threshold Voltage Falling l 1.185 EN/UVLO Rising Hysteresis 1.220 1.250 20 EN/UVLO Input Low Voltage IVIN Drops Below 1µA 0.4 EN/UVLO Pin Bias Current Low EN/UVLO = 1.15V 2.5 EN/UVLO Pin Bias Current High EN/UVLO = 1.30V VMODE OUTPUT Low V µA V mV V 3 3.8 µA 40 200 nA IVMODE = 0.5mA 300 mV FAULT OUTPUT Low IFAULT = 0.5mA 300 mV SYNC Pin Resistance to GND LT3796 Only SYNC Input Low Threshold LT3796 Only SYNC Input High Threshold LT3796 Only TGEN Pin Resistance to GND LT3796-1 Only TGEN Input Low Threshold LT3796-1 Only TGEN Input High Threshold LT3796-1 Only 40 kΩ 0.4 V 1.5 40 V kΩ 0.4 V 1.5 V 3796fb 4 For more information www.linear.com/3796 LT3796/LT3796-1 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C, VIN = 24V, EN/UVLO = 24V, CTRL = 2V, PWM = 5V, unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX UNITS Gate Driver tr NMOS GATE Driver Output Rise Time CL = 3300pF, 10% to 90% 20 tf NMOS GATE Driver Output Fall Time CL = 3300pF, 10% to 90% 18 NMOS GATE Output Low (VOL) ns ns 0.05 NMOS GATE Output High (VOH) V INTVCC – 0.05 V tr Top GATE Driver Output Rise Time CL = 300pF 50 ns tf Top GATE Driver Output Fall Time CL = 300pF 100 ns Top Gate On Voltage (VISP -VTG) ISP = 48V 7 8 V Top Gate Off Voltage (VISP -VTG) PWM = 0V, ISP = 48V 0 0.3 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: Do not apply a positive or negative voltage source to TG and GATE pins, otherwise permanent damage may occur. Note 3: Operating maximum for INTVCC is 8V. Note 4: The LT3796E and LT3796E-1 are guaranteed to meet specified performance from 0°C to 125°C. Specifications over the –40°C to 125°C operating temperature range are assured by design, characterization and correlation with statistical process controls. The LT3796I and LT3796I-1 are guaranteed to meet performance specifications over the –40°C to 125°C operating temperature range. The LT3796H and LT3796H-1 are guaranteed 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 5: Measured in servo. See Figure 9 for details. Note 6: See Duty Cycle Considerations in the Applications Information section. 3796fb For more information www.linear.com/3796 5 LT3796/LT3796-1 Typical Performance Characteristics 253 200 150 100 50 0 0 0.2 0.6 0.8 VCTRL (V) 0.4 1.0 1.2 254 251 250 249 248 251 250 249 248 0 20 60 40 VISP (V) 246 –50 –25 100 80 0 25 50 75 100 125 150 TEMPERATURE (°C) 37961 G03 37961 G02 V(ISP-ISN) Threshold at CTRL = 0.6V vs Temperature V(ISP-ISN) Threshold vs FB Voltage 128 300 127 250 1.27 VFB vs Temperature FB1 126 125 124 123 122 –50 –25 0 25 50 75 150 1.24 50 1.2 1.15 TEMPERATURE (°C) 1.25 900 376 374 372 75 100 125 150 TEMPERATURE (°C) 100 125 150 VREF Voltage vs Temperature PWM = 5V 800 2.04 700 2.03 ISP 500 400 2.01 1.99 200 1.98 1.97 ISN 0 20 40 60 IREF = –100µA 2.00 300 0 IREF = 0µA 2.02 600 80 100 VISP, VISN (V) 37961 G06 75 2.05 100 50 50 37961 G05 VREF (V) ISP, ISN BIAS CURRENT (µA) 378 25 TEMPERATURE (°C) ISP/ISN Input Bias Current vs VISP , VISN 380 25 0 37961 G04 ISP/ISN Overcurrent Protection Threshold vs Temperature 0 1.23 –50 –25 1.3 VFB (V) 37961 G03a 370 –50 –25 1.25 100 0 1.1 100 125 150 1.26 FB2 200 VFB (V) V(ISP-ISN) THRESHOLD (mV) V(ISP-ISN) (mV) 252 247 37961 G01 ISP/ISN OVERCURRENT THRESHOLD (mV) ISP = 48V CTRL = 2V 253 252 247 1.4 V(ISP-ISN) Full-Scale Threshold vs Temperature V(ISP-ISN) Threshold vs VISP V(ISP-ISN) THRESHOLD (mV) 250 V(ISP-ISN) THRESHOLD (mV) V(ISP-ISN) THRESHOLD (mV) 300 V(ISP-ISN) Threshold vs VCTRL TA = 25°C, unless otherwise noted. 1.96 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 37961 G07 37961 G08 3796fb 6 For more information www.linear.com/3796 LT3796/LT3796-1 Typical Performance Characteristics VREF vs VIN 2.05 100 IREF = 0µA TA = 25°C, unless otherwise noted. Switching Frequency vs Temperature RT vs Switching Frequency 440 2.02 RT (kΩ) VREF (V) 2.03 2.01 10 2.00 1.99 420 410 400 390 380 1.98 1.97 370 0 20 60 40 80 1 100 VIN (V) 360 –50 –25 0 100 200 300 400 500 600 700 800 900 1000 SWITCHING FREQUENCY (kHz) 37961 G09 2.5 450 50 75 100 125 150 37961 G11 Quiescent Current vs VIN 2000 VISMON vs V(ISP-ISN) 1800 2.0 300 250 200 150 100 1600 1400 VISMON (mV) VIN CURRENT (mA) 350 1.5 1.0 200 400 600 800 1000 0 1200 0 20 60 40 80 3.0 EN/UVLO (V) 2.5 2.0 1.5 1.0 118 1.27 117 1.26 116 1.25 EN/UVLO RISING THRESHOLD 1.24 1.23 EN/UVLO FALLING THRESHOLD 1.19 –50 –25 37961 G14 500 115 114 113 112 111 109 0 25 50 75 100 125 150 TEMPERATURE (°C) TEMPERATURE (°C) 400 110 1.20 100 125 150 300 37961 G13 1.28 1.21 0.5 200 SENSE Current Limit Threshold vs Temperature 1.22 75 100 V(ISP-ISN) (mV) EN/UVLO Falling/Rising Threshold vs Temperature 3.5 50 0 37961 G12 EN/UVLO Hysteresis Current vs Temperature 25 0 100 VIN (V) 37961 G11a 0 800 200 PWM = 0V SS VOLTAGE (mV) 0 –50 –25 1000 400 50 0 1200 600 0.5 SENSE THRESHOLD (mV) SW FREQUENCY (kHz) 25 TEMPERATURE (°C) 400 0 0 37961 G10 Switching Frequency vs SS Voltage EN/UVLO HYSTERESIS CURRENT (µA) RT = 19.6k 430 SWITCHING FREQUENCY (kHz) 2.04 108 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 37961 G15 37961 G16 3796fb For more information www.linear.com/3796 7 LT3796/LT3796-1 Typical Performance Characteristics SENSE Current Limit Threshold vs Duty Cycle 120 110 105 0 20 40 60 80 VIN = 24V 100 80 60 40 20 0 100 100 INTVCC CURRENT LIMIT (mA) INTVCC CURRENT LIMIT (mA) SENSE THRESHOLD (mV) 115 0 20 60 40 80 60 50 –50 –25 100 8 1600 7 1400 3 2 1 125°C 80 25°C 800 0°C 600 0 100 7.7 7.6 7.5 400 7.4 –55°C –40°C 0 5 VIN (V) 10 15 7.3 –50 –25 20 0 INTVCC LOAD (mA) 25 50 37961 G22 V(CSP-CSN) Offset Voltage vs Temperature Current Sense Amplifier Gain Error vs Temperature 0.6 2 2.0 ICSOUT = 100µA 1.5 0 ICSOUT = 10µA 0.2 ICSOUT = 50µA 0 –1 –2 1.0 ICSOUT = 100µA GAIN ERROR (%) V(CSP-CSN) (mV) V(CSP-CSN) (mV) 0.4 ICSOUT = 50µA 0 20 40 60 80 100 –0.2 –50 –25 37961 G23 0.5 ICSOUT = 50µA 0 –0.5 0 25 50 75 –1.5 100 125 150 TEMPERATURE (°C) VS (V) ICSOUT = 10µA –1.0 ICSOUT = 10µA SEE NOTE 5 FOR TEST SETUP 100 125 150 37961 G21 V(CSP-CSN) Offset Voltage with Different ICSOUT vs VS ICSOUT = 100µA 75 TEMPERATURE (°C) 37961 G20 1 100 125 150 7.8 75°C 200 60 75 7.9 150°C 1000 4 50 INTVCC vs Temperature VIN = 6V 1200 5 25 8.0 INTVCC (V) 1800 INTVCC DROPOUT (mV) 9 40 0 37961 G19 INTVCC Dropout Voltage vs Current, Temperature 6 INTVCC (V) VIN = 48V 70 37961 G18 INTVCC vs VIN 20 80 TEMPERATURE (°C) 37961 G17 0 90 VIN (V) DUTY CYCLE (%) 0 INTVCC Current Limit vs Temperature INTVCC Current Limit vs VIN 120 100 TA = 25°C, unless otherwise noted. –2 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 37961 G24 37961 G25 3796fb 8 For more information www.linear.com/3796 LT3796/LT3796-1 Typical Performance Characteristics Top Gate (PMOS) Rise/Fall Time vs Capacitance NMOS Gate Rise/Fall Time vs Capacitance Current Sense Amplifier Gain vs Frequency 30 160 800 25 140 700 120 600 15 TIME (ns) 10 5 0 –5 100 RISE TIME 80 60 FALL TIME 40 –10 VS = 48V, RIN = 1k –15 ROUT = 10k, VSENSE = 100mV (NOTE 5) –20 0.1 1 10 100 0.01 TIME (ns) 20 GAIN (dB) TA = 25°C, unless otherwise noted. 0 0 10 20 30 40 85V 75V 50 0 0 1 2 100ns/DIV PMOS VISHAY SILICONIX Si7113DN 3 4 5 6 7 8 9 10 CAPACITANCE (nF) 37961 G27 Top Gate Driver Rising Edge TG RISE TIME CAPACITANCE (nF) 37961 G26 5V 0V 300 100 FREQUENCY (kHz) PWM FALL TIME 400 200 20 1000 10000 500 37961 G28 Top Gate Driver Falling Edge PWM 5V 0V TG 85V 75V 3796 G29 100ns/DIV 37961 G30 PMOS VISHAY SILICONIX Si7113DN 3796fb For more information www.linear.com/3796 9 LT3796/LT3796-1 Pin Functions ISP (Pin 1): Connection Point for the Positive Terminal of the Current Feedback Resistor (RLED). Also serves as positive rail for TG pin driver. ISN (Pin 2): Connection Point for the Negative Terminal of the Current Feedback Resistor (RLED). TG (Pin 3): Top Gate Driver Output. An inverted PWM signal drives series PMOS device between VISP and (VISP – 7V) if VISP > 7V. An internal 7V clamp protects the PMOS gate by limiting VGS. Leave TG unconnected if not used. GND (Pins 4, 17, 21, 22, Exposed Pad Pin 29): Ground. These pins also serve as current sense input for control loop, sensing negative terminal of current sense resistor in the source of the N-channel MOSFET. Solder the exposed pad directly to ground plane. ISMON (Pin 5): ISP/ISN Current Report Pin. The LED current sensed by ISP/ISN inputs is reported as VISMON = ILED • RLED • 4. Leave ISMON pin unconnected if not used. When PWM is low, ISMON is driven to ground. Bypass with a 47nF capacitor or higher if needed. FB2 (Pin 6): Voltage Loop Feedback 2 Pin. This pin is connected to the internal transconductance amplifier positive input node. The internal transconductance amplifier with output VC regulates FB2 to 1.25V through the DC/ DC converter. On LT3796 only, the FB2 has an additional feature. If FB2 is driven above 1.3V, the TG pin is pulled high to turn off the external PMOS and GATE pin is driven to GND to turn off the external N-channel MOSFET. Connect to GND if not used. FB1 (Pin 7): Voltage Loop Feedback 1 Pin. FB1 is intended for constant-voltage regulation or for LED protection/open LED detection. The internal transconductance amplifier with output VC regulates FB1 to 1.25V (nominal) through the DC/DC converter. If the FB1 input is regulating the loop and V(ISP-ISN) is less than 25mV (normal), the VMODE pull-down is asserted. This action may signal an open LED fault. If FB1 is driven above the 1.3V (by an external power supply spike, for example), the FAULT pull-down is asserted, the GATE pin is pulled low to turn off the external N-channel MOSFET and the TG pin is driven high to protect the LEDs from an overcurrent event. Do not leave the FB1 pin open. If not used, connect FB1 to GND. VC (Pin 8): Transconductance Error Amplifier Output Pin. Used to stabilize the control 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. Do not leave this pin open. CTRL (Pin 9): Current Sense Threshold Adjustment Pin. Regulating threshold V(ISP-ISN) is 0.25 • VCTRL plus an offset for 0.1V < VCTRL < 1V. For VCTRL > 1.2V the current sense threshold is constant at the full-scale value of 250mV. For 1V < VCTRL < 1.2V, the dependence of the current sense threshold upon VCTRL transitions from a linear function to a constant value, reaching 98% of full-scale value by VCTRL = 1.1V. Connect CTRL to VREF for the 250mV default current threshold. Do not leave this pin open. Pull CTRL pin to GND for zero LED current. VREF (Pin 10): Voltage Reference Output Pin. Typically 2.015V. This pin drives a resistor divider for the CTRL pin, either for analog dimming or for temperature limit/ compensation of LED load. It can supply up to 100μA. SS (Pin 11): Soft-Start Pin. This pin modulates oscillator frequency and compensation pin voltage (VC) clamp. The soft-start interval is set with an external capacitor. The pin has a 28μA (typical) pull-up current source to an internal 2.5V rail. This pin can be used as fault timer. Provided the SS pin has exceeded 1.7V, the pull-up current source is disabled and a 2.8µA pull-down current enabled when any one of the following fault conditions happen: 1. LED overcurrent 2. INTVCC undervoltage 3. Thermal limit The SS pin must be discharged below 0.2V to reinitiate a soft-start cycle. Switching is disabled until SS is recharged. RT (Pin 12): Switching Frequency Adjustment Pin. Set the frequency using a resistor to GND (for resistor values, see the Typical Performance curve or Table 2). Do not leave the RT pin open. 3796fb 10 For more information www.linear.com/3796 LT3796/LT3796-1 Pin Functions SYNC (Pin 13) LT3796: The SYNC pin is used to synchronize the internal oscillator to an external logic level signal. If SYNC is used, the RT 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. Use a 50% duty cycle waveform to drive this pin. If not used, tie this pin to GND. TGEN (Pin 13) LT3796-1: Top Gate Driver Enable Pin. TGEN signal low causes the TG pin to transition high and turn off a PMOS switch, independent of the PWM input. Unlike PWM, TGEN low does not put the switching regulator in an idle state. TGEN can be used in combination with TG and a PMOS switch to deactivate one of two output current regulation loops. Tie the TGEN pin to 1.5V or higher if its function is not used. There is an equivalent 90k resistor from TGEN to ground internally. PWM (Pin 14): PWM Input Signal Pin. A signal low turns off switching, idles the oscillator, disconnects the VC pin from all internal loads, and makes the TG pin high. There is an equivalent 500k resistor from PWM pin to GND internally. If not used, tie this pin to VREF. FAULT (Pin 15): An open-collector pull-down on FAULT asserts when any of the following conditions happen: 1. FB1 overvoltage (VFB1 > 1.3V), 2. INTVCC undervoltage, 3. LED overcurrent (V(ISP-ISN) > 375mV), or 4. Thermal shutdown. If all faults are removed, FAULT flag returns high. Fault status is only updated during PWM high state and latched during PWM low state. FAULT remains asserted until the SS pin is discharged below 0.2V for cases 2, 3 and 4 above. VMODE (Pin 16): An open-collector pull-down on VMODE asserts if the FB1 input is above 1.19V (typical), and V(ISP-ISN) is less than 25mV (typical). To function, the pin requires an external pull-up resistor. VMODE status is updated only during PWM high state and latched during PWM low state. SENSE (Pin 18): The current sense input for the control loop. Kelvin connect this pin to the positive terminal of the switch current sense resistor, RSENSE, in the source of the N-channel MOSFET. The negative terminal of the current sense resistor should be Kelvin connected to the GND plane of the IC. GATE (Pin 19): N-Channel MOSFET Gate Driver Output. Switches between INTVCC and GND. It is driven to GND during shutdown, fault or idle states. INTVCC (Pin 20): Regulated Supply for Internal Loads, GATE Driver and Top Gate (PMOS) Driver. Supplied from VIN and regulates to 7.7V (typical). INTVCC must be bypassed with a 4.7μF capacitor placed close to the pin. Connect INTVCC directly to VIN if VIN is always less than or equal to 7V. VIN (Pin 23): Input Supply Pin. Must be locally bypassed with a 0.22μF (or larger) capacitor placed close to the IC. EN/UVLO (Pin 24): Enable and Undervoltage Lockout Pin. An accurate 1.22V falling threshold with externally programmable hysteresis detects when power is OK to enable switching. Rising hysteresis is generated by the external resistor divider and an accurate internal 3μA pull-down current. Above the threshold (but below 6V), EN/UVLO input bias current is sub-μA. Below the falling threshold, a 3μA pull-down current is enabled so the user can define the hysteresis with the external resistor selection. An undervoltage condition resets soft-start. Tie to 0.4V, or less, to disable the device and reduce VIN quiescent current below 1μA. VS (Pin 25): Current Sense Amplifier Power Supply Pin. This pin supply current to the current sense amplifier and can operate from 3V to 100V. CSN (Pin 26): Negative Current Sense Input Terminal. CSN remains functional for voltages up to 100V. Typically connected to VS and CSP as shown in Figure 9. CSP (Pin 27): Positive Current Sense Input Terminal. The internal sense amplifier sinks current from CSP to regulate it to the same potential as CSN. A resistor (RIN1) tied from VIN to CSP sets the output current ICSOUT = VSNS/RIN1. VSNS is the voltage developed across RSNS. See Figure 9. CSOUT (Pin 28): Current Sense Amplifier Output. CSOUT pin sources the current that is drawn from CSP. Typically is output to an external resistor to GND. 3796fb For more information www.linear.com/3796 11 LT3796/LT3796-1 Block Diagram EN/UVLO FB1 A1 – VC SHDN PWM ISP-7V VIN 1.25V + 1.25V 1.5V VLED X1 7.7V INTVCC 10µA AT FB1 = 1.25V LT3796-1 ONLY SCILMB A5 – A4 SHORT-CIRCUIT DETECT – LDO + gm 3µA ISMON TG ISP A2 – + 1.22V TGEN (LT3796-1 ONLY) TGOFFB 2.5V + 10µA ISP + 100mV –+ X4 ISN – – + + 1.1V CTRL FB2 gm EAMP FAULTB + R A7 A6 – 10µA AT A6+ = A6– S PWM COMPARATOR ILIM A8 – + – + 1.25V 5.5V 113mV A9 A9 gm VS GATE DRIVER Q 10µA AT FB2 = 1.25V 5.5V CSP 2.5V – CSN 28µA THERMAL SHDN A11 + PWM CSOUT ISENSE SS SS AND FAULT LOGIC FAULTB FAULT 1mA + 1.3V – – GND RAMP GENERATOR 2.8µA 100kHz TO 1MHz OSCILLATOR INTVCC SCILMB FB1 SENSE A9 A10 TGOFFB 5.5V + VMODE A3 OVFB1 1.19V 0VFB COMPARATOR + FB2 INTVCC 100µA 1V A16 0VFB2 1.3V – LT3796 ONLY + + – – A12 A7 A13 FREQ PROG VREF FB1 + VLED – A14 A7 100mV – C/10 COMPARATOR WITH 200mV HYSTERESIS A15 2.015V + + SS RT SYNC (LT3796 ONLY) 37961 BD LT3796/LT3796-1 Block Diagram 3796fb 12 For more information www.linear.com/3796 LT3796/LT3796-1 Operation The LT3796/LT3796-1 are constant-frequency, current mode controllers with a low side NMOS gate driver. The operation of the LT3796/LT3796-1 is best understood by referring to the Block Diagram. In normal operation, with the PWM pin low, the GATE pin is driven to GND, the TG pin is pulled high to ISP to turn off the PMOS disconnect switch, the VC pin goes 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 TG pin transitions low after a short delay. At the same time, the internal oscillator wakes up and generates a pulse to set the PWM latch, turning on the external power N-channel 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 negative 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 difference 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 113mV (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, any fault condition, i.e. FB1 overvoltage (VFB1 > 1.3V), LED over current, or INTVCC undervoltage (INTVCC < 4V), the GATE pin is pulled down to GND immediately. 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 FB1 and FB2 pins. If FB1 and FB2 are both lower than the reference voltage, the switch current increases; if FB1 or FB2 is higher than the reference voltage, the switch demand current decreases. The LED current sense feedback interacts with the voltage feedback so that neither FB1 or FB2 exceeds the internal reference and the voltage between ISP and ISN does 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, FB1 and FB2 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 VREF . Two LED specific functions featured on the LT3796/LT37961 are controlled by the voltage feedback FB1 pin. First, when the FB1 pin exceeds a voltage 60mV lower (–5%) than the FB1 regulation voltage and V(ISP-ISN) is less than 25mV (typical), the pull-down driver on the VMODE pin is activated. This function 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 FB1 pin exceeds the FB1 regulation voltage by 50mV (4% typical), the FAULT pin is activated. LT3796/LT3796-1 feature a PMOS disconnect switch driver. The PMOS disconnect switch can be used to improve the PWM dimming ratio, and operate as fault protection as well. Once a fault condition is detected, the TG pin is pulled high to turnoff the PMOS switch. The action isolates the LED array from the power path, preventing excessive current from damaging the LEDs. A standalone current sense amplifier is integrated in the LT3796/LT3796-1. It can work as input current limit or open LED protection. The detailed information can be found in the Application Information section. 3796fb For more information www.linear.com/3796 13 LT3796/LT3796-1 Applications Information INTVCC Regulator Bypassing and Operation The INTVCC pin requires a capacitor for stable operation and to store the charge for the large GATE switching currents. Choose a 10V rated low ESR, X7R or X5R ceramic capacitor for best performance. A 4.7μF ceramic capacitor is adequate for many applications. Place the capacitor close to the IC to minimize the trace length to the INTVCC pin and also to the IC ground. is to allow the user to program the rising hysteresis. The following equations should be used to determine the values of the resistors: R1+ R2 VIN(FALLING) = 1.22 • R2 V =V + 3µA • R1 IN(RISING) IN(FALLING) VIN An internal current limit on the INTVCC output protects the LT3796/LT3796-1 from excessive on-chip power dissipation. The minimum value of this current limit should be considered when choosing the switching N-channel MOSFET and the operating frequency. IINTVCC can be calculated from the following equation: LT3796/LT3796-1 R1 EN/UVLO R2 37961 F01 Figure 1. IINTVCC = QG • fOSC Careful choice of a lower QG MOSFET allows higher switching frequencies, leading to smaller magnetics. The INTVCC pin has its own undervoltage disable (UVLO) set to 4V (typical) to protect the external FETs from excessive power dissipation caused by not being fully enhanced. If the INTVCC pin drops below the UVLO threshold, the GATE pin is forced to 0V, TG pin is pulled high and the soft-start pin will be reset. If the input voltage, VIN, will not exceed 7V, then the INTVCC pin should be connected to the input supply. Be aware that a small current (typically 10μA) loads the INTVCC in shutdown. If VIN is normally above, but occasionally drops below the INTVCC regulation voltage, then the minimum operating VIN is close to 6V. This value is determined by the dropout voltage of the linear regulator and the 4V INTVCC undervoltage lockout threshold mentioned above. LED Current Programming Programming the Turn-On and Turn-Off Thresholds with the EN/UVLO Pin ILED = 0, VCTRL = 0V The falling UVLO value can be accurately set by the resistor divider. A small 3μA pull-down current is active when EN/ UVLO is below the threshold. The purpose of this current The LED current is programmed by placing an appropriate value current sense resistor RLED between the ISP and ISN pins. Typically, sensing of the current should be done at the top of the LED string. If this option is not available, then the current may be sensed at the bottom of the string. The CTRL pin should be tied to a voltage higher than 1.2V to get the full-scale 250mV (typical) threshold across the sense resistor. The CTRL pin can also be used to dim the LED current to zero, although relative accuracy decreases with the decreasing voltage sense threshold. When the CTRL pin voltage is less than 1V, the LED current is: ILED = VCTRL – 100mV , 0.1V < VCTRL ≤ 1V RLED • 4 When the CTRL pin voltage is between 1V and 1.2V, the LED current varies with CTRL, but departs from the previous equation by an increasing amount as the CTRL voltage increases. Ultimately above 1.2V, the LED current no 3796fb 14 For more information www.linear.com/3796 LT3796/LT3796-1 Applications Information longer varies with CTRL. The typical V(ISP-ISN) threshold vs CTRL is listed in the Table 1. Table 1. V(ISP-ISN) Threshold vs CTRL VCTRL (V) V(ISP-ISN) (mV) 1 225 1.05 236 1.1 244.5 1.15 248.5 1.2 250 regulation. FB2 does not have this extra comparator. The output voltage can be set by selecting the values of R3 and R4 (see Figure 2) according to the following equation: VOUT =1.25 • R3+R4 R4 VOUT LT3796/LT3796-1 R3 FB1 When CTRL is higher than 1.2V, the LED current is regulated to: ILED = R4 37961 F02 Figure 2. Feedback Resistor Connections for Boost and SEPIC Applications 250mV RLED The CTRL pin should not be left open (tie to VREF if not used). The CTRL pin can also be used in conjunction with a thermistor to provide overtemperature protection for the LED load, or with a resistor divider to VIN to reduce output power and switching current when VIN is low. The presence of a time varying differential voltage signal (ripple) across ISP and ISN at the switching frequency is expected. The amplitude of this signal is increased by high LED load current, low switching frequency and/or a smaller value output filter capacitor. Programming Output Voltage (Constant-Voltage Regulation) or Open LED/Overvoltage Threshold The LT3796/LT3796-1 have two voltage feedback pins, FB1 and FB2. Either one can be used for a boost or SEPIC application. The difference between these two pins is FB1 has a comparator that senses when FB1 exceeds VFB – 60mV (VMODE threshold) and asserts the VMODE output if V(ISP-ISN) is less than 25mV. This indicates that the output is in voltage regulation mode and not current For a boost type LED driver, set the resistor from the output to the FB1 pin such that the expected VFB1 during normal operation does not exceed 1.15V. For an LED driver of buck mode or a buck-boost mode configuration, the FB voltage is typically level shifted to a signal with respect to GND as illustrated in Figure 3. The output can be expressed as: R5 R6 +R7 • for Figure 3a R8 R6 R9 + VBE(Q1) for Figure 3b or VOUT = 1.25 • R10 VOUT = 1.25 • VS LT3796/ LT3796-1 + R5 CSP R6 VOUT RLED LED STRING R7 CSN – CSOUT FB1 37961 F03a R8 Figure 3a. Feedback Resistor Connection for Buck Mode or Buck-Boost Mode LED Driver 3796fb For more information www.linear.com/3796 15 LT3796/LT3796-1 Applications Information + RSENSE R9 VOUT LT3796/ LT3796-1 – Q1 LED ARRAY FB1 37961 F03b R10 Figure 3b. Feedback Resistor Connection for Buck Mode or Buck-Boost Mode LED Driver Using External PNP recommended to limit the transient short-circuit current. With the PNP helper, the short-circuit current can be limited to 2A, whereas the short-circuit current can reach to 20A without the PNP helper as shown in Figure 5 and Figure 6 respectively. Refer to boost LED driver with output short-circuit protection and LED current monitor for the test schematic. Note that the impedance of the short-circuit cable affects the peak current. L1 D1 VIN C1 Open LED Detection VIN GATE SENSE LT3796/ LT3796-1 The LT3796/LT3796-1 provide an open-collector status pin, VMODE, that pulls low when the FB1 pin is above 1.19V and V(ISP-ISN) is less than 25mV. If the open LED clamp voltage is programmed correctly using the resistor divider, then the FB1 pin should never exceed 1.15V when LEDs are connected, therefore, the only way for the FB1 pin to be within 60mV of the 1.25V regulation voltage is for an open LED event to have occurred. C2 M1 RSNS ISP Q1 RLED ISN TG M2 LED+ LED STRING LED Over Current Protection Feature GND (BOOST) OR VIN (BUCK-BOOST MODE) The ISP and ISN pins have a short-circuit protection feature independent of the LED current sense feature. This feature prevents the development of excessive switching currents and protects the power components. The short-circuit protection threshold (375mV, typ) is designed to be 50% higher than the default LED current sense threshold. Once the LED over current is detected, the GATE pin is driven to GND to stop switching, and TG pin is pulled high to disconnect the LED array from the power path. A typical LED short-circuit protection scheme for boost or buck-boost mode converter is shown in Figure 4. The Schottky diode D2 should be put close to the drain of M2 on the board. It protects the LED+ node from swinging well below ground when being shorted to ground through a long cable. Usually, the internal protection loop takes about 1µs to respond. Including PNP helper Q1 is D2 37961 F04 Figure 4. The Simplified LED Short-Circuit Protection Schematic for Boost/Buck-Boost Mode LED Driver LED+ 50V/DIV IM2 10A/DIV FAULT 10V/DIV 1µs/DIV 37961 F05 Figure 5. Short-circuit Current without PNP Helper 3796fb 16 For more information www.linear.com/3796 LT3796/LT3796-1 Applications Information A second method uses the PWM pin to modulate the LED current between zero and full current to achieve a precisely programmed average current, without the possibility of color shift that occurs at low current in LEDs. To make PWM dimming more accurate, the switch demand current is stored on the VC node during the quiescent phase when PWM is low. This feature minimizes recovery time when the PWM signal goes high. To further improve the recovery time, a disconnect switch should be used in the LED current path to prevent the output capacitor from discharging during the PWM signal low phase. The minimum PWM on or off time depends on the choice of operating frequency through the RT input. For best current accuracy, the minimum PWM high time should be at least three switching cycles (3μs for fSW = 1MHz). LED+ 50V/DIV IM2 1A/DIV FAULT 10V/DIV 1µs/DIV 37961 F06 Figure 6. Short-circuit Current with PNP Helper VIN RLED M2 LED+ Q1 VIN ISP LED STRING ISN TG D3 C2 LED– LT3796/LT3796-1 D2 L1 D1 GATE M1 C1 SENSE 37961 F07 RSNS Figure 7. The Simplified LED Short-Circuit Protection Schematic for Buck Mode Converter Similar to boost, Schottky diodes D2, D3 and PNP transistor Q1 are recommended to protect short-circuit event in the buck mode. PWM Dimming Control for Brightness There are two methods to control the LED current for dimming using the LT3796/LT3796-1. One method uses the CTRL pin to adjust the current regulated in the LEDs. A low duty cycle PWM signal can cause excessive start-up times if it were allowed to interrupt the soft-start sequence. Therefore, once start-up is initiated by PWM > 1V, it will ignore a logical disable by the external PWM input signal. The device will continue to soft-start with switching and TG enabled until either the voltage at SS reaches the 1.0V level, or the output current reaches one-fourth of the fullscale current. At this point the device will begin following the dimming control as designated by PWM. If at any time an output overcurrent is detected, GATE and TG will be disabled even as SS continues to charge. Programming the Switching Frequency The RT frequency adjust pin allows the user to program the switching frequency from 100kHz to 1MHz to optimize efficiency/performance or external component size. Higher frequency operation yields smaller component size but increases switching losses and gate driving current, and may not allow sufficiently high or low duty cycle operation. Lower frequency operation gives better performance at the cost of larger external component size. For an appropriate 3796fb For more information www.linear.com/3796 17 LT3796/LT3796-1 Applications Information RT resistor value see Table 2. An external resistor from the RT pin to GND is required—do not leave this pin open. respectively. The following equations express the minimum/ maximum duty cycle: Table 2. Typical Switching Frequency vs RT Value (1% Resistor) Min Duty Cycle = minimum on-time • switching frequency RT(kΩ) 1000 6.65 900 7.50 800 8.87 700 10.2 600 12.4 500 15.4 400 19.6 300 26.1 200 39.2 100 82.5 Max Duty Cycle = 1 – minimum off-time • switching frequency 350 300 250 TIME (ns) fosc(kHz) MIN ON-TIME 200 MIN OFF-TIME 150 100 Frequency Synchronization 50 The LT3796 switching frequency can be synchronized to an external clock using the SYNC pin. For proper operation, the RT 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 must have the aligned rising edges to achieve the optimized high PWM dimming ratio. If the SYNC pin is not used, it should be connected to GND. Duty Cycle Considerations Switching duty cycle is a key variable defining converter operation, therefore, its limits must be considered when programming the switching frequency for a particular application. The fixed minimum on-time and minimum off-time (see Figure 8) and the switching frequency define the minimum and maximum duty cycle of the switch, 0 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 37961 F08 Figure 8. Typical Minimum On- and Off-Time vs Temperature When calculating the operating limits, the typical values for on/off-time in the data sheet should be increased by at least 100ns to allow margin for PWM control latitude, GATE rise/fall times and SW node rise/fall times. Setting Input Current Limit The LT3796/LT3796-1 have a standalone current sense amplifier. It can be used to limit the input current. As shown in Figure 9, the input current signal is converted to voltage output at CSOUT pin. When the CSOUT voltage exceeds FB2 regulation voltage, the GATE is pulled low, and the converter stops switching. The input current limit is calculated as follows: IIN = 1.25 • RIN1 ROUT • RSNS 3796fb 18 For more information www.linear.com/3796 LT3796/LT3796-1 Applications Information For buck applications, filter components, RIN2(OPT) and COPT, are recommended to be placed close to LT3796/ LT3796-1 to suppress the substantial transient signal or noise at across CSN and CSP pins. For boost and buckboost applications, RIN2(OPT) and COPT are not required. +VSNS– RSNS TJ = TA + [VIN • (IQ + fSW • QG) •θJA] where TA is the ambient temperature, IQ is the VIN operating current of the part (2.5mA typical) and θJA is the package thermal impedance (30°C/W for the TSSOP package). For example, an application with TA(MAX) = 85°C, VIN(MAX) = 60V, fSW = 400kHz, and having a N-channel MOSFET with QG = 20nC, the maximum IC junction temperature will be approximately: IIN VIN TO LOAD RIN2(OPT) RIN1 COPT CSN CSP – VS TJ = 85°C + [60V • (2.5mA + 400kHz • 20nC) • 30°C/W] VS ≈ 104°C + LT3796/LT3796-1 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. CSOUT FB2 37961 F03 CFILT ROUT Figure 9. Setting Input Current Limit Thermal Considerations The LT3796/LT3796-1 are rated to a maximum input voltage of 100V. Careful attention must be paid to the internal power dissipation of the IC at higher input voltages to ensure that a junction temperature of 150°C is not exceeded. This junction limit is especially important when operating at high ambient temperatures. The majority of the power dissipation in the IC comes from the supply current needed to drive the gate capacitance of the external power N-channel MOSFET. This gate drive current can be calculated as: IGATE = fSW • QG A low QG power MOSFET should always be used when operating 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 junction temperature, TJ of the IC can be estimated by: It is best if the copper plane is extended on either the top or bottom layer of the PCB to have the maximum exposure to air. Internal ground layers do not dissipate thermals as much as top and bottom layer copper does. See recommended layout as an example. 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 3796fb For more information www.linear.com/3796 19 LT3796/LT3796-1 Applications Information require a lower value capacitor than a buck mode converter. Assuming that a 100mV input voltage ripple is acceptable, the required capacitor value for a boost converter can be estimated as follows (TSW = 1/fOSC): V 1µF CIN (µF) =ILED(A) • LED • TSW (µs) • VIN A • µs • 2.8 Therefore, a 2.2µF capacitor is an appropriate selection for a 400kHz boost regulator with 12V input, 48V output and 500mA load. With the same VIN voltage ripple of less than 100mV, the input capacitor for a buck converter can be estimated as follows: CIN(µF)=ILED(A)• VLED (VIN – V LED) 10µF • T (µs)• SW A • µs VIN2 A 10µF input capacitor is an appropriate selection for a 400kHz buck mode converter with 24V input, 12V output and 1A load. In the buck mode configuration, the input capacitor has large pulsed currents due to the current returned through the Schottky diode when the switch is off. It is important to place the capacitor as close as possible to the Schottky diode and to the GND return of the switch (i.e., the sense resistor). It is also important to consider the ripple current rating of the capacitor. For best reliability, this capacitor should have low ESR and ESL and have an adequate ripple current rating. The RMS input current for a buck mode LED driver is: IIN(RMS) = ILED • √(1–D)D D= V LED VIN where D is the switch duty cycle. Table 3. Recommended Ceramic Capacitor Manufacturers MANUFACTURER WEB TDK www.tdk.com Kemet www.kemet.com Murata www.murata.com Taiyo Yuden www.t-yuden.com AVX www.avx.com Output Capacitor Selection The selection of the output capacitor depends on the load and converter configuration, i.e., step-up or step-down and the operating frequency. For LED applications, the equivalent resistance of the LED is typically low and the output filter capacitor should be sized to attenuate the current ripple. Use of an X7R type ceramic capacitor is recommended. To achieve the same LED ripple current, the required filter capacitor is larger in the boost and buck-boost mode applications than that in the buck mode applications. Lower operating frequencies will require proportionately higher capacitor values. Power MOSFET Selection For applications operating at high input or output voltages, the power N-channel MOSFET switch is typically chosen for drain voltage VDS rating and low gate charge QG. Consideration of switch on-resistance, RDS(ON), is usually secondary because switching losses dominate power loss. The INTVCC regulator on the LT3796/LT3796-1 has a fixed current limit to protect the IC from excessive power dissipation at high VIN, so the MOSFET should be chosen so that the product of QG at 7.7V and switching frequency does not exceed the INTVCC current limit. For driving LEDs be careful to choose a switch with a VDS rating that exceeds the threshold set by the FB pin in case of an open load fault. Several MOSFET vendors are listed 3796fb 20 For more information www.linear.com/3796 LT3796/LT3796-1 Applications Information in Table 4. The MOSFETs used in the application circuits in this data sheet have been found to work well with the LT3796/LT3796-1. Consult factory applications for other recommended MOSFETs. Table 4. MOSFET Manufacturers VENDOR WEB Vishay Siliconix www.vishay.com Fairchild www.fairchildsemi.com International Rectifier www.irf.com Infineon www.infineon.com High Side PMOS Disconnect Switch Selection A high side PMOS disconnect switch with a minimum VTH of –1V to –2V is recommended in most LT3796/LT3796-1 applications to optimize or maximize the PWM dimming ratio and protect the LED string from excessive heating during fault conditions as well. The PMOS disconnect switch is typically selected for drain-source voltage VDS, and continuous drain current ID. For proper operations, VDS rating must exceed the open LED regulation voltage set by the FB1 pin, and ID rating should be above ILED. Schottky Rectifier Selection The power Schottky diode conducts current during the interval when the switch is turned off. Select a diode rated for the maximum SW voltage. If using the PWM feature for dimming, it is important to consider diode leakage, which increases with the temperature, from the output during the PWM low interval. Therefore, choose the Schottky diode with sufficiently low leakage current. Table 5 has some recommended component vendors. Table 5. Schottky Rectifier Manufacturers VENDOR WEB On Semiconductor www.onsemi.com Diodes, Inc www.diodes.com Central Semiconductor www.centralsemi.com Rohm Semiconductor www.rohm.com Sense Resistor Selection The resistor, RSENSE, between the source of the external N-channel MOSFET and GND should be selected to provide adequate switch current to drive the application without exceeding the 113mV (typical) current limit threshold on the SENSE pin of LT3796/LT3796-1. For buck mode applications, select a resistor that gives a switch current at least 30% greater than the required LED current. For buck mode, select a resistor according to: RSENSE(BUCK) ≤ 0.07V ILED For buck-boost mode, select a resistor according to: RSENSE(BUCK − BOOST ) ≤ VIN • 0.07V (VIN +VLED)ILED For boost, select a resistor according to: RSENSE(BOOST ) ≤ VIN • 0.07V VLED • ILED The placement of RSENSE should be close to the source of the NMOS FET and GND of the LT3796/LT3796-1. The SENSE input to LT3796/LT3796-1 should be a Kelvin connection to the positive terminal of RSENSE. 70mV is used in the equations above to give some margin below the 113mV (typical) sense current limit threshold. Inductor Selection The inductor used with the LT3796/LT3796-1 should have a saturation current rating appropriate to the maximum switch current selected with the RSENSE resistor. Choose an inductor value based on operating frequency, input and output voltage to provide a current mode signal on SENSE 3796fb For more information www.linear.com/3796 21 LT3796/LT3796-1 Applications Information of approximately 20mV magnitude. The following equations are useful to estimate the inductor value (TSW = 1/fOSC): LBUCK = TSW •RSENSE • VLED (V IN – VLED ) V IN • 0.02V LBUCK, BOOST = TSW •RSENSE • VLED • V IN (VLED +V IN) • 0.02V Soft-Start Capacitor Selection For many applications, it is important to minimize the inrush current at start-up. The built-in soft-start circuit significantly reduces the start-up current spike and output voltage overshoot. The soft-start interval is set by the soft-start capacitor selection according to the equation: T •R • V (V – V ) LBOOST = SW SENSE IN LED IN VLED • 0.02V Table 6 provides some recommended inductor vendors. Table 6. Inductor Manufacturers VENDOR WEB Sumida www.sumida.com Würth Elektronik www.we-online.com Coiltronics www.cooperet.com Vishay www.vishay.com Coilcraft www.coilcraft.com TSS = CSS • 2V 28µA A typical value for the soft-start capacitor is 0.1µF. The soft-start pin reduces the oscillator frequency and the maximum current in the switch. Soft-start also operates as fault protection, which forces the converter into hiccup or latchoff mode. Detailed information is provided in the Fault Protection: Hiccup Mode and Latchoff Mode section. Fault Protection: Hiccup Mode and Latchoff Mode Loop Compensation The LT3796/LT3796-1 use an internal transconductance error amplifier whose VC output compensates the control loop. The external inductor, output capacitor and the compensation 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 22nF 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. If an LED overcurrent condition, INTVCC undervoltage, or thermal limit happens, an open-drain pull-down on FAULT asserts. The TG pin is pulled high to disconnect the LED array from the power path, and the GATE pin is driven low. If the soft-start pin is charging and still below 1.7V, then it will continue to do so with a 28µA source. Once above 1.7V, the pull-up source is disabled and a 2.8µA pull-down is activated. While the SS pin is discharging, the GATE is forced low. When SS pin is discharged below 0.2V, a new cycle is initiated. This is referred as hiccup mode operation. If the fault still exists when SS crosses below 0.2V, then a full SS charge/discharge cycle has to complete before switching is enabled and the FAULT flag is deasserted. If a resistor is placed between VREF pin and SS pin to hold SS pin higher than 0.2V during a fault, then the LT3796/ LT3796-1 will enter latchoff mode with GATE pin low, TG pin high and FAULT pin low. To exit latchoff mode, the EN/ UVLO pin must be toggled low to high. 3796fb 22 For more information www.linear.com/3796 LT3796/LT3796-1 Applications Information Board Layout The high speed operation of the LT3796/LT3796-1 demands careful attention to board layout and component placement. The exposed pad of the package is the 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 LT3796/LT3796-1. Likewise, the ground terminal of the bypass capacitor for the INTVCC regulator should be placed near the GND of the switching path. Typically, this requirement results in the external switch being closest to the IC, along with the INTVCC 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 FB1, FB2, RT and VC, as they may pick up switching noise. Since there is a small variable DC input bias current to the ISN and ISP inputs, resistance in series with these pins should be minimized to avoid creating an offset in the current sense threshold. Likewise, minimize resistance in series with the SENSE input to avoid changes (most likely reduction) to the switch current limit threshold. Figure 10 is a suggested two sided layout for a boost converter. Note that the 4-layer layout is recommended for best performance. Please contact the factory for the reference layout design. 3796fb For more information www.linear.com/3796 23 LT3796/LT3796-1 Applications Information X VIN VIA VIN RSNS1 X VIAS TO GROUND PLANE X ROUTING ON THE 2nd LAYER C3 VIA FROM ISP R6 VIA FROM ISN X 1 VIA FROM TG X VIA FROM OUT R7 ISMON R8 CC RC R2 X VIA FROM VIN R1 VREF C6 C4 27 X 3 26 C1 R5 L1 4 25 X 5 24 6 23 X 7 22 8 21 9 20 10 19 11 C1 28 LT3796 X 2 12 RT C1 VIA FROM VIN R3 R4 VIA FROM VIN C5 INTVCC VIA X X 1 18 29 2 17 SYNC 13 16 PWM 14 15 GATE VIA 3 8 7 M1 6 X 4 R9 R10 X VIA FROM INTVCC RSNS 5 D1 VIA FROM GATE C2 C2 C2 X OUT VIA C2 TG VIA D2 LED+ 5 4 6 3 7 8 M2 Q1 X 2 RLED 1 37961 F10 ISN VIA X X ISP VIA COMPONENT DESIGNATIONS REFER TO BOOST LED DRIVER WITH OUTPUT SHORT CIRCUIT PROTECTION AND LED CURRENT MONITOR Figure 10. Boost Converter Suggested Layout 3796fb 24 For more information www.linear.com/3796 LT3796/LT3796-1 Typical Applications Boost LED Driver with Output Short Circuit Protection and LED Current Monitor VIN 9V TO 60V 100V (TRANSIENT) C1 2.2µF ×3 R1 1M OPTIONAL INPUT CURRENT REPORTING R3 499k VS GATE R9 100k RSNS 15mΩ LT3796 PWM FB1 SYNC SYNC ISP ISMON FAULT INTVCC VMODE VREF R11 OPTIONAL FOR FAULT LATCHOFF R11 402k (OPT) RLED 620mΩ M2 TG VMODE SS FB2 RC 10k C6 0.1µF D2 C5 4.7µF RT VC INTVCC 85V LED RT 31.6k 250kHz CC 10nF 37961 TA02a Fault (Short LED) Protection without R11: Hiccup Mode Fault (Short LED) Protection with R11: Latchoff Mode SS 2V/DIV SS 2V/DIV LED+ 50V/DIV LED+ 50V/DIV FAULT 10V/DIV FAULT 10V/DIV IM2 1A/DIV IM2 1A/DIV 50ms/DIV Q1 ISN C4 0.1µF FAULT M1: INFINEON BCS160N10NS3-G M2: VISHAY SILICONIX Si7113DN L1: COILTRONICS DR127-220 D1: DIODES INC PDS5100 D2: VISHAY ES1C Q1: ZETEX FMMT589 LED: CREE XLAMP XR-E UP TO 400mA GND PWM LED CURRENT REPORTING R10 100k R8 13.7k M1 SENSE CSOUT INTVCC CSN CSP CTRL R6 40.2k C2 2.2µF ×4 100V R7 1M EN/UVLO R4 97.6k CSOUT C3 10nF D1 R5 2k VIN R2 118k L1 22µH RSNS1 50mΩ IIN 37961 TA02b 50ms/DIV 37961 TA02c 3796fb For more information www.linear.com/3796 25 LT3796/LT3796-1 Typical Applications Buck Mode LED Driver with Open LED Flag and LED Current Reporting VIN 24V TO 80V RLED 100mΩ LED+ M2 2.5A R3 49.9k R1 1M VS VIN R2 61.9k ISN CSP VREF CSN L1 33µH R6 59k FB1 FB2 LED CURRENT REPORTING C5 0.1µF SENSE INTVCC R8 100k M1 GATE ISMON C2 4.7µF ×2 25V R5 1M CSOUT LT3796 PWM 18V LED TG EN/UVLO CTRL PWM ISP R4 49.9k D1 C1 2.2µF ×3 100V RSNS 15mΩ R9 100k FAULT GND FAULT VMODE VMODE INTVCC VC RT RC 10k CC 4.7nF M1: VISHAY SILICONIX Si7454DP M2: VISHAY SILICONIX Si7113DN D1: DIODES INC PDS3100 L1: COILTRONICS HC9-330 LED: CREE XLAMP XM-L SYNC C3 4.7µF SS RT 19.6k 400kHz INTVCC C4 0.1µF 37961 TA03a Efficiency vs VIN 100 EFFICIENCY (%) 95 90 85 80 75 70 20 30 40 50 60 70 80 VIN (V) 37961 TA03b 3796fb 26 For more information www.linear.com/3796 LT3796/LT3796-1 Typical Applications SEPIC LED Driver Using FB2 for Input Overvoltage Protection C1 2.2µF ×3 100V R1 953k VIN VREF OPTIONAL INPUT CURRENT REPORTING CSOUT PWM M1 C2 10µF ×3 35V R10 909k R11 40.2k SENSE RSNS 15mΩ LT3796 FB2 R7 40.2k CSN GATE CTRL C3 0.1µF CSP L1B EN/UVLO R5 100k R3 20k VS D1 • R6 2k R4 511k R2 75k L1A 22µH RSNS1 50mΩ • IIN VIN 8V TO 60V C6 2.2µF 100V GND CSOUT FB1 PWM ISP UP TO 1A RLED 250mΩ ISN ISMON LED CURRENT REPORTING TG C7 0.1µF INTVCC R8 100k M2 R9 100k FAULT 22V LED FAULT VMODE VMODE INTVCC VC M1: VISHAY SILICONIX Si7456DP M2: VISHAY Si7415DN L1: COILTRONICS DRQ127-220 D1: DIODES INC PDS5100 LED: CREE XLAMP XR-E RT SYNC INTVCC C5 4.7µF SS RT 19.6k 400kHz RC 4.99k CC 10nF C4 0.1µF 37961 TA04a Efficiency vs VIN 100 EFFICIENCY (%) 95 90 85 80 75 70 0 10 20 30 40 50 60 VIN (V) 37961 TA04b 3796fb For more information www.linear.com/3796 27 LT3796/LT3796-1 Typical Applications SEPIC Sealed Lead Acid (SLA) Battery Charger RSNS1 50mΩ IIN D2 15V C1 4.7µF 50V R1 10k R2 806k R6 2k R4 357k VIN R3 20k CSN VS CSP R11 10k NTC + BAT RSNS 15mΩ LT3796 FB2 GND FB1 CSOUT CSOUT C3 10nF R12 30.1k VCHARGE = 14.6V VFLOAT = 13.5V AT 25°C SENSE VREF CTRL OPTIONAL INPUT CURRENT REPORTING BAT R13 93.1k M1 GATE OUT RSNS2 C2 250mΩ 10µF L1B EN/UVLO R5 100k D1 • M2 L1A 33µH • VIN 8V TO 40V 100V (TRANSIENT) C6 2.2µF 100V R7 40.2k VREF OUTPUT CURRENT REPORTING PWM OUT ISN BAT C4 0.1µF R9 113k R10 10.2k M3 ISMON C7 0.1µF VMODE SS SYNC VMODE R8 49.9k TG INTVCC VC M1: VISHAY SILICONIX Si7456DP M2: VISHAY SUD19P06-60-E3 M3: ZETEX ZXM61N03F L1: COILCRAFT MSD1260-333 D1: ON SEMI MBRS260T3G D2: CENTRAL SEMI CMDZ15L R11: MURATA NCP18XH103F03RB ISP RT FAULT C5 4.7µF RT 19.6k 400kHz RC 499Ω INTVCC R14 49.9k FAULT CC 10nF 37961 TA05a VCHARGE, VFLOAT vs Temperature 17.5 17.0 VCHARGE, VFLOAT (V) 16.5 16.0 15.5 VCHARGE 15.0 14.5 14.0 VFLOAT 13.5 13.0 12.5 –40 –30–20 –10 0 10 20 30 40 50 60 70 80 TEMPERATURE (°C) 37961 TA05b 3796fb 28 For more information www.linear.com/3796 LT3796/LT3796-1 Typical Applications 28VIN to 28V SuperCap Charger with Input Current Limit and Charge Done Flag RSNS1 150mΩ 1.33A MAX R1 20k VIN VS L1B CSP CSN EN/UVLO OUTPUT CURRENT REPORTING ISMON VREF RSNS 33mΩ LT3796 GND CSOUT FB1 FB2 ISP R6 100k INTVCC C6 4.7µF FAULT CHGDONE 1.67A MAX SUPERCAP TG FAULT R9 24.9k ISN INTVCC R7 100k C2 4.7µF ×2 50V R8 536k RSNS2 150mΩ SS C4 0.1µF VOUT = 0V TO 28V SENSE SYNC R2 124k M1 GATE PWM INPUT CURRENT REPORTING AND LIMIT CSOUT C3 0.1µF D1 • C1 10µF C7 0.1µF C6 2.2µF ×2 L1A 33µH • VIN 28V VMODE CTRL VREF VC RT RC 499Ω VOUT L1: COILCRAFT MSD1260-333 D1: ON SEMI MBRS260T3G M1: VISHAY SILICONIX Si7850 Q1: ZETEX FMMT591A R3 499k C5 0.1µF R4 30.1k R10 499k CC 22nF Q1 R5 1M RT 19.6k 400kHz 37961 TA06a Input and Output Current vs Output Voltage 1800 IOUT INPUT/OUTPUT CURRENT (mA) 1600 1400 1200 IIN 1000 800 600 400 200 0 0 5 10 15 20 25 30 VOUT (V) 37961 TA06b 3796fb For more information www.linear.com/3796 29 LT3796/LT3796-1 Typical Applications SEPIC Converter with RWIRE Compensation and Output Current Limit C2 10µF L1A 22µH OUT RSNS1 250mΩ D1 • VIN 12V 1:1 • C1 10µF M1 C3 10µF L1B + RWIRE C4 100µF 25V VLOAD 12V, 1A CURRENT LIMIT RSNS 33mΩ VIN C8 0.1µF GATE GND ISP ISMON ISN PWM VREF SENSE EN/UVLO LT3796 SYNC R3 154k SS VS CTRL VREF INTVCC R7 100k R2 38.3k CSN FB2 C5 0.1µF R1 38.3k C7 1µF OUT CSP R6 100k R4 287k CSOUT FAULT FAULT VMODE FB1 VMODE TG VC RT RT 19.6k 400kHz R5 12.4k INTVCC RC 24.9k INTVCC CC 10nF L1: WÜRTH 744871220 D1: ZETEX ZLLS2000TA M1: VISHAY SILICONIX Si4840DY C6 4.7µF 37961 TA08a Line Impedance Compensation 13.0 RWIRE = 0.5Ω Load Step Response VOUT 800mA IOUT 500mA/DIV 12.5 VOUT/VLOAD (V) VLOAD 12.0 11.5 VOUT 500mV/DIV (AC) 11.0 500µs/DIV 10.5 10.0 200mA 0 200 400 600 800 1000 37961 TA08c 1200 ILOAD (mA) 37961 TA08b 3796fb 30 For more information www.linear.com/3796 LT3796/LT3796-1 Typical Applications Solar Panel Driven SLA Battery Charger with Maximum Power Point Tracking WÜRTH SOLAR PANEL VOC = 37.5V VMPP = 28V BAT RSNS1 250mΩ • M2 D1 D2 15V 1:1 C1 4.7µF 50V • R4 301k INTVCC C2 10µF L1B R1 10k R10 30.1k R9 10k NTC VCHARGE = 14.6V VFLOAT = 13.5V AT 25°C + BAT R5 137k R2 475k VIN VS CSN CSP R3 20k SENSE CTRL CSOUT C3 0.1µF M1 GATE EN/UVLO RSNS 15mΩ LT3796 CSOUT R6 100k R11 93.1k GND FB1 FB2 PWM VREF ISP OUT ISN BAT R8 113k C6 0.1µF R12 10.2k M3 ISMON SS C4 0.1µF VMODE VMODE SYNC TG M1: VISHAY SILICONIX Si7456DP M2: VISHAY SUD19P06-60-E3 M3: ZETEX ZXM61N03F L1: COILCRAFT MSD1260-333 D1: ON SEMI MBRS260T3G D2: CENTRAL SEMI CMDZ15L R9: MURATA NCP18XH103F03RB R7 49.9k INTVCC VC RT RC 499Ω CC 22nF FAULT C5 4.7µF RT 19.6k 400kHz INTVCC R12 49.9k FAULT 37961 TA09a ICHARGE vs VIN 1.2 1.0 0.8 ICHARGE (A) VIN OUT C6 2.2µF 100V L1A 33µH 0.6 0.4 0.2 0 20 25 30 35 40 VIN (V) 37961 TA09b 3796fb For more information www.linear.com/3796 31 LT3796/LT3796-1 Typical Applications Boost LED Driver for Twin LEDs with Detection and Protection for Faulty LED VIN 8V TO 20V L1 22µH D1 C1 2.2µF ×3 RSNS 15mΩ R1 499k EN/UVLO R2 97.6k LED STRING 1 CURRENT REPORTING FB1 VS C2 0.1µF ISMON LT3796-1 FAULT INTVCC FAULT VMODE M2 TG TGEN INTVCC C5 4.7µF VMODE VC SS SS C3 0.1µF Q1 R10 10k CC 22nF R11 10k D4 M3 SS Q2 D3 9 LEDs STRING 1 9 LEDs STRING 2 D2 RT RC 1k INTVCC R9 20k ISN CTRL R5 100k RLED2 500mΩ RLED1 500mΩ VREF R4 100k R8 10k ISP FB2 PWM INTVCC CSP CSN CSOUT R3 40.2k R6 1M R7 22.6k VIN GATE SENSE GND LED STRING 2 CURRENT REPORTING C6 2.2µF ×4 100V M1 RT 31.6k 250kHz 37961 TA10a M1: VISHAY SILICONIX Si7460DP M2: VISHAY SILICONIX Si7415DN D1: DIODES INC PDS3100 D2, D3, D4: NXP PMEG6010CEJ Q1, Q2: ZETEX FMMT589 L1: WÜRTH 744 355 122 1 LED: CREE XLAMP XR-L Short One LED from String 1 ILED2 500mA/DIV ILED1 500mA/DIV Short One LED from String 2 500mA ILED2 500mA/DIV 500mA 500mA 500mA ILED1 500mA/DIV ISMON 1V/DIV ISMON 1V/DIV CSOUT 1V/DIV CSOUT 1V/DIV 20ms/DIV 625mA 37961 TA10b 1V 1.25V 1ms/DIV 37961 TA10c 3796fb 32 For more information www.linear.com/3796 LT3796/LT3796-1 Typical Applications SEPIC LED Driver with 100:1 Analog Dimming C1 2.2µF ×3 M1: INFINEON BSC16ON1ONS3-G M2: VISHAY SILICONIX Si7461DP M3: ZETEX ZXM6IN02F L1: COILTRONICS DR Q 127-220 D1: DIODES INC PDS5100 D2: NXP PMEG6010CEJ LED: CREE XLAMP XR-E M1 3 R4 113k 2 R2 97.6k C3 0.1µF 8 1 LTC1541 OP AMP – C2 0.1µF 5 6 + LTC1541 COMP RLED1 270mΩ FB2 ISN VREF INTVCC CTRL FAULT R11 200k R10 7.87k 7 M3 M2 TG PWM C4 0.1µF R9 30.1k C5 0.1µF RLED2 3.32Ω CSOUT 4 VREF R14 6.65k CSP ISP R6 10k C8 4.7µF C6 0.1µF 1.2V INTERNAL REFERENCE R16 100k 22V LED VMODE RT VC TGEN SS INTVCC R15 100k FAULT VMODE – RC 1k CC 22nF RT 19.6k 400kHz 37961 TA11a ILED vs VCTRL_IN 1200 1000 800 ILED (mA) CTRL_IN VS LT3796-1 R12 1M R13 23.7k VIN GATE SENSE GND EN/UVLO FB1 ISMON C7 2.2µF ×4 50V CSN R8 100k R7 100k RSNS 15mΩ R1 499k + R17 10k • L1B LED CURRENT REPORTING R5 1k R3 INTVCC 200k D2 • VIN 8V TO 20V VREF C3 INTVCC 2.2µF ×2 50V D1 L1A 22µH 600 400 200 0 0 2 4 6 8 VCTRL_IN (V) 37961 TA11b 3796fb For more information www.linear.com/3796 33 LT3796/LT3796-1 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. FE Package 28-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663 Rev J) 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) 0.50 – 0.75 (.020 – .030) NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS 2. DIMENSIONS ARE IN MILLIMETERS (INCHES) 3. DRAWING NOT TO SCALE 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0.25 REF 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 J 1012 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 3796fb 34 For more information www.linear.com/3796 LT3796/LT3796-1 Revision History REV DATE DESCRIPTION A 1/13 Added LT3796-1 option PAGE NUMBER B 3/14 Clarified Schematic 1 to 36 33 3796fb 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/3796 35 LT3796/LT3796-1 Typical Application Buck-Boost Mode LED Driver with Open LED Clamp and Output Voltage Limit L1 68µH R4 715k C1 2.2µF ×2 R5 20k R6 200k VIN R2 187k CSN PWM INTVCC R10 100k C4 0.1µF VS RSNS 33mΩ LT3796 FB2 PWM ISP 100 R7 1M PWM = VREF 95 R8 13.3k GND FB1 Efficiency vs VIN M1 SENSE CTRL RLED 1Ω ISMON ISN R9 100k FAULT VIN GATE CSOUT R3 249k CSP EN/UVLO VREF C3 4.7µF ×2 C2 1µF R1 1M LED CURRENT REPORTING D1 M2 TG EFFICIENCY (%) VIN 9V TO 55V 75V (TRANSIENT) 90 85 80 75 FAULT VMODE VMODE VC M1: FAIRCHILD SEMICONDUCTOR FDM3622 M2: ZETEX ZXMP6A13F L1: WÜRTH 744066680 D1: IRF 10BQ100 LED: CREE XLAMP XR-E INTVCC SYNC SS RT RT 19.6k 400kHz RC 4.99k C6 0.1µF 70 INTVCC C5 4.7µF 25V LED 250mA 10 20 30 40 50 60 VIN (V) 37961 TA07b VIN CC 10nF 0 37961 TA07a Related Parts PART NUMBER DESCRIPTION COMMENTS LT3755/LT3755-1 High Side 60V, 1MHz LED Controller with LT3755-2 True Color 3,000:1 PWM Dimming VIN: 4.5V to 40V, VOUT Range: 5V to 60V, True Color PWM, Analog = 3000:1, ISD < 1µA, 3mm × 3mm QFN-16, MSOP-16E Packages LT3756/LT3756-1 High Side 100V, 1MHz LED Controller with VIN: 6V to 100V, VOUT Range: 5V to 100V, True Color PWM, Analog = 3000:1, ISD < 1µA, 3mm × 3mm QFN-16, MSOP-16E Packages LT3756-2 True Color 3,000:1 PWM Dimming LT3743 Synchronous Step-Down 20A LED Driver with Three-State LED Current Control VIN: 5.5V to 36V, VOUT Range: 5.5V to 35V, True Color PWM, Analog = 3000:1, ISD < 1µA, 4mm × 5mm QFN-28, TSSOP-28E Packages LT3791 60V, Synchronous Buck-Boost LED Driver Controller VIN: 4.7V to 60V, VOUT Range: 1.2V to 60V, True Color PWM, Analog, ISD < 1µA, TSSOP-38E Package LT3791-1 60V, Synchronous Buck-Boost Controller VIN: 4.7V to 60V, VOUT Range: 1.2V to 60V, ISD < 1µA, TSSOP-38E Package LT3517 1.3A, 2.5MHz High Current LED Driver with 3,000:1 Dimming VIN: 3V to 30V, True Color PWM, Analog = 3000:1, ISD < 1µA, 4mm × 4mm QFN-16 Package LT3518 2.3A, 2.5MHz High Current LED Driver with 3,000:1 Dimming VIN: 3V to 30V, True Color PWM, Analog = 3000:1, ISD < 1µA, 4mm × 4mm QFN-16 Package LT3474/LT3474-1 36V, 1A (ILED), 2MHz, Step-Down LED Driver VIN: 4V to 36V, VOUT Range = 13.5V, True Color PWM = 400:1, ISD < 1µA, TSSOP-16E Package LT3475/LT3475-1 Dual 1.5A(ILED), 36V, 2MHz, Step-Down LED Driver VIN: 4V to 36V, VOUT Range = 13.5V, True Color PWM, Analog = 3000:1, ISD < 1µA, TSSOP-20E Package 3796fb 36 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 For more information www.linear.com/3796 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/3796 LT 0314 REV B • PRINTED IN USA LINEAR TECHNOLOGY CORPORATION 2012