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FEATURES
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LT3492 Triple Output LED Driver with 3000:1 PWM Dimming DESCRIPTION
The LT(R)3492 is a triple output DC/DC converter designed to operate as a constant-current source and is ideal for driving LEDs. The LT3492 works in buck, boost or buckboost mode. The LT3492 uses a fixed frequency, current mode architecture resulting in stable operation over a wide range of supply and output voltages. A frequency adjust pin allows the user to program switching frequency between 330kHz and 2.1MHz to optimize efficiency and external component size. The external PWM input provides 3000:1 LED dimming on each channel. Each of the three channels has a built-in gate driver to drive an external LED-disconnect P-channel MOSFET, allowing high dimming range. The output current range of each channel of the LT3492 is programmed with an external sense resistor. The CTRL pin is used to adjust the LED current either for analog dimming or overtemperature protection.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. 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, and others pending.
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True Color PWMTM Dimming Delivers Up to 3000:1 Dimming Ratio Built-In Gate Driver for PMOS LED Disconnect Three Independent Driver Channels with 600mA, 60V Internal Switches Operates in Buck, Boost, Buck-Boost Modes CTRL Pin Accurately Sets LED Current Sense Threshold Over a Range of 10mV to 100mV Adjustable Frequency: 330kHz to 2.1MHz Open LED Protection Wide Input Voltage Range: Operation from 3V to 30V Transient Protection to 40V Surface Mount Components 28-Lead (4mm x 5mm) QFN and TSSOP Packages
APPLICATIONS
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RGB Lighting Billboards and Large Displays Automotive and Avionic Lighting Constant-Current Sources
TYPICAL APPLICATION
High Dimming Ratio Triple Output Buck-Mode LED Power Supply
PVIN 58V ISP1 330m ISN1 TG1 TG2 ISP2 330m ISN2 ISP3 330m ISN3 TG3 PWM 5V/DIV 10 LEDs 0.3A 0.47F 0.47F 0.3A 0.3A 0.47F ILED 0.2A/DIV IL 0.5A/DIV 1s/DIV SW1 ISP1-3 ISN1-3 VIN PWM1-3 SHDN SW2 SW3 TG1-3 VC1-3 VREF CTRL1-3 150k FADJ 49.9k 1.3MHz GND OVP1-3
3492 TA01a 3492 TA01b
1F 3
3000:1 PWM Dimming at 100Hz
33H
33H
33H
VIN 3V TO 24V
1F
LT3492
10k 680pF
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LT3492 ABSOLUTE MAXIMUM RATINGS
(Note 1)
VIN (Note 4) ...............................................................40V SW1-SW3, ISN1-ISN3, ISP1-ISP3 ............................60V TG1-TG3 ...............................................ISP - 10V to ISP PWM1-PWM3 ...........................................................20V VREF, CTRL1-CTRL3, FADJ, VC1-VC3, OVP1-OVP3....2.5V SHDN (Note 4) ...........................................................VIN
Operating Junction Temperature Range (Note 2).................................................. -40C to 125C Max Junction Temperature.................................... 125C Storage Temperature Range TSSOP ............................................... -65C to 150C UFD.................................................... -65C to 125C
PIN CONFIGURATION
TOP VIEW SHDN PWM3 PWM2 PWM1 VREF CTRL3 CTRL2 CTRL1 FADJ 1 2 3 4 5 6 7 8 9 GND 29 28 VIN PWM2 27 TG3 26 ISN3 25 ISP3 24 SW3 23 SW2 22 ISN2 21 ISP2 20 TG2 19 SW1 18 ISN1 17 ISP1 16 TG1 15 OVP1 PWM1 1 VREF 2 CTRL3 3 CTRL2 4 CTRL1 5 FADJ 6 VC3 7 VC2 8 9 10 11 12 13 14 OVP3 OVP2 OVP1 ISN1 VC1 TG1 GND 29 TOP VIEW PWM3 SHDN ISN3 22 ISP3 21 SW3 20 SW2 19 ISP2 18 ISN2 17 TG2 16 SW1 15 ISP1 TG3 VIN
28 27 26 25 24 23
VC3 10 VC2 11 VC1 12 OVP3 13 OVP2 14
FE PACKAGE 28-LEAD PLASTIC TSSOP TJMAX = 125C, JA = 30C/W, JC = 10C/W EXPOSED PAD (PIN 29) IS GND, MUST BE SOLDERED TO PCB
UFD PACKAGE 28-LEAD (4mm 5mm) PLASTIC QFN TJMAX = 125C, JA = 34C/W, JC = 2.7C/W EXPOSED PAD (PIN 29) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH LT3492EFE#PBF LT3492IFE#PBF LT3492EUFD#PBF LT3492IUFD#PBF TAPE AND REEL LT3492EFE#TRPBF LT3492IFE#TRPBF LT3492EUFD#TRPBF LT3492IUFD#TRPBF PART MARKING* LT3492FE LT3492FE 3492 3492 PACKAGE DESCRIPTION 28-Lead Plastic TSSOP 28-Lead Plastic TSSOP 28-Lead (4mm x 5mm) Plastic QFN 28-Lead (4mm x 5mm) Plastic QFN TEMPERATURE RANGE -40C to 125C -40C to 125C -40C to 125C -40C to 125C
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/
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LT3492 ELECTRICAL CHARACTERISTICS
PARAMETER VIN Operation Voltage VIN Undervoltage Lockout Full-Scale LED Current Sense Voltage One-Tenth Scale LED Current Sense Voltage ISPn/ISNn Operating Voltage VREF Output Voltage VREF Line Regulation Quiescent Current in Shutdown Quiescent Current Idle Quiescent Current Active (Not Switching) Switching Frequency IREF = 200A, Current Out of Pin 3V VIN 40V, IREF = 10A SHDN = 0V PWM1-PWM3 = 0V VC1-VC3 = 0V FADJ = 1.5V FADJ = 0.5V FADJ = 0.1V FADJ = 1.5V (2.1MHz) FADJ = 0.5V (1.3MHz) FADJ = 0.1V (330kHz) Current Out of Pin, CTRL1-3 = 0.1V Current Out of Pin, FADJ = 0.1V Current Out of Pin, OVP1-3 = 0.1V 0.95 PWM1-3 = 0V ISP1-3 = 48V ISP1-3 = 48V (Note 3) ISW = 500mA (Note 3) SHDN = 0V, SW = 5V 180 PWM1-3 = 0V SHDN = 0V 600 -20 1800 1000 280 73 80 0.1 6 11 2100 1300 340 78 87 97 20 20 10 1 0 10 200 1000 340 2 250 1 1 1300 100 100 100 1.05 20
l
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 5V, SHDN = 5V, PWM1-3 = 5V, FADJ = 0.5V, CTRL1-3 = 1.5V, OVP1-3 = 0V, unless otherwise noted.
CONDITIONS (Note 4) ISP1-3 = 48V CTRL1-3 = 100mV, ISP1-3 = 48V MIN 3 2.1
l
TYP
MAX 30 2.4 103 104 13 60 2.04 0.03 10 8 15 2400 1600 400
UNITS V V mV mV mV V V %/V A mA mA kHz kHz kHz % % % nA nA nA V nA M S mA mV A A A A
98 96 7 2.5 1.96
100 10 2
Maximum Duty Cycle
CTRL1-3 Input Bias Current FADJ Input Bias Current OVP1-3 Input Bias Current OVP1-3 Threshold VC1-3 Idle Input Bias Current VC1-3 Output Impedance EAMP gm (IVC/VCAP-LED) SW1-3 Current Limit SW1-3 VCESAT SW1-3 Leakage Current ISP1-3 Input Bias Current ISP1-3, ISN1-3 Idle Input Bias Current ISP1-3, ISN1-3 Input Bias Current in Shutdown
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LT3492 ELECTRICAL CHARACTERISTICS
PARAMETER SHDN Input Low Voltage SHDN Input High Voltage SHDN Pin Current PWM1-3 Input Low Voltage PWM1-3 Input High Voltage PWM1-3 Pin Current Gate Off Voltage (ISP1-3-TG1-3) Gate On Voltage (ISP1-3-TG1-3) Gate Turn-On Delay Gate Turn-Off Delay Current Into Pin ISP1-3 = 60V, PWM1-3 = 0V ISP1-3 = 60V , CLOAD = 300pF ISP1-3 = 60V (Note 5) , CLOAD = 300pF ISP1-3 = 60V (Note 5) 5.5 1.2 160 0.1 6.5 110 110 210 0.3 7.5 SHDN = 5V, Current Into Pin 1.5 65 120 0.4
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 5V, SHDN = 5V, PWM1-3 = 5V, FADJ = 0.5V, CTRL1-3 = 1.5V, OVP1-3 = 0V, unless otherwise noted.
CONDITIONS MIN TYP MAX 0.4 UNITS V V A V V A V V ns ns
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 LT3492E is guaranteed to meet performance specifications from 0C to 125C junction temperature. Specifications over the -40C to 125C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LT3492I is guaranteed over the full -40C to 125C operating junction temperature range.
Note 3: Current flows into pin. Current limit and switch VCESAT is guaranteed by design and/or correlation to static test. Note 4: Absolute maximum voltage at the VIN and SHDN pins is 40V for nonrepetitive 1 second transients, and 30V for continuous operation. Note 5: Gate turn-on/turn-off delay is measured from 50% level of PWM voltage to 90% level of gate on/off voltage.
TYPICAL PERFORMANCE CHARACTERISTICS
Quiescent Current
14 12 INPUT CURRENT (mA) 10 8 6 4 2 0 0 10 VC = GND, NOT SWITCHING 20 VIN (V)
3492 G01
(TA = 25C unless otherwise noted) Switch Current Limit vs Duty Cycle
1200 1000 800 600 400 200 0
Switch On Voltage
600 PWM1-3 = 5V SWITCH CURRENT LIMIT (mA) 0 200 600 800 400 SWITCH CURRENT (mA) 1000
3492 G02
500 SWITCH VOLTAGE (mV) 400 300 200 100 0
PWM1-3 = 0V
30
40
0
20
60 40 DUTY CYCLE (%)
80
100
3492 G03
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LT3492 TYPICAL PERFORMANCE CHARACTERISTICS
Switch Current Limit vs Temperature
1200 1000 CURRENT LIMIT (mA) 800 VREF (V) 600 400 200 0 -50 -25 2.04 2.03 2.02 2.01 2.00 1.99 1.98 1.97 50 25 75 0 TEMPERATURE (C) 100 125 1.96 -50 -25 0 75 50 25 TEMPERATURE (C) 100 125 SWITCH FREQUENCY (kHz)
(TA = 25C unless otherwise noted)
Reference Voltage vs Temperature
2250 2000 1750
Switch Frequency vs FADJ
1500
1250 1000 750 500 250 0 0 0.2 0.4 0.6 0.8 FADJ (V) 1.0 1.2
3492 G06
3492 G04
3492 G05
Switch Frequency vs Temperature
1.4 FADJ = 0.5V VISP-VISN THRESHOLD (mV) 100 80 60 40 20 1.0 -50 -25 0 50 25 75 0 TEMPERATURE (C) 100 125 SWITCH FREQUENCY (MHz) 120
VISP-VISN Threshold vs CTRL
VISP = 24V VISP-VISN TRHESHOLD (mV) 103 102 101 100 99 98 97
VISP-VISN Threshold vs VISP
CTRL = 1.2V
1.3
1.2
1.1
0
0.2
0.4
0.6 0.8 CTRL (V)
1
1.2
3492 G08
0
10
20
30 VISP (V)
40
50
60
3492 G09
3492 G07
VISP-VISN Threshold vs Temperature
103 102 101 100 60V 99 98 97 -50 -25 TG 50V CTRL = 1.2V VISP = 24V 5V PWM 0V
PMOS Turn On Waveforms
PMOS Turn Off Waveforms
VISP-VISN THRESHOLD (mV)
5V PWM 0V
60V TG 50V
50 25 75 0 TEMPERATURE (C)
100
125
3492 G10
VISP = 60V QG FET = 6nC
200ns/DIV
3492 G11
VISP = 60V QG FET = 6nC
200ns/DIV
3492 G12
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LT3492 PIN FUNCTIONS
CTRL1, CTRL2, CTRL3: LED Current Adjustment Pins. Sets voltage across external sense resistor between ISP and ISN pins of the respective converter. Setting CTRL voltage to be less than 1V will control the current sense voltage to be one-tenth of CTRL voltage. If CTRL voltage is higher than 1V, the default current sense voltage is 100mV. The CTRL pin must not be left floating. FADJ: Switching Frequency Adjustment Pin. Setting FADJ voltage to be less than 1V will adjust switching frequency up to 2.1MHz. If FADJ voltage is higher than 1V, the default switching frequency is 2.1MHz. The FADJ pin must not be left floating. GND: Signal Ground and Power Ground. Solder exposed pad directly to ground plane. ISN1, ISN2, ISN3: Noninverting Input of Current Sense Error Amplifier. Connect directly to LED current sense resistor terminal for current sensing of the respective converter. ISP1, ISP2, ISP3: Inverting Input of Current Sense Error Amplifier. Connect directly to other terminal of LED current sense resistor terminal of the respective converter. OVP1, OVP2, OVP3: Open LED Protection Pins. A voltage higher than 1V on OVP turns off the internal main switch of the respective converter. Tie to ground if not used. PWM1, PWM2, PWM3: Pulse Width Modulated Input. Signal low turns off the respective converter, reduces quiescent supply current and causes the VC pin for that converter to become high impedance. PWM pin must not be left floating; tie to VREF if not used. SHDN: Shutdown Pin. Used to shut down the switching regulator and the internal bias circuits for all three converters. Tie to 1.5V or greater to enable the device. Tie below 0.4V to turn off the device. SW1, SW2, SW3: Switch Pins. Collector of the internal NPN power switch of the respective converter. Connect to external inductor and anode of external Schottky rectifier of the respective converter. Minimize the metal trace area connected to this pin to minimize electromagnetic interference. TG1, TG2, TG3: The Gate Driver Output Pin for Disconnnect P-Channel MOSFET. One for each converter. When the PWM pin is low, the TG pin pulls up to ISP to turn off the external MOSFET. When the PWM pin is high, the external MOSFET turns on. ISPn-TGn is limited to 6.5V to protect the MOSFET. Leave open if the external MOSFET is not used. VC1, VC2, VC3: Error Amplifier Compensation Pins. Connect a series RC from these pins to GND. VIN: Input Supply Pin. Must be locally bypassed. Powers the internal control circuitry. VREF: Reference Output Pin. Can supply up to 200A. The nominal Output Voltage is 2V.
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LT3492 BLOCK DIAGRAM
D1 VSENSE ILED LED ARRAY M1 C2 VIN C1 L1
+
-
RSENSE
ISP1 R3 OVP1 R4 RC CC PWM1 VC1
ISN1
TG1 A7 MOSFET DRIVER
PWM1
SW1
+ +
V1 R1 2k A1
EAMP 1V
-
- -
VC SR LATCH A3 A2 SLOPE R S Q
VIN 1V CTRL1
CTRL BUFFER R2 20k
VIN C3
VIN SHDN VREF
INTERNAL REGULATOR AND UVLO
VIN 200A RAMP GENERATOR
A9
Q2
OSCILLATOR
R5
C4
R6
Figure 1. LT3492 Block Diagram Working in Boost Configuration
+
2V REFERENCE
FADJ SHARED COMPONENTS
3492 BD
+
+ + -
A8
Q3
+
A4 PWM COMPARATOR ISENSE A10
-
A6 NPN DRIVER A5 Q1
+ -
GND
REPLICATED FOR EACH CHANNEL
-
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LT3492 APPLICATIONS INFORMATION
Operation The LT3492 uses a fixed frequency, current mode control scheme to provide excellent line and load regulation. Operation can be best understood by referring to the Block Diagram in Figure 1. The oscillator, ramp generator, reference, internal regulator and UVLO are shared among the three converters. The control circuitry, power switch etc., are replicated for each of the three converters. Figure 1 shows the shared circuits and only converter 1 circuits. If the SHDN pin is logic low, the LT3492 is shut down and draws minimal current from VIN. If the SHDN pin is logic high, the internal bias circuits turn on. The switching regulators start to operate when their respective PWM signal goes high. The main control loop can be understood by following the operation of converter 1. The start of each oscillator cycle sets the SR latch, A3, and turns on power switch Q1. The signal at the noninverting input (SLOPE node) of the PWM comparator A2 is proportional to the sum of the switch current and oscillator ramp. When SLOPE exceeds VC (the output of the error amplifier A1), A2 resets the latch and turns off the power switch Q1 through A4 and A5. In this manner, A10 and A2 set the correct peak current level to keep the output in regulation. Amplifier A8 has two noninverting inputs, one from the 1V internal voltage reference and the other one from the CTRL1 pin. Whichever input is lower takes precedence. A8, Q3 and R2 force V1, the voltage across R1, to be one tenth of either 1V or the voltage of CTRL1 pin, whichever is lower. VSENSE is the voltage across the sensing resistor, RSENSE, which is connected in series with the LEDs. VSENSE is compared to V1 by A1. If VSENSE is higher than V1, the output of A1 will decrease, thus reducing the amount of current delivered to LEDs. In this manner the current sensing voltage VSENSE is regulated to V1. Converters 2 and 3 are identical to converter 1. PWM Dimming Control The LED array can be dimmed with pulse width modulation using the PWM1 pin and an external P-channel MOSFET, M1. If the PWM1 pin is pulled high, M1 is turned on by internal driver A7 and converter 1 operates nominally. A7 limits ISP1-TG1 to 6.5V to protect the gate of M1. If the PWM1 pin is pulled low, Q1 is turned off. Converter 1 stops operating, M1 is turned off, disconnects the LED array and stops current draw from output capacitor C2. The VC1 pin is also disconnected from the internal circuitry and draws minimal current from the compensation capacitor CC. The VC1 pin and the output capacitor store the state of the LED current until PWM1 is pulled up again. This leads to a highly linear relationship between pulse width and output light, and allows for a large and accurate dimming range. A P-channel MOSFET with smaller total gate charge (QG) improves the dimming performance, since it can be turned on and off faster. Use a MOSFET with a QG lower than 10nC, and a minimum VTH of -1V to -2V. Don't use a Low VTH PMOS. To optimize the PWM control of all the three channels, the rising edge of all the three PWM signals should be synchronized. In the applications where high dimming ratio is not required, M1 can be omitted to reduce cost. In these conditions, TG1 should be left open. The PWM dimming range can be further increased by using CTRL1 pin to linearly adjust the current sense threshold during the PWM1 high state. Loop Compensation Loop compensation determines the stability and transient performance. The LT3492 uses current mode control to regulate the output, which simplifies loop compensation. To compensate the feedback loop of the LT3492, a series resistor-capacitor network should be connected from the VC pin to GND. For most applications, the compensation capacitor should be in the range of 100pF to 2.2nF The com. pensation resistor is usually in the range of 5k to 50k. To obtain the best performance, tradeoffs should be made in the compensation network design. A higher value of compensation capacitor improves the stability and dimming range (a larger capacitance helps hold the VC voltage when the PWM signal is low). However, a large compensation capacitor also increases the start-up time and the time to recover from a fault condition. Similarly, a larger compensation resistor improves the transient response but may reduce the phase margin. A practical approach is to start with one of the circuits in this data sheet that
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LT3492 APPLICATIONS INFORMATION
is similar to your application and tune the compensation network to optimize the performance. The stability, PWM dimming waveforms and the start-up time should be checked across all operating conditions. Open-LED Protection The LT3492 has open-LED protection for all the three converters. As shown in Figure 1, the OVP1 pin receives the output voltage (the voltage across the output capacitor) feedback signal from an external resistor divider. OVP1 voltage is compared with a 1V internal voltage reference by comparator A6. In the event the LED string is disconnected or fails open, converter 1 output voltage will increase, causing OVP1 voltage to increase. When OVP1 voltage exceeds 1V, the power switch Q1 will turn off, and cause the output voltage to decrease. Eventually, OVP1 will be regulated to 1V and the output voltage will be limited. In the event one of the converters has an open-LED protection, the other converters will continue functioning properly. Switching Frequency and Soft-Start The LT3492 switching frequency is controlled by FADJ pin voltage. Setting FADJ voltage to be less than 1V will reduce switching frequency. If FADJ voltage is higher than 1V, the default switching frequency is 2.1MHz. In general, a lower switching frequency should be used where either very high or very low switch duty cycle is required or higher efficiency is desired. Selection of a higher switching frequency will allow use of low value external components and yield a smaller solution size and profile. As a cautionary note, operation of the LT3492 at a combination of high switching frequency with high output voltage and high switch current may cause excessive internal power dissipation. Consideration should be given to selecting a switching frequency less than 1MHz if these conditions exist. Connecting FADJ pin to a lowpass filter (R5 and C4 in Figure 1) from the REF pin provides a soft-start function. During start-up, FADJ voltage increases slowly from 0V to the setting voltage. As a result, the switching frequency increases slowly to the setting frequency. This function limits the inrush current during start-up. Input Capacitor Selection For proper operation, it is necessary to place a bypass capacitor to GND close to the VIN pin of the LT3492. A 1F or greater capacitor with low ESR should be used. A ceramic capacitor is usually the best choice. In the buck mode configuration, the capacitor at PVIN has large pulsed currents due to the current returned though the Schottky diode when the switch is off. For the best reliability, this capacitor should have low ESR and ESL and have an adequate ripple current rating. The RMS input current is: IIN(RMS) =ILED *
(1- D) * D
where D is the switch duty cycle. A 1F ceramic type capacitor placed close to the Schottky diode and the ground plane is usually sufficient for each channel. Output Capacitor Selection The selection of output filter capacitor depends on the load and converter configuration, i.e., step-up or step-down. For LED applications, the equivalent resistance of the LED is typically low, and the output filter capacitor should be large enough to attenuate the current ripple. To achieve the same LED ripple current, the required filter capacitor value is larger in the boost and buck-boost mode applications than that in the buck mode applications. For the LED buck mode applications at 1.3MHz, a 0.22F ceramic capacitor is usually sufficient for each channel. For the LED boost and buck-boost applications at 1.3MHz, a 1F ceramic capacitor is usually sufficient for each channel. Lower switching frequency requires proportionately higher capacitor values. If higher LED current ripple can be tolerated, a lower output capacitance can be selected to reduce the capacitor's cost and size. Use only ceramic capacitors with X7R or X5R dielectric, as they are good for temperature and DC bias stability of the capacitor value. All ceramic capacitors exhibit loss of capacitance value with increasing DC voltage bias, so it may be necessary to choose a higher value capacitor to get the required capacitance at the operation voltage. Always check that the voltage rating of the capacitor is sufficient. Table 1 shows some recommended capacitor vendors.
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LT3492 APPLICATIONS INFORMATION
Table 1. Ceramic Capacitor Manufacturers
VENDOR Taiyo Yuden AVX Murata Kemet TDK TYPE Ceramic Ceramic Ceramic Ceramic Ceramic SERIES X5R, X7R X5R, X7R X5R, X7R X5R, X7R X5R, X7R CooperET SD20 SD25 Taiyo Yuden NP04SZB TDK VLF5014A Wurth Electronics 7447789133 Coilcraft M556132
Table 2. Surface Mount Inductors
PART NUMBER Sumida CDRH4D28 CDRH5D28 VALUE (H) 15 22 33 15 22 33 15 22 15 22 33 22 DCR ( MAX) 0.149 0.122 0.189 0.1655 0.2053 0.2149 0.180 0.210 0.32 0.46 0.24 0.19 IRMS (A) 0.76 0.9 0.75 1.25 1.12 1.11 0.95 0.77 0.97 0.51 1.22 1.45 7.3 x 7.3 x 3.2 6.1 x 6.1 x 3.2 SIZE W x L x H (mm3) 5.0 x 5.0 x 3.0 6.0 x 6.0 x 3.0
Inductor Selection Inductor value is selected based on switching frequency and desired transient response. The data sheet applications show appropriate selections for a 1.3MHz switching frequency. Proportionately higher values may be used if a lower switching frequency is selected. Several inductors that work well with the LT3492 are listed in Table 2. However, there are many other manufacturers and devices that can be used. Consult each manufacturer for more detailed information and their entire range of parts. Ferrite core inductors should be used to obtain the best efficiency. Choose an inductor that can handle the necessary peak current without saturating, and ensure that the inductor has a low DCR (copper-wire resistance) to minimize I2R power losses. An inductor with a magnetic shield should be used to prevent noise radiation and cross coupling among the three channels. Diode Selection The Schottky diode conducts current during the interval when the switch is turned off. Select a diode VR rated for the maximum SW voltage. It is not necessary that the forward current rating of the diode equal the switch current limit. The average current, IF , through the diode is a function of the switch duty cycle. Select a diode with forward current rating of: IF = IL * (1 - D) where IL is the inductor current. 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 sufficient low leakage current at hot temperature. Table 3 shows several Schottky diodes that work well with the LT3492.
5.0 x 5.0 x 2.0 5.0 x 5.0 x 2.5 4.0 x 4.0 x 1.8
4.5 x 4.7 x 1.4
Table 3. Schottky Diodes
PART NUMBER ZETEX ZLLS350 ZLLS400 DIODES B1100 ROHM RB160M-60 60 1.0 PMDU/SOD-123 100 1.0 SMA 40 40 0.38 0.52 SOD523 SOD323 VR (V) IF (A) PACKAGE
Undervoltage Lockout The LT3492 has an undervoltage lockout circuit that shuts down all the three converters when the input voltage drops below 2.1V. This prevents the converter from switching in an erratic mode when powered from a low supply voltage. Programming the LED Current An important consideration when using a switch with a fixed current limit is whether the regulator will be able to supply the load at the extremes of input and output voltage range. Several equations are provided to help determine
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LT3492 APPLICATIONS INFORMATION
this capability. Some margin to data sheet limits is included, along with provision for 200mA inductor ripple current. For boost mode converters: IOUT(MAX ) 0.4A VIN(MIN) VOUT(MAX ) Thermal Considerations The LT3492 is rated to a maximum input voltage of 30V for continuous operation, and 40V for nonrepetitive one second transients. Careful attention must be paid to the internal power dissipation of the LT3492 at higher input voltages and higher switching frequencies/output voltage to ensure that a junction temperature of 125C is not exceeded. This is especially important when operating at high ambient temperatures. Consider driving VIN from 5V or higher to ensure the fastest switching edges, and minimize one source of switching loss. 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 LT3492. Board Layout The high speed operation of the LT3492 demands careful attention to board layout and component placement. The exposed pad of the package is the only GND terminal of the IC and is important for thermal management of the IC. Therefore, it is crucial to achieve a good electrical and thermal contact between the exposed pad and the ground plane of the board. Also, in boost configuration, the Schottky rectifier and the capacitor between GND and the cathode of the Schottky are in the high frequency switching path where current flow is discontinuous. These elements should be placed so as to minimize the path between SW and the GND of the IC. To reduce electromagnetic interference (EMI), it is important to minimize the area of the SW node. Use the GND plane under SW to minimize interplane coupling to sensitive signals. To obtain good current regulation accuracy and eliminate sources of channel to channel coupling, the ISP and ISN inputs of each channel of the LT3492 should be run as separate lines back to the terminals of the sense resistor. Any resistance in series with ISP and ISN inputs should be minimized. Avoid extensive routing of high impedance traces such as OVP and VC. Make sure these sensitive signals are star coupled to the GND under the IC rather than a GND where switching currents are flowing. Finally, the bypass capacitor on the VIN supply to the LT3492 should be placed as close as possible to the VIN terminal of the device.
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For buck mode converters: ILED(MAX) 0.4A For SEPIC and buck-boost mode converters: IOUT(MAX ) 0.4A VIN(MIN) ( VOUT(MAX ) + VIN(MIN) )
If some level of analog dimming is acceptable at minimum supply levels, then the CTRL pin can be used with a resistor divider to VIN (as shown in the Block Diagram) to provide a higher output current at nominal VIN levels. The LED current of each channel is programmed by connecting an external sense resistor RSENSE in series with the LED load, and setting the voltage regulation threshold across that sense resistor using CTRL input. If the CTRL voltage, VCTRL, is less than 1V, the LED current is: ILED = VCTRL 10 * RSENSE 100mV RSENSE
If VCTRL is higher than 1V, the LED current is: ILED =
The CTRL pins should not be left open. The CTRL pin can also be used in conjunction with a PTC thermistor to provide overtemperature protection for the LED load as shown in Figure 2.
VREF 45k 2V 50k CTRL1-3
470 PTC
3492 F02
Figure 2
11
LT3492 TYPICAL APPLICATIONS
Minimum BOM Buck Mode LED Driver
PVIN 58V ISP1 330m ISN1 ISP2 330m ISN2 ISP3 330m ISN3 C1-C3 1F 3
10 LEDs
0.3A C4 C5 0.22F 0.22F
0.3A
0.3A C6 0.22F
L1 33H
D1
D2
L2 33H
L3 33H
D3
VIN 5V C7 1F
SW1 ISP1-3 ISN1-3 VIN PWM1-3 SHDN
SW2
SW3
TG1-3 VC1-3 VREF CTRL1-3 150k FADJ 49.9k 1.3MHz 470pF 10k
LT3492
GND
OVP1-3
3492 TA07a
C1-C3, C7: MURATA GRM31CR72A105KA01L C4-C6: MURATA GRM21BR71H224KA01 D1-D3: DIODES B1100 L1-L3: TDK VLF5014AT-330MR50
300:1 PWM Dimming at 100Hz
95 PWM 5V/DIV ILED 0.5A/DIV IL 0.5A/DIV 5s/DIV
3492 TA07b
Efficiency
90 85 EFFICIENCY (%) 80 75 70 65 60 55 50 0 20 80 60 40 PWM DUTY CYCLE (%) 100
3492 TA07c
3492f
12
LT3492 TYPICAL APPLICATIONS
Triple Boost 100mA x 12 LED Driver
PVIN 12V L1 22H C1 2.2F 3
L2 22H
L3 22H
D1 C2 1F ISP1 1 ISN1 TG1 M1 TG2 C3 1F
D2 ISP2 1 ISN2 M2 TG3 C4 1F
D3 ISP3 1 ISN3 M3
1M 12 LEDs 100mA OVP1 20k 12 LEDs 100mA
1M OVP2 20k 12 LEDs 100mA
1M OVP3 20k
VIN 5V
C5 1F
SW1 ISP1-3 ISN1-3 VIN PWM1-3 SHDN
SW2
SW3
LT3492
TG1-3 OVP1-3 VC1-3 VREF CTRL1-3 FADJ
18.2k 2.2nF 150k
GND C1: MURATA GRM31MR71C225KA35 C2-C4: MURATA GRM31CR72A105KA01L C5: MURATA GRM31MR71H105KA88 D1-D3: DIODES B1100 L1-L3: TDK VLF5014AT-220MR62 M1-M3: ZETEX ZXMP6A13F
49.9k
3492 TA03a
1.3MHz
1000:1 PWM Dimming at 100Hz
85 PWM 5V/DIV EFFICIENCY (%) 80 75 ILED 0.1A/DIV IL 0.5A/DIV 2s/DIV
3492 TA03b
Efficiency vs PWM Duty Cycle
70 65 60 55 50 0 20 40 60 80 100
3492 TA03c
PWM DUTY CYCLE (%)
3492f
13
LT3492 TYPICAL APPLICATIONS
Dual Boost LED Driver
PVIN 12V L1 22H
C1 2.2F 3
L2 22H
L3 22H
D1 C2 1F ISP1 1 ISN1 C3 1F
D2 ISP2 1 ISN2 C4 1F
D3 ISP3 1 ISN3
M1
M2
1M 12 LEDs 100mA OVP1 20k 12 LEDs 200mA
1M OVP2-3 20k
SW1 TG1 ISP1-3 ISN1-3 VIN PWM1-3 SHDN
SW2
SW3 TG2
VIN 3V TO 12V
C5 1F
LT3492
OVP1-3 TG3 VC1-3 VREF CTRL1-3 FADJ
OPEN 18.2k 2.2nF 150k
GND C1: MURATA GRM31MR71C225KA35 C2-C4: MURATA GRM31CR72A105KA01L C5: MURATA GRM31MR71H105KA88 D1-D3: DIODES B1100 L1-L3: TDK VLF5014AT-220MR62 M1, M2: ZETEX ZXMP6A13F
49.9k
3492 TA04
1.3MHz
1000:1 PWM Dimming at 100Hz for 200mA LEDs
PWM 5V/DIV ILED 0.2A/DIV
Efficiency vs PWM Duty Cycle for 200mA LEDs
85 80 75 EFFICIENCY (%) 70 65 60 55 50 0 20 40 60 80 100
3492 TA04c
IL2 IL3 0.5A/DIV 2s/DIV
3492 TA04b
PWM DUTY CYCLE (%)
3492f
14
LT3492 TYPICAL APPLICATIONS
Triple Boost 100mA x 9 LED Driver with VIN Controlled Dimming
VIN 5V TO 16V 357k CTRL1-3 40.2k D1 C2 1F ISP1 1 ISN1 TG1 M1 TG2 M2 C3 1F D2 ISP2 1 ISN2 TG3 M3 C4 1F D3 ISP3 1 ISN3 L1 15H
C1 2.2F 3
L2 15H
L3 15H
750k 9 LEDs 100mA OVP1 20k 9 LEDs 100mA
750k OVP2 20k 9 LEDs 100mA
750k OVP3 20k
C5 1F
SW1 ISP1-3 ISN1-3 VIN PWM1-3 SHDN
SW2 LT3492
SW3
TG1-3 OVP1-3 VC1-3 VREF FADJ CTRL1-3
3492 TA08
18.2k 2.2nF 430k
GND C1: MURATA GRM31MR71C225KA35 C2-C5: MURATA GRM31MR71H105KA88 D1-D3: ZETEX ZLLS400TA L1-L3: TAIYO YUDEN NP04SZB 150M M1-M3: ZETEX ZXMP6A13F
100k 1MHz
LED Current Decreasing with VIN
110 100 90 EFFICIENCY (%) 80 ILED (mA) 70 60 50 40 30 20 2 6 10 VIN (V)
3492 TA08b
Efficiency vs VIN
90 80 70 60 50 40 30 20 10
14
18
0 4 8 VIN (V)
3492 TA08c
12
16
3492f
15
LT3492 TYPICAL APPLICATIONS
Triple LED Driver Driving LED Strings in Buck, Boost and Buck-Boost Modes
VIN 10V TO 16V C1 3.3F 3 ISP1 330m ISN1 TG1 D2 M1 C3 1F ISP2 1 ISN2 2 LEDs 0.3A C2 0.47F 10 LEDs L1 6.8H D1 0.1A TG2 M2 ISN3 1 ISP3 D3 825k OVP2 20k C4 0.1F VIN C5 1F 3.9M OVP3 100k TG3 M3
L2 22H
L3 33H
4 LEDs
0.1A
SW1 ISP1-3 ISN1-3 VIN PWM1-3 SHDN
SW2
SW3
LT3492
TG1-3 OVP2-3 VC1-3 VREF CTRL1-3 150k FADJ 49.9k 1.3MHz 2.2nF 18.2k
GND C1: MURATA GRM55DR71H335KA0193 C2: MURATA GRM21BR71H474KA88 C3, C5: MURATA GRM31MR71H105KA88 C4: MURATA GRM21BR71H104KA01B D1: DIODES DFLS130 D2, D3: ROHM RB160M-60 L1: TDK VLF5014AT-6R8MR99 L2: TDK VLF5014AT-229MR62 L3: TDK VLF5014AT-330MR50 M1: ZETEX ZXMP3A13F M2, M3 ZETEX ZXMP6A13F
OVP1
3492 TA05
3000:1 PWM Dimming at 100Hz for CH1 (Buck Mode)
PWM 5V/DIV
3000:1 PWM Dimming at 100Hz for CH2 (Boost Mode)
PWM 5V/DIV
ILED 0.5A/DIV IL 0.5A/DIV 1s/DIV
3492 TA05b
ILED 0.1A/DIV IL 0.5A/DIV 1s/DIV
3492 TA05c
3000:1 PWM Dimming at 100Hz for CH3 (Buck-Boost Mode)
PWM 5V/DIV
ILED 0.1A/DIV IL 0.5A/DIV 1s/DIV
3492 TA05d
3492f
16
LT3492 TYPICAL APPLICATIONS
Triple Buck Mode LED Driver with Open LED Protection
PVIN 48V ISP1 330m ISN1 TG1 M1 TG2 M2 ISP2 330m ISN2 M3 ISP3 330m ISN3 TG3 C1-C3 1F 3
80.6k 10 LEDs 0.3A C4 0.47F 10 LEDs C5 0.47F 0.3A
80.6k
80.6k 10 LEDs 0.3A C6 0.47F
5.6k M4 OVP1 2k L1 22H
5.6k M5 M6 OVP2 OVP1 2k 2k L2 22H
5.6k L3 22H
D1
D2
D3
SW1 ISP1-3 ISN1-3 VIN PWM1-3 SHDN
SW2
SW3
VIN 5V
C7 1F
LT3492
TG1-3 OVP1-3 VC1-3 VREF CTRL1-3 FADJ
10k 470pF 430k
GND C1-C3, C7: MURATA GRM31CR72A105KA01L C4-C6: MURATA GRM21BR72A474KA73 D1-D3: ROHM RB160M-60 L1-L3: TDK VLF5014AT-220MR62 M1-M3: ZETEX ZXMP6A13F M4-M6: PHILIPS BC858B
100k
3492 TA02
1MHz
2000:1 PWM Dimming at 100Hz
PWM 5V/DIV ILED 0.5A/DIV IL 0.5A/DIV 1s/DIV
3492 TA02b
Efficiency vs PWM Duty Cycle for 200mA LEDs
95 90 85 EFFICIENCY (%) 80 75 70 65 60 55 50 0 20 80 60 40 PWM DUTY CYCLE (%) 100
3492 TA02c
3492f
17
LT3492 PACKAGE DESCRIPTION
FE Package 28-Lead Plastic TSSOP (4.4mm)
(Reference LTC DWG # 05-08-1663)
Exposed Pad Variation EB
4.75 (.187) 9.60 - 9.80* (.378 - .386) 4.75 (.187) 28 2726 25 24 23 22 21 20 19 18 1716 15 6.60 0.10 4.50 0.10
SEE NOTE 4
2.74 (.108) 0.45 0.05 1.05 0.65 BSC
RECOMMENDED SOLDER PAD LAYOUT
EXPOSED PAD HEAT SINK ON BOTTOM OF PACKAGE 0.10
6.40 2.74 (.252 (.108) BSC
1 2 3 4 5 6 7 8 9 10 11 12 13 14 1.20 (.047) MAX
0 -8
4.30 - 4.50* (.169 - .177)
0.25 REF
0.09 - 0.20 (.0035 - .0079)
0.50 - 0.75 (.020 - .030)
0.65 (.0256) BSC
0.195 - 0.30 (.0077 - .0118) TYP
0.05 - 0.15 (.002 - .006)
FE28 (EB) TSSOP 0204
NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS 2. DIMENSIONS ARE IN MILLIMETERS (INCHES) 3. DRAWING NOT TO SCALE
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
3492f
18
LT3492 PACKAGE DESCRIPTION
UFD Package 28-Lead Plastic QFN (4mm x 5mm)
(Reference LTC DWG # 05-08-1712 Rev B)
0.70
0.05
4.50
0.05 3.10
0.05 2.50 REF 2.65 0.05 3.65 0.05
PACKAGE OUTLINE
0.25 0.05 0.50 BSC 3.50 REF 4.10 0.05 5.50 0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 4.00 0.10 (2 SIDES) PIN 1 TOP MARK (NOTE 6) 0.75 0.05 R = 0.05 TYP
2.50 REF R = 0.115 TYP 27 28
PIN 1 NOTCH R = 0.20 OR 0.35 45 CHAMFER
0.40 1 2
0.10
5.00 0.10 (2 SIDES)
3.50 REF 3.65 0.10 2.65 0.10
(UFD28) QFN 0506 REV B
0.200 REF 0.00 - 0.05
0.25
0.05
0.50 BSC BOTTOM VIEW--EXPOSED PAD
NOTE: 1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WXXX-X). 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE
3492f
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.
19
LT3492 TYPICAL APPLICATION
Triple Buck-Boost Mode 100mA x 4 LED Driver
PVIN 10V TO 16V C1 2.2F 100mA L1 22H 4 LEDs L2 22H 100mA 4 LEDs L3 22H 100mA 4 LEDs
3000:1 PWM Dimming at 100Hz
TG3 M3 3.9M OVP3 100k ILED 0.1A/DIV IL 0.5A/DIV 1s/DIV
3492 TA06b
TG1
M1 3.9M OVP1 100k
TG2
M2 3.9M OVP2 100k
PWM 5V/DIV
ISN1 1 ISP1 D1 C2 0.1F
ISN2 1 ISP2 D2 C4 0.1F
ISN3 1 ISP3 D3 C6 0.1F
C3 1F PVIN
C5 1F PVIN
C7 1F PVIN
VIN 5V TO 16V
C8 1F
SW1 ISP1-3 ISN1-3 VIN PWM1-3 SHDN
SW2 LT3492
SW3
TG1-3 OVP1-3 VC1-3 VREF CTRL1-3 150k FADJ
18.2k 2.2nF
3492 TA06
GND C1: MURATA GRM31MR71E225KA93 C2, C4, C6: MURATA GRM21BR71H104KA01B C3, C5, C7: MURATA GRM31MR71H105KA88 C8: MURATA GRM31MR71E105KA93 D1-D3: ROHM RB160M-60 L1-L3: TDK VLF5014AT-220MR62 M1-M3: ZETEX ZXMP6A13F
49.9k 1.3MHz
RELATED PARTS
PART NUMBER LT3496 LT3474 LT3475 LT3476 LT3477 LT3478/LT3478-1 LT3486 LT3517 LT3518 LT3755/LT3755-1 LT3756-1 LTC(R)3783 DESCRIPTION Triple 0.75A, 2.1MHz, 45V LED Driver 36V, 1A (ILED), 2MHz, Step-Down LED Driver Dual 1.5A (ILED), 36V, 2MHz Step-Down LED Driver COMMENTS VIN: 3V to 30V, VOUT(MAX) = 45V, Dimming = 3000:1, ISD < 1A, 4mm x 5mm QFN and TSSOP16E Packages VIN: 4V to 36V, VOUT(MAX) = 13.5V, True Color PWM Dimming = 400:1, ISD < 1A, TSSOP16E Package VIN: 4V to 36V, VOUT(MAX) = 13.5V, True Color PWM Dimming = 3000:1, ISD < 1A, TSSOP20E Package
Quad Output 1.5A, 36V, 2MHz High Current LED Driver VIN: 2.8V to 16V, VOUT(MAX) = 36V, True Color PWM Dimming = 1000:1, ISD < 10A, 5mm x 7mm QFN Package with 1000:1 Dimming 3A, 42V, 3MHz Boost, Buck-Boost, Buck LED Driver 4.5A, 42V, 2.5MHz High Current LED Driver with 3000:1 Dimming Dual 1.3A, 2MHz High Current LED Driver 1.5A, 2.5MHz, 45V LED Driver 2.3A, 2.5MHz, 45V LED Driver 40VIN , 75VOUT, Full Featured LED Controller 100V High Current LED Controller High Current LED Controller VIN: 2.5V to 25V, VOUT(MAX) = 40V, Dimming = Analog/PWM, ISD < 1A, QFN and TSSOP20E Packages VIN: 2.8V to 36V, VOUT(MAX) = 42V, True Color PWM Dimming = 3000:1, ISD < 3A, TSSOP16E Package VIN: 2.5V to 24V, VOUT(MAX) = 36V, True Color PWM Dimming = 1000:1, ISD < 1A, 5mm x 3mm DFN and TSSOP16E Packages VIN: 3V to 30V, VOUT(MAX) = 45V, Dimming = 3000:1, ISD < 1A, 4mm x 4mm QFN and TSSOP16E Packages VIN: 3V to 30V, VOUT(MAX) = 45V, Dimming = 3000:1, ISD < 1A, 4mm x 4mm QFN and TSSOP16E Packages VIN: 4.5V to 40V, VOUT(MAX) = 75V, True Color PWM Dimming = 3000:1, ISD < 1A, 3mm x 3mm QFN-16 and MS16E Packages VIN: 6V to 100V, VOUT(MAX) = 100V, True Color PWM Dimming = 3000:1, ISD < 1A, 3mm x 3mm QFN-16 and MS16E Packages VIN: 3V to 36V, VOUT(MAX) = Ext FET, True Color PWM Dimming = 3000:1, ISD < 20A, 5mm x 4mm QFN10 and TSSOP16E Packages
3492f LT 1109 * PRINTED IN USA
20 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2009

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