DESIGN FEATURES L 4.5A Monolithic LED Drivers with 3000:1 Dimming are Ideal for a Wide Range of High Power LED Applications by Mark W. Marosek Introduction The LT3478 and LT3478-1 are monolithic step-up DC/DC converters specifically designed to drive high brightness LEDs with a constant current over a wide programmable range. They are extremely easy to use and include programmable features for optimizing performance, reliability, size and overall solution cost. These devices can operate in boost, buckmode boost and buck-boost mode LED driver topologies. Depending on the topology, they can provide up to 4A of LED current, a level unmatched by other monolithic LED drivers. The LT3478 and LT3478-1 are ideal for high power LED applications, including automotive and avionic lighting, and are available in a 16-pin thermally enhanced TSSOP package with either E-grade or I-grade temperature ratings. The LT3478 and LT3478-1 operate similarly to conventional current mode boost converters, but use LED current (instead of output voltage) as the main source of feedback for the control loop. The block diagram in Figure 2 shows the major functions of each part. Both parts use high side LED current sensing to extend operation to buck and buck-boost modes. The LT3478-1 saves space and cost by integrating the current sense resistor and limits maximum LED current to 1.05A. The LT3478 uses an external sense resistor to allow programming of maximum LED current up to 4A. boards and airplane cockpits, require very high levels of PWM dimming. The LT3478 and LT3478-1 offer a 3000:1 PWM dimming range (preserving LED color) in addition to an optional 10:1 analog dimming range. Current control for dimming is an important feature, but it is just as important to avoid overdriving LEDs beyond their maximum rated current. The LT3478 and LT3478-1 make it easy to set the maximum current and to derate the maximum current relative to temperature. Programming the LED Current for Protection and Dimming Maximum LED Current The LT3478 and LT3478-1 control maximum LED current using the voltage at the CTRL1 pin, unless the device is set to derate the maximum LED current relative to temperature (using CTRL2 pin described below). The voltage at CTRL1 pin can be set using a simple resistor divider from LEDs are a desirable lighting solution in part because of their wide dimming range via simple current control. For instance, environments with the potential for very low ambient light conditions, such as automotive dashL1 10µH VIN 8V TO 16V C1 4.7µF 25V VIN VS L D1 C2 10µF 25V SW SHDN OUT 100 VREF R1 45.3k CTRL2 LT3478-1 700mA LED 95 EFFICIENCY (%) OVPSET R4 54.9k CTRL1 R2 130k PWM SS CSS 1µF L1: CDRH104R-100NC D1: PDS560 Q1: Si2318DS LEDs: LUXEON III (WHITE) VC RT CC 0.1µF RT 69.8k 90 85 fOSC = 500kHz 80 3.3V 0V ILED = 700mA fOSC = 500kHz PWM DUTY CYCLE = 100% 100Hz 10 Q1 PWM DIMMING RATIO = 1000:1 6 LEDs LUXEON III (WHITE) 8 12 VIN (V) 14 16 R3 10k Figure 1. Automotive TFT LCD backlight, 15W, 6 LEDs at 700mA, boost LED driver Linear Technology Magazine • June 2007 13 L DESIGN FEATURES SHDN VS 11 L 4 SS 5 10µA 9.5mΩ + – + 1.4V VIN VOUT 6 OVERVOLTAGE DETECT – 57mV OVPSET INRUSH CURRENT PROTECTION UVLO REF 1.24V 3 1, 2 VC – SW 16 + 100Ω RSENSE 0.1Ω (INTERNAL FOR LT3478-1) SOFT-START RSENSE (EXTERNAL FOR LT3478) LED 7 PWM DETECT VREF 10 S Q Q1 R LED PWM + + + – – + LED Σ 1000Ω 1V PWM + 13 GM + CTRL2 LED SLOPE COMP Q2 – 12 LED + 1.05V – CTRL1 OSC 14 RS – TO OVERVOLTAGE DETECT CIRCUIT 8 15 OVPSET RT 17 9 EXPOSED PAD (GND) VC Figure 2. LT3478 and LT3478-1 block diagram 13 R2 12 LT3478/LT3478-1 VREF VOUT (LT3478) RSENSE CTRL2 CTRL1 LED R1 Figure 3. Programming maximum LED current LED CURRENT (mA) 1400 TA = 25°C CTRL2 = VREF (FOR LT3478 SCALE BY 0.1Ω/RSENSE) 1050 LT3478-1 700 350 VREF 0 0 0.35 0.70 CTRL1 (V) 1.05 1.40 Figure 4. LED current vs CTRL1 voltage 14 VREF (see Figure 3), from an external voltage source, or by connecting it directly to the VREF pin for maximum current. Figure 4 shows LED current versus CTRL1 pin voltage. Temperature-Based Derating of the Maximum LED Current To ensure optimum reliability, LED manufacturers specify curves of maximum allowed LED current versus temperature (Figure 5). If the LED current is not derated relative to temperature, it is possible to permanently damage the LED. The LT3478 and LT3478-1 enable temperature derating via the CTRL2 pin. Simply connect CTRL2 to VREF via a temperature-dependent resistor divider as shown in Figure 6. As the temperature rises, the voltage at CTRL2 falls. When CTRL2 falls below CTRL1, the voltage at CTRL2 takes over in setting the maximum LED current (Figure 7). 900 800 If FORWARD CURRENT (mA) 10 700 LUXEON V EMITTER CURRENT DERATING CURVE 600 500 EXAMPLE LT3478-1 PROGRAMMED LED CURRENT DERATING CURVE 400 300 200 100 0 0 25 50 75 TA AMBIENT TEMPERATURE (°C) 100 LUXEON V EMITTER (GREEN, CYAN, BLUE, ROYAL BLUE) θJA = 20°C/W Figure 5. LED current derating curve vs ambient temperature Linear Technology Magazine • June 2007 DESIGN FEATURES L R4 13 12 R1 VREF LT3478/LT3478-1 CTRL2 CTRL1 OPTION A TO D R3 RY RNTC RNTC A RX RNTC B RY RNTC C D The temperature at which LED current begins to decrease and the rate of decrease are selectable by the resistor network/values chosen. Table 1 lists several NTC resistor manufacturers. Murata Electronics notably provides an online simulator to select the required resistor combinations as shown in Figure 6 including a catalog describing the NTC resistor specifications. Figure 5 shows an example of LT3478-1 programmed LED current falling versus temperature using the option C, shown in Figure 6, with R4 = 19.3k, RY = 3.01k and RNTC = 22k (NCP15XW223J0SRC). A more detailed description of how to determine these values by hand calculation is given in the LT3478 and LT3478-1 data sheet. Analog Dimming Many LED applications require accurate brightness control. LED brightness can be reduced by simply decreasing the programmed LED VS L CTRL2 Contact Murata Electronics North America www.murata.com TDK Corporation www.tdk.com Digi-Key www.digikey.com COUT SW current, but reducing the operating current of the LED changes the color of the LED. This method is known as analog dimming and is available in the LT3478 and LT3478-1 by reducing the voltage at the CTRL1 pin to as low as 0.1V (10:1 dimming from 1V). If color preservation is important, then PWM dimming is a better option. PWM Dimming PWM dimming (Figures 8 and 9) yields high dimming ratios with no current-related LED color change. PWM dimming is implemented in the LT3478 and LT3478-1 via the PWM pin. When the PWM pin is active high (TPWM(ON)) or low, the LED current is either at its maximum or off, respectively. The LED on time, and hence the average current, is controlled by the duty cycle of the PWM pin. Because the LED is always operating at the same current (maximum set by CTRL1), and only the average current changes, dimming is achieved without changing the color of the LED. PWM dimming is not new, but the ability to achieve high PWM dimming ratios (requiring extremely low PWM duty cycles) is challenging. The LT3478 and LT3478-1 use a patented architecture to achieve PWM dimming ratios exceeding 3000:1 at 100Hz. The application circuit and waveforms shown 1000 900 800 CTRL1 700 600 500 400 CTRL2 300 200 LED CURRENT = MINIMUM 100 OF CTRL1, CTRL2 R3 = OPTION C 0 0 25 50 75 TA AMBIENT TEMPERATURE (°C) 100 Figure 7. CTRL1 and CTRL2 voltages vs temperature. The voltage at CTRL1 sets the maximum LED current until the voltage at CTRL2 falls below that of CTRL1. At that point (here at 25°C) CTRL2 takes over and derates the maximum current to rising temperature. in Figures 10, 11 and 12 show a PWM dimming ratio that can actually exceed 3000:1 if PWM on time is reduced to only 3 switching cycles (TPWM(ON) < 3.3µs for fPWM = 100Hz). The simplified waveforms in Figure 10 and guidelines listed below explain the relationship between PWM duty cycle, PWM frequency, PWM dimming ratio and LED current. Strategies for achieving maximum possible PWM dimming using the PWM pin fall out of the relation: PWM DIMMING RATIO 1 = MINIMUM PWM DUTY CYCLE 1 = TPWM(OON)MIN • f PWM qFor a PWM frequency (fPWM) of 100Hz, a PDR of 3000 implies a PWM on time of 3.3µs. qThe lower the PWM frequency, the greater the PWM dimming ratio (for a fixed PWM on time). However, there are limits to how VOUT SHDN VREF Manufacturer RX Figure 6. Programming LED current derating curve vs temperature (RNTC located on LED’s circuit board) VIN 1100 Table 1. NTC resistor manufacturers/distributors CTRL1, CTRL2 PIN VOLTAGES (mV) 10 R2 TPWM TPWM(ON) (LT3478) LT3478/ LT3478-1 RSENSE (= 1/fPWM) PWM CTRL1 OVPSET RT LED VC PWM PWM DIMMING CONTROL Figure 8. PWM dimming control Linear Technology Magazine • June 2007 INDUCTOR CURRENT LED CURRENT MAX ILED Figure 9. PWM dimming waveforms 15 L DESIGN FEATURES VIN VS L 1000 SW SHDN 100 OUT VREF CTRL2 100k LT3478-1 LED VIN = 12V 6 LEDS AT 700mA PWM FREQ=100Hz fOSC = 1.67MHz 4.7µF LED CURRENT (mA) 3.3µF PDS560 2.2µH 12V 700mA CTRL1 10 1 OVPSET TA=25°C CTRL1=0.7V CTRL2=VREF 130k PWM SS 1µF VC RT 0.1µF 0 11k 1 10 100 1000 PWM DIMMING RATIO 10000 Figure 11. LED current versus PWM dimming ratio for the circuit in Figure 10 3.3V 0V 100Hz PWM DIMMING RATIO = 3000:1 Q1 voltage limits the maximum output voltage, given by: Maximum output voltage = OVPSET • 41 Figure 10. Boost LED driver optimized for high PWM dimming ratio (3000:1): 15W, 6 LEDs at 700mA low the PWM frequency can be operated since the human eye can see flicker below about 80Hz. qHigher programmed switching frequency (fOSC) improves PDR but reduces efficiency and increases internal heating. In general, TPWM(ON)MIN = 3 • 1/fOSC (approximately 3 switch cycles). qLeakage currents from the output capacitor should be minimized. The LT3478 and LT3478-1 both turn off any circuitry running from VOUT when the PWM pin is low. qFor an even wider dimming range, the PWM and analog dimming features can be combined, where TDR = PDR • ADR where TDR = Total Dimming Ratio PDR = PWM Dimming Ratio ADR = Analog Dimming Ratio A PDR of 3000:1 and an ADR of 10:1 (CTRL = 0.1V) yields a TDR of 30,000:1. Open LED Protection The output voltage has a programmable maximum to avoid damaging the LEDs due to a disconnect (open LED) followed by a reconnect. During LED disconnect, the converter can go open loop and drive the output voltage so high that the internal power switch is damaged. Most LED drivers have a fixed maximum output voltage to save the switch, but this may be too high for the reconnected string of LEDs. The LT3478 and LT3478-1 provide a programmable overvoltage protection (OVP) level to limit output voltage based on the number of series connected LEDs. The OVPSET pin OVPSET voltage can be derived from VREF by it’s own resistor divider or by adding one resistor to the divider used to define CTRL1 voltage. OVPSET program level should not exceed 1V to ensure the switch voltage does not exceed 42V. Robust Operation: Fault Detection and Soft-Start For robust performance during hotplugging, startup, or during normal operation, the LT3478 and LT3478-1 monitor system parameters for any of the following faults: VIN < 2.8V, SHDN < 1.4V, inductor inrush current greater than 6A, and/or output voltage greater than programmed OVP. On detection of any of these faults, the LT3478 and LT3478-1 stop switching immediately and the soft-start pin is discharged (Figure 13). When all faults are removed and the SS pin has SW SS PWM 5V/DIV FAULTS TRIGGERING SOFT-START LATCH WITH SW TURNED OFF IMMEDIATELY: IL 1A/DIV VIN < 2.8V OR SHDN < 1.4V OR VOUT > OVP OR I(INDUCTOR) > 6A ILED 1A/DIV 1µs/DIV Figure 12. PWM dimming waveforms for the circuit in Figure 10 16 0.65V (ACTIVE THRESHOLD) 0.25V (RESET THRESHOLD) 0.15V SOFT-START LATCH RESET: SOFT-START LATCH SET: SS < 0.25V AND VIN > 2.8V AND SHDN > 1.4V AND VOUT < OVP AND I(INDUCTOR) < 6A Figure 13. LT3478/ LT3478-1 fault detection and SS pin timing diagram Linear Technology Magazine • June 2007 DESIGN FEATURES L VIN 3.8V TO 6.5V NiMH 4× C1 10µF 10V L1 6.8µH VIN ON OFF VS D1 L SHDN 80 ILED = 1A fOSC = 500kHz 75 PWM DUTY CYCLE = 100% C2 4.7µF 16V SW OUT CTRL2 R1 100k LT3478-1 Q2 1A LED CTRL1 R4 510Ω OVPSET L1: CDRH105R-6R8 D1: B320 Q1: Si2302ADS Q2: Si2315BDS LED: LUXEON III (WHITE) R2 34k PWM SS CSS 1µF 3.3V 0V VC 70 EFFICIENCY (%) VREF 65 60 RT CC 0.1µF 55 RT 69.8k R5 510Ω 50 SINGLE LED LUXEON III (WHITE) 3 4 5 VIN (V) fOSC = 500kHz 1kHz 6 7 Q1 PWM DIMMING RATIO = 200:1 R3 10k Figure 14. Portable camera flash: 4W single LED at 1A buck-boost mode LED driver been discharged to at least 0.25V, an internal 12µA supply charges the SS pin with a rate programmed using an external capacitor CSS. A gradual ramp up of SS pin voltage is equivalent to a ramp up of switch current limit until SS exceeds the VC pin voltage. Conclusion The LT3478 and LT3478-1 are ideal for boost, buck or buck-boost mode LED applications requiring high LED current operation and high PWM dimming ratios. The high 4.5A peak switch curPVIN 32V High Efficiency: Separate Inductor and IC Supplies, Programmable fOSC, 60mΩ Switch The LT3478 and LT3478-1 can use separate supplies for the IC and the inductor to optimize efficiency and switch duty cycle range. Detection of inductor inrush current uses VS and L pins independent of the VIN supply of the IC (Figure 2). This allows VIN to be supplied from the lowest available supply (at least 2.8V) in the system to minimize efficiency lost in the power switch driver. The inductor can then be powered from a supply (between 2.8V and 36V) better suited to the duty cycle and power requirements of the LED load. The switching frequency of the power switch can be tailored to achieve the optimum inductor size and efficiency performance required for the system. The 60mΩ switch further improves efficiency by keeping switch losses to a minimum for high duty cycle operation. rent limit combined with a new patent pending PWM dimming architecture allow the LT3478 and LT3478-1 to provide high PWM dimming ratios for LED currents up to 4A. L C1 3.3µF 50V RSENSE 0.068Ω 1.5A 4 LEDs R4 365Ω TYPICAL EFFICIENCY = 90% FOR CONDITIONS/COMPONENTS SHOWN (PWM DUTY CYCLE = 100%, TA =25°C) C3 10µF 25V Q2 L1 10µH VIN 3.3V C2 4.7µF 10V D1 VIN VS L OUT LED SW SHDN L1: CDRH105R-100 D1: PDS560 Q1: 2N7002 Q2: Si2319DS LEDs: LXK2 (WHITE) Q1 PWM R3 10k VREF R1 24k R5 510Ω LT3478 CTRL2 PWM CTRL1 DIMMING RATIO = 3000:1 OVPSET 3.3V R2 100k SS CSS 1µF VC RT CC 0.1µF 0V 100Hz RT 69.8k fOSC = 500kHz Figure 15. High powered LED lighting: 24W, 4 LEDs at 1.5A buck-boost mode LED driver Linear Technology Magazine • June 2007 17