LT3956 - 80VIN, 80VOUT Constant-Current, Constant-Voltage Converter
Document overview
- Manufacturer or author: Linear Technology Corporation
- PDF pages: 20
Technical content
FeaTures
applicaTions
DescripTion
80VIN, 80VOUT Constant-Current, Constant-Voltage Converter The L T®3956 is a DC/DC converter designed to operate as a constant-current source and constant-voltage regulator . It is ideally suited for driving high current LEDs. It features an internal low side N-channel power MOSFET rated for 84V at 3.3A and driven 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 T3956 senses 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. 94% Efficient 25W White LED Headlamp Driver n 3000:1 T rue Color PWMTM Dimming n Wide Input Voltage Range: 4.5V to 80V n Output Voltage Up to 80V n Internal 3.3A/84V Switch n Constant-Current and Constant-Voltage Regulation n 250mV 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 Constant-Voltage Loop Status Pin n PWM Disconnect Switch Driver n CTRL Pin Adjusts High Side Current Sense Threshold n Low Shutdown Current: <1µA n Programmable Soft-Start n Available in the 36-Lead (5mm × 6mm) QFN Package n High Power LED n Battery Charger n Accurate Current Limited Voltage Regulator VIN SW L T3956 22µH D1 GNDVC INTVCC EN/UVLO PGND VREF ISP 332k 100k INTVCC 332k2.2µF 2.2µF 4.7nF VIN, 6V TO 60V (80V TRANSIENT) 47nF 100k 34k28.7k 375kHz 4.7µF 40.2k CTRL 16.2k 0.68/uni03A9370mA INTVCC 25W LED STRING
3956 TA01a
VIN (V) 0 20 EFFICIENCY (%) 100 40 60 80
3956 TA01b
L, L T , L TC, L TM, Linear Technology and the Linear logo are registered trademarks and 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 and 7321203.
Operating Junction Temperature Range (Note 1) orDer inForMaTion LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE L T3956EUHE#PBF L T3956EUHE#TRPBF 3956 36-Lead (5mm × 6mm) Plastic QFN –40°C to 125°C L T3956IUHE#PBF L T3956IUHE#TRPBF 3956 36-Lead (5mm × 6mm) Plastic QFN –40°C to 125°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 . For more information on lead free part marking, go to: http://www.linear .com/leadfree/ This product is only offered in trays. For more information go to: http://www.linear .com/packaging/ PARAMETER CONDITIONS MIN TYP MAX UNITS VIN Minimum Operating Voltage VIN Tied to INTVCC l 4.5 V VIN Shutdown IQ EN/UVLO = 0V EN/UVLO = 1.15V 0.1 1 µA µA VIN 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 ≤ 80V 0.006 %/V pin conFiguraTion 12 13 14 TOP VIEW GND SW UHE PACKAGE 36-LEAD (5mm s 6mm) PLASTIC QFN 15 16 17 36 35 34 33 32 31 30 1NC EN/UVLO INTVCC GND VIN SW SW NC ISP ISN FB GND PWMOUT SW SW RT SS VMODE PWM VREF CTRL VC PGND PGND PGND PGND PGND PGND 209 TJMAX = 125°C, θJA = 43°C/W , θJC = 5°C/W EXPOSED PAD (PIN 37) IS GND, MUST BE SOLDERED TO PCB EXPOSED PAD (PIN 38) IS SW , MUST BE SOLDERED TO PCB elecTrical characTerisTics The l denotes the specifications which apply over the full operating temp- erature range, otherwise specifications are at TA = 25°C. VIN = 24V , EN/UVLO = 24V , CTRL = 2V , PWM = 5V , unless otherwise noted.
elecTrical characTerisTics The l denotes the specifications which apply over the full operating temp- erature 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 SW Pin Leakage SW = 48V 5 10 µA SW Pin Current Limit l 3.3 3.9 4.6 A SW Pin Voltage Drop I(SW) = 2A 220 mV SS Pull-Up Current Current Out of Pin 8 10 13 µA Error Amplifier Full-Scale Current Sense Threshold ( V(ISP–ISN)) FB = 0V , ISP = 48V , CTRL ≥ 1.2V l 240 250 257 mV Current Sense Threshold at CTRL = 1V ( V(ISP–ISN)) CTRL = 1V , FB = 0V , ISP = 48V l 217 225 231 mV Current Sense Threshold at CTRL = 0.5V ( V(ISP–ISN)) CTRL = 0.5V l 96 100 103 mV Current Sense Threshold at CTRL = 0.1V ( V(ISP–ISN)) CTRL = 0.1V , FB = 0V , ISP = 48V l –2.5 0 4.5 mV CTRL Range for Current Sense Threshold Adjustment 0 1.1 V CTRL Input Bias Current Current Out of Pin, CTRL = 0V 50 100 nA Current Sense Amplifier Input Common Mode Range ( VISN) 2.9 80 V ISP/ISN Short-Circuit Threshold ( V(ISP–ISN)) ISN = 0V 300 335 370 mV ISP/ISN Short-Circuit Fault Sensing Common Mode Range ( VISN) 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 gm 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 = 0V , 48V l 1.220 1.232 1.250 1.250 1.270 1.265 V V FB Amplifier gm FB = VFB, ISP = ISN 480 µS FB Pin Input Bias Current Current Out of Pin, FB = 1V 40 100 nA FB Voltage Loop Active Threshold VMODE Falling VFB – 65mV VFB – 50mV VFB – 40mV V FB Overvoltage Threshold PWMOUT Falling VFB + 50mV VFB + 60mV VFB + 80mV V Oscillator Switching Frequency RT = 100k RT = 10k l 90 925 100 1000 125 1050 kHz kHz SW Minimum Off-Time 170 ns SW Minimum On-Time 200 ns Linear Regulator INTVCC Regulation Voltage 7 7.15 7.3 V Dropout (VIN – INTVCC) IINTVCC = –10mA, VIN = 7V 1 V INTVCC Undervoltage Lockout l 4.1 4.4 V INTVCC Current Limit 14 17 25 mA INTVCC Current in Shutdown EN/UVLO = 0V , INTVCC = 7V 8 12 µA
elecTrical characTerisTics The l denotes the specifications which apply over the full operating temp- erature 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 Logic Inputs/Outputs PWM Threshold Voltage l 0.85 1.35 1.8 V PWM Pin Resistance to GND 45 60 kΩ EN/UVLO Threshold Voltage Falling l 1.185 1.220 1.245 V EN/UVLO Rising Hysteresis 20 mV EN/UVLO Input Low Voltage IVIN Drops Below 1µA 0.4 V EN/UVLO Pin Bias Current Low EN/UVLO = 1.15V 1.7 2.1 2.5 µA EN/UVLO Pin Bias Current High EN/UVLO = 1.30V 10 100 nA VMODE Output Low (VOL) IVMODE = 1mA 200 mV VMODE Pin Leakage FB = 0V , VMODE = 12V 0.1 5 µA PWMOUT Driver tr PWMOUT Driver Output Rise Time CL = 560pF 35 ns tf PWMOUT Driver Output Fall Time CL = 560pF 35 ns PWMOUT Output Low (VOL) 0.05 V PWMOUT 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 T3956E is 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 T3956I is guaranteed to meet performance specifications over the –40°C to 125°C operating junction temperature range. Note 3: For VIN below 6V , the EN/UVLO pin must not exceed VIN for proper operation. V(ISP–ISN) Threshold vs VCTRL V(ISP–ISN) Threshold vs VISP with Reduced CTRL Voltage V(ISP–ISN) Full-Scale Threshold vs Temperature CTRL VOL TAGE (V) –50 V(ISP– ISN) THRESHOLD (mV) 150 250 0.5 1 1.5 300 100 200
3956 G01
ISP VOL TAGE (V) V(ISP– ISN) THRESHOLD (mV) 101 20 40 8060 103 100 102
3956 G02
CTRL = 0.5V 242 246 250 256 254 244 248 252
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V(ISP– ISN) THRESHOLD (mV) TEMPERATURE (°C) –50 0 50 75–25 25 100 125 CTRL = 2V Typical perForMance characTerisTics TA = 25°C, unless otherwise noted.
Typical perForMance characTerisTics FB Regulation Voltage (VFB) vs Temperature VREF Voltage vs Temperature VREF Voltage vs VIN Switching Frequency vs RT Switching Frequency vs Temperature EN/UVLO Hysteresis Current vs Temperature TA = 25°C, unless otherwise noted. RT (k) SWITCHING FREQUENCY (kHz)
3956 G07
3956 G04
VFB (V) TEMPERATURE (°C) –50 0 50 75–25 25 100 125 1.20 1.22 1.24 1.26 1.28 1.21 1.23 1.25 1.27
3956 G05
VREF (V) TEMPERATURE (°C) –50 0 50 75–25 25 100 125 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 2.04 1.97 1.99 2.01 2.03
3956 G06
3956 G08
SWITCHING FREQUENCY (kHz) TEMPERATURE (°C) –50 0 50 75–25 25 100 125 300 400 500 350 450 RT = 26.7k
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TEMPERATURE (°C) –50 0 50 75–25 25 100 125 1.6 IEN/UVLO (µA) 2.0 2.4 1.8 2.2 Quiescent Current vs VIN SW Pin Current Limit vs Temperature EN/UVLO Threshold vs Temperature
3956 G12
EN/UVLO VOLTAGE (V) TEMPERATURE (°C) 1.18 1.22 1.28 1.20 1.24 1.26 EN/UVLO RISING EN/UVLO FALLING –50 0 50 75–25 25 100 125 VIN (V) VIN CURRENT (mA) 1.0 2.0 0.5 1.5
3956 G10
PWM = 0V CURRENT LIMIT (A) 4.2 4.0 3.8 3.6 4.4
3956 G11
TEMPERATURE (°C) –50 0 50 75–25 25 100 125
Typical perForMance characTerisTics Quiescent Current vs Switching Frequency INTVCC Current Limit vs Temperature INTVCC Voltage vs Temperature TA = 25°C, unless otherwise noted. INTVCC Dropout Voltage vs INTVCC Current SW Pin Current Limit vs Duty Cycle LED Current Sense Threshold vs FB Voltage
3956 G13
VIN CURRENT (mA) SWITCHING FREQUENCY (kHz) 0 400 800200 600 1000
3956 G14
INTVCC CURRENT LIMIT (mA) TEMPERATURE (°C) –50 0 50 75–25 25 100 125 NOT SWITCHING
3956 G15
INTVCC (V) TEMPERATURE (°C) –50 0 50 75–25 25 100 125 7.0 7.2 7.4 7.1 7.3 DUTY CYCLE (%) 2.5 SW PIN CURRENT LIMIT (A) 3.5 4.5 3.0 4.0
3956 G16
FB VOL TAGE (V)
3956 G17
125.0 312.5 62.50 187.5 250.0V(ISP–ISN) THRESHOLD (mV) VCTRL = 2V LDO CURRENT (mA) –2.5 LDO DROPOUT (V) –2.0 –1.5 –1.0 –0.5 3 6 9 12 15
3956 G18
–40°C 25°C 125°C PWMOUT Waveform Switch On-Resistance vs Temperature ISP/ISN Input Bias Current vs CTRL Voltage CTRL (V) INPUT BIAS CURRENT (µA)
3956 G19
0 0.5 1 1.5 2 ISP ISN TEMPERATURE (°C) –50 ON-RESISTANCE (mΩ) 120 140 160 100 –25 250 50 75 100 125 180
3956 G20
3956 G21
CPWMOUT = 2.2nF
FB: Voltage Loop Feedback Pin. FB is intended for con - stant-voltage regulation or for LED protection/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 VMODE pull-down is asserted. This action may signal an open LED fault. If FB is driven above the FB threshold (by an external power supply spike, for example), the VMODE 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. ISN: 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 = 250mV/RLED when VCTRL > 1.2V or ILED = (VCTRL –100mV)/(4 • RLED) if VCTRL < 1V. Input bias current is typically 20µA. Below 3V , ISN is an input to the short-circuit protection feature that forces GATE to 0V if ISP exceeds ISN by more than 350mV (typ). ISP: Connection point for the positive terminal of the current feedback resistor . Input bias current for this pin depends on CTRL pin voltage, as shown in the Typical Performance Characteristics. ISP is an input to the short-circuit protec- tion feature when ISN is less than 3V . VC: T ransconductance Error Amplifier Output Pin. This pin is 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. CTRL: Current Sense Threshold Adjustment Pin. Regula- ting threshold V(ISP – ISN) is 0.25 • V CTRL plus an offset for 0V < 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 V CTRL 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 threshold. Do not leave this pin open. NC: No Internal Connection. These pins may be left floating or connected to an adjacent pin. EN/UVLO: Shutdown and Undervoltage Detect Pin. An accurate 1.22V falling threshold with externally program- mable 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 1.24V (nominal) threshold (but below 6V), EN/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 VIN quiescent current below 1µA. INTVCC: Regulated supply for internal loads, GATE driver and PWMOUT driver . Supplied from VIN and regulates to 7.15V (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 . GND: Ground. The exposed pad, Pin 37, is ground and must be soldered directly to the ground plane. VIN: Input Supply Pin. Must be locally bypassed with a 0.22µF (or larger) capacitor to PGND placed close to the IC. SW: The exposed pad, Pin 38, is the drain of the switch- ing N-channel MOSFET and must be connected to the external inductor . PGND: Source terminal of switch and the GND input to the switch current comparator . Kelvin connect to the GND plane close to the IC using Pin 12. Pins 13 to 17 should be connected externally to the PGND terminals of components in the switching path. See the Board Layout section. PWMOUT: Buffered Version of the PWM Signal. This pin is used to drive the LED load disconnect N-channel MOSFET 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 (VFB) plus 60mV (typical). The PWMOUT pin is driven from INTVCC. Use of a MOSFET with gate cut-off voltage higher than 1V is recommended.
– + ++– ++– FREQ PROG 1.25V SSCLAMP 1.1V CTRL VREF EN/UVLO ISP ISN 350mV 20k 170k 140µA 2.1µA CTRL BUFFER gm EAMP PWM COMPARATOR DRIVER 1mA (MAX) ISENSE gm A10 OVFB COMPARATOR 1.25V FB SHORT-CIRCUIT DETECT SCILMB SCILMB PWMOUT PWM 1.25V VIN INTVCC VC A2 R Q S RAMP GENERATOR 100kHz TO 1MHz OSCILLATOR A8 7.15V LDO SW PGND 3956 BD VMODE GND 1.2V FB 1.22V 1.31V RTSS SHDN 10µA 10µA AT FB = 1.25V VC TSD 165°C FAUL T LOGIC 10µA 10µA AT A1+ = A1– VREF: 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: 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 INTVCC. VMODE: An open-collector pull-down on VMODE asserts if the FB input is greater than the FB regulation threshold minus 50mV (typical). To function, the pin requires an external pull-up resistor . When the PWM input is low and the DC/DC converter is idle, the VMODE condition is latched to the last valid state when the PWM input was high. When PWM input goes high again, the VMODE pin will be updated. This pin may be used to report an open LED fault. Use a pull-up current less than 1mA. SS: Soft-Start Pin. This pin modulates oscillator fre - quency and compensation pin voltage (V C) clamp. The soft-start interval is set with an external capacitor . 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 EN/UVLO pin) or thermal limit. RT: Switching Frequency Adjustment Pin. Set the fre - quency using a resistor to GND (for resistor values, see the Typical Performance curve or Table 1). Do not leave the RT pin open. pin FuncTions
The LT3956 is a constant-frequency, current mode converter with a low side N-channel MOSFET switch. The switch 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 power switch is turned off and the PWMOUT pin is 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 generates a pulse to set the PWM latch, turning on the internal power MOSFET switch. A voltage input proportional to the switch current, sensed by an internal current sense resistor , is added to a stabilizing slope compensation ramp and the resulting switch-current sense signal is fed into the positive termi- nal 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 compensa- tion 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 is not allowed to exceed the current limit threshold of 3.9A (typical). If the SW pin exceeds the current limit threshold, the SR latch is reset regardless of the output state of the PWM compara- tor . Likewise, at an ISP/ISN common mode voltage less than 3V , the difference 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 335mV (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 VREF . T wo LED specific functions featured on the L T3956 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 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 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 the overvolt- age threshold (1.31V typical), then an externally driven overvoltage event may have caused the FB pin to be too high and the VMODE pull-down will be deactivated until the FB pin drops below the overvoltage threshold.
INTVCC Regulator Bypassing and Operation The INTVCC pin requires a capacitor for stable operation and to store the charge for the switch driver and PWMOUT pin switching currents. Choose a 10V rated low ESR, X7R or X5R ceramic capacitor for best performance. A 4.7µF capacitor will be 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. An internal current limit on the INTV CC output protects the L T3956 from excessive on-chip power dissipation. The INTVCC pin has its own undervoltage disable (UVLO) set to 4.1V (typical) to protect the internal MOSFET from excessive power dissipation caused by not being fully en- hanced. If the INTVCC pin drops below the UVLO threshold, the PWMOUT pin will be forced to 0V , the power switch turned off and the soft-start pin will be reset. If the input voltage, VIN, will not exceed 7V , then the INTVCC pin could be connected to the input supply. This action allows the L T3956 to operate from as low as 4.5V . Be aware that a small current (less than 12μA) will load the INTVCC in shutdown. Otherwise, the minimum operating VIN value is determined by the dropout voltage of the linear regulator and the 4.4V (4.1V typical) INTVCC undervoltage lockout threshold mentioned above. Programming the Turn-On and Turn-Off Thresholds With the EN/UVLO Pin The falling UVLO value can be accurately set by the resistor divider . A small 2.1µA pull-down current is active when EN/UVLO is below the falling threshold. The purpose of this current is to allow the user to program the rising hysteresis. The following equations should be used to determine the values of the resistors: V R R RIN FALLING, . •= +1 22 1 2 V µA R VIN RISING IN FALLING, ,. •= +2 1 1 EN/UVLO L T3956 VIN
3956 F01
The LED current is programmed by placing an appropri- ate 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, but take caution that the minimum ISN value does not fall below 3V , which is the lower limit of the LED current regulation function. 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: I V m V RLED CTRL LED = − 100 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 CTRL = 1.2V , the LED current no longer varies with CTRL. At CTRL = 1.1V , the actual value of I LED is ~98% of the equation’s estimate. When VCTRL is higher than 1.2V , the LED current is regu- lated to: I mV RLED LED = 250 The CTRL pin should not be left open (tie to V REF 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 V IN to reduce output power and switching current when V IN 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 . Some level of ripple signal is acceptable: the compensation capacitor on the VC pin filters the signal so the average difference between ISP and ISN is regulated to the user-programmed value. Ripple voltage amplitude (peak-to-peak) in excess of
required at high duty cycle. output current at nominal VIN levels.
3956 F02
Figure 2. Feedback Resistor Connection
3956 F03
Figure 3. Feedback Resistor Connection for the ISN voltage range where these two functions overlap. ence falls below the threshold.
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
3956 F04
Figure 4. Typical Switch Minimum On The L T3956 is rated to a maximum input voltage of 80V .
will be enabled after the device temperature drops 10°C. momentary overload conditions.
- 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 (43°C/W for the 5mm × 6mm QFN package). For example, an application with TA(MAX) = 85°C, VIN(MAX) = 60V , fSW = 400kHz, and having an average switching current of 2.5A at 70% duty cycle, the maximum IC junction temperature will be approximately: (1.7mA + 400kHz • 7nC)] • 43°C/W= 123°C The Exposed Pads on the bottom of the package must be soldered to a plane. These should then be connected to inter- nal copper planes with thermal vias placed directly under the package to spread out the heat dissipated by the IC. Open LED Detection The L T3956 provides an open-drain status pin, VMODE, that pulls low when the FB pin is within ~50mV of its 1.25V regulated voltage. If the open LED clamp voltage is pro- grammed 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. 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: C I V V T µF A µ sIN µF LED A OUT IN Therefore, a 4.7µF capacitor is an appropriate selection for a 400kHz boost regulator with 12V input, 48V output and 1A load. With the same VIN voltage ripple of 100mV , the input capaci- tor for a buck converter can be estimated as follows: C I T µF A µ sIN µF LED A S W µs( ) ( ) ( )• • .
- = 4 7 A 10µF input capacitor is an appropriate selection for a 400kHz buck mode converter with a 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. In this buck converter case it is important to place the capacitor as close as possible to the Schottky diode and to the PGND return of the switch. 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) = ( )I D DLED • – •1 where D is the switch duty cycle.
Table 2. Recommended Ceramic Capacitor Manufacturers
Table 3. Schottky Rectifier Manufacturers
- .0 6 LBUCK-BOOST = +( ) T V V V V A SW LED IN LED IN
- •
- .0 6 L T V V V V ABOOST SW IN LED IN LED = ( )• –
- .0 6 Table 4 provides some recommended inductor vendors.
Table 4. Inductor Manufacturers
resistor and capacitor determine the loop stability. ing fast transients on the input supply to the converter . such as FB and VC, as they may pick up switching noise. the current sense threshold. Figure 5. Boost Converter Suggested Layout
3956 F05
94% Efficient 25W White LED Headlamp Driver Typical applicaTions PWM Waveforms for 25W Headlamp Driver VIN SW L T3956 22µH D1 GNDVC INTVCC EN/UVLO PGND VREF ISP 332k 100k INTVCC 332k CVIN 2.2µF COUT 2.2µF CC 4.7nF VIN 6V TO 60V (80V TRANSIENT) CSS 47nF 100k RC 20k RT 28.7k 375kHz CVCC 4.7µF 40.2k CTRL 16.2k RS 0.68/uni03A9370mA INTVCC
3956 TA02a
M1: VISHAY SILICONIX Si2328DS D1: DIODES INC PDS5100 L1: COIL TRONICS DR125-220 C1, C2: MURATA GRM42-2x7R225 SEE SUGGESTED LAYOUT (FIGURE 5) 25W LED STRING (CURRENT DERATED FOR VIN < 11V) 5µs/DIV PWM ILED 200mA/DIV ILI 1A/DIV
3956 TA02b
VOUT = 68V VIN = 15V
Buck-Boost Mode LED Driver L T3956 68µH GNDVC INTVCC EN/UVLO FB PGND VREF ISP 0.1µF VIN 9V TO 45V VIN VIN VOUT L1: COILCRAFT MSS1038-683 D1: ON SEMICONDUCTOR MBRS3100T3 M1: ZETEX ZXM6IP03F Q1: ZETEX FMMT493 187k 35.7k 300kHz 3.4k 10nF 4.7µF CTRL 750/uni03A9 680m/uni03A9 619k 10k 4.7µF 35V
3956 TA03a
4.7µF 1µF 100V 24V LED STRING 350mA 100k INTVCC INTVCC VIN SW Efficiency vs VIN VIN (V) 0 10 EFFICIENCY (%) 100 30 40
3956 TA03b
28VIN /0V to 28V SEPIC SuperCap Charger with Input Current Limit VIN L T3956 L1A 33µH 1:1 GNDVC INTVCC INTVCC EN/UVLO SW VREF ISP 10k 1µF 10nF VIN 28V ≤ 1.2A VOUT 0V TO 28V L1: WÜRTH ELEKTRONIK 744871330 D1: ON SEMI MBRS36OT Q1: MMBTA42 C1, C3, C4: TAIYO-YUDEN GMK 3I6BJ106 28.7k 375kHz 10µF PGND L1B 4.7µF CTRL 30.1k 10µF
3956 TA04a
40.2k 2k 59k 25k 536k 14k 10µF 200m/uni03A9 Input and Output Current vs Output Voltage VOUT (V) INPUT/OUTPUT CURRENT (A)0.5 1.0 1.5 2.0 3.0 5 10 15 20
3956 TA04b
2.5 INPUT OUTPUT
90% Efficient, 20W SEPIC LED Driver VIN L T3956 L1A 33µH 1:1 GNDVC INTVCC EN/UVLO SW VREF ISP 100k INTVCC 4.7µF 50V 10nF0.01µF L1: COIL TRONICS DRQ127-330 D1: VISHAY PDS5100 M1: ZETEX ZXM61N03F VIN 8V TO 50V 185k 250k 25k 10µF 35V PGND L1B 15k 28.7k 375kHz 4.7µF 10V CTRL 0.25/uni03A9 2.2µF (50V) 20W LED STRING CURRENT DERATED FOR VIN < 13V
3956 TA05a
56.2k VIN (V) 0 10 EFFICIENCY (%) 100 30 40
3956 TA05b
90W Buck Mode LED Driver , 80VIN/60VOUT VIN VIN L T3956 GNDVC INTVCC VREF INTVCC EN/UVLO SW ISP 100k INTVCC 0.01µF0.1µF D1: VISHAY 10MQ100N L1: WÜRTH ELEKTRONIK 744066330 M1: VISHAY SILICONIX Si7113DN Q1: ZETEX FMMT593 Q2: ZETEX FMMT493 C1, C2: MURATA GRM42-2x7R225 VIN 64V TO 80V 20k 28.7k 375kHz 4.7µF 0.1/uni03A9 1.5A
16 WHITE
LEDs, 90W
3956 TA06a
2.2µF 2.2µF 267k 200k 10k 33µH 200k 470/uni03A9 24.3k 13k VIN (V) EFFICIENCY (%) 100 72 76
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. package DescripTion 5.00 p 0.10 6.00 p 0.10 NOTE: 1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE 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 0.20mm 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 p 0.10 363530 31 32 33 34 27 2 121314151617 BOTTOM VIEW—EXPOSED PAD
2.00 REF
1.50 REF
0.75 p 0.05 R = 0.125 TYP R = 0.10 TYP PIN 1 NOTCH R = 0.30 OR 0.35 s 45o CHAMFER 0.25 p 0.05
0.50 BSC
0.200 REF
0.00 – 0.05 (UHE28MA) QFN 0110 REV C RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 0.70 p0.05 4.10 p 0.05 5.50 p 0.05 PACKAGE OUTLINE 1.88 p 0.10 1.53 p 0.10 5.10 p 0.05 6.50 p 0.05 Variation: UHE28MA 36-Lead Plastic QFN (5mm s 6mm) (Reference LTC DWG # 05-08-1836 Rev C) 3.00 p 0.10 3.00 p 0.10 0.12 p 0.10 1.88 p 0.05 1.53 p 0.05 3.00 p 0.05 3.00 p 0.05 0.48 p 0.05 0.12 p 0.05 0.48 p 0.10 0.25 p0.05
Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear .com LINEAR TECHNOLOGY CORPORATION 2010 LT 0510 • PRINTED IN USA relaTeD parTs Typical applicaTion Buck Mode 1A LED Driver with High Dimming Ratio and Open LED Reporting Efficiency vs VIN PART NUMBER DESCRIPTION COMMENTS L T3756/L T3756-1/ L T3756-2 100VIN, 100VOUT, Full Featured 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 T3755/L T3755-1/ L T3755-2 40VIN, 75VOUT, Full Featured LED Controller VIN: 4.5V to 40V , VOUT(MAX) = 60V , T rue Color PWM Dimming = 3000:1, ISD < 1µA, 3mm × 3mm QFN-16 and MS16E Packages L T3474 36V , 1A (ILED), 2MHz, Step-Down LED Driver VIN: 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 VIN: 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 VIN: 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 VIN L T3956 GNDVC INTVCC EN/UVLO FB VIN INTVCC ISP 2.2nF 0.1µF VIN 24V TO 80V 28.7k 375kHz 47k 100k 4.7µF 0.1/uni03A9 4.7µF 1µF 33µH
3956 TA07a
61.9k D1: DIODES INC B1100/B L1: WÜRTH 74456133 M1: VISHAY SILICONIX Si5435BDC Q1: ZETEX FMMT493 Q2: ZETEX FMMT593 C1: TDKC3226X7R2A105K C2: TDKC3225X7RIE475K 200k 200k200k 20k
6 WHITE LEDs
30.1k 10k VIN (V) EFFICIENCY (%) 100 40 60