L DESIGN FEATURES LED Driver Yields 3000:1 True Color PWM Dimming with Any Buck, Boost or Buck-Boost Topology from a Wide 3V–40V Input Range by Xin Qi Introduction High power LEDs are quickly expanding their reach as a light source for TV projection, scanners, and various automotive and avionic products. All require a constant LED current, whether in buck, boost, buck-boost or SEPIC configurations. Pulse Width Modulation (PWM) is the preferred dimming method for these LED systems to preserve LED color over a wide dynamic range of light intensities. The LT3518 is a highly integrated 2.3A full-featured LED driver capable of providing 3000:1 True Color PWM™ dimming ratio in a variety of topologies for high power LED driver applications. The LT3518 features a 45V power switch, 100mV high side current PWM PWM OSC OSC SW SW TD TD = 200ns Figure 1. Regular LED driver timing diagram Figure 2. LT3518 timing diagram sense and accurate open LED protection. It combines a traditional voltage feedback loop and a current feedback loop to operate as a constant current and/or constant voltage source. The programmable soft-start limits inrush current during startup, preventing input current spikes. The LT3518’s wide operating input range of 3V to 40V makes it ideally suitable for automotive applications. The 10:1 analog dimming range further extends the total dimming range to 30,000:1. A PMOS disconnect switch driver is integrated to improve the transient response to the PWM control signal. The programmable operating frequency of 250kHz to 2.5MHz allows optimization of the external components for efficiency or component size. To reduce switching noise interference, the LT3518 is synchronizable to an external clock. LED ARRAY RSENSE(EXT) CFILT(EXT) PMOS PVIN R1 MAIN SWITCH DRIVER R2 Q1 MAIN SWITCH NMOS M1 PWM LED DRIVER PWM Figure 3. External PMOS disconnect switch driver for a conventional LED driver LED ARRAY CFILT(EXT) RSENSE(EXT) PMOS PVIN (VISP) VISP PMOS DRIVER MAIN SWITCH DRIVER VISP – 7V Q1 MAIN SWITCH LT3518 PWM Figure 4. LT3518 internal PMOS driver 20 Highly Effective PWM Dimming Control Alignment of Internal Clock and External PWM signal Most LED drivers operate with an independent, free-running internal oscillator. Each switching cycle begins when the internal oscillator transitions from high to low. When PWM dimming, the switch is turned off when the PWM signal is low. After the PWM signal is driven high, the switch has to wait for the next oscillator high-low transition to turn on, as shown in Figure 1. The turn on delay varies from 0 to one full oscillator cycle, which limits Linear Technology Magazine • October 2007 DESIGN FEATURES L L1 8.2µH VIN 8V TO 16V SHDN VIN PWM PWM SYNC SYNC TGEN C1 2.2µF CTRL SW ISP LT3518 ISN TG CTRL FB RT SS RT 16.9k ONE WIRE CONNECTION FOR LED STRING. THE OTHER SIDE OF LED STRING CAN BE RETURNED TO GROUND ANYWHERE. RSENSE 330mΩ VREF VC C4 0.1µF D1 M1 R1 1M GND LED2 300mA R2 30k C3 0.1µF C2 6.8µF LED1 LED8 C1: KEMET C1206C225K2RAC C2: TDK C5750X7R1H685M C3, C4: MURATA GRM21BR71H104KA01B D1: ZETEX ZLLS2000TA L1: TOKO B992AS-8R2N LEDS: LUXEON I (WHITE) M1: ZETEX ZXMP6A13GTA Figure 5. 1-wire boost 300mA LED driver with LED open protection the achievable PWM dimming ratio. This extra cycle becomes an obstacle when high PWM dimming ratios are required. The LT3518 adopts a new timing scheme, illustrated in Figure 2, to run the converter. Instead of using a freerunning oscillator, the LT3518 aligns the internal oscillator to the external PWM signal. When the PWM signal is low, the internal clock is disabled. The PWM rising edge wakes up the internal oscillator with a fixed 200ns delay. In this manner, the LT3518 has a fast response to the PWM input signal, thus improving the achievable PWM dimming ratio. PMOS Disconnect Switch Driver Recent LED driver designs disable all internal loads to the VC pin during the PWM low period, which preserves the charge state of the VC pin on the external compensation capacitor. This feature reduces the transient recovery time, further increasing the achievable PWM dimming ratio. However, to achieve the best PWM dimming ratio for a buck/buck-boost mode LED driver, other ICs still rely on several additional external components to drive a PMOS disconnect switch. As The LT3518’s wide operating input range of 3V to 40V makes it ideally suitable for automotive applications. shown in Figure 3, a typical PMOS disconnect switch driver consists of an NMOS transistor and a level shift resistor network formed by R1 and R2. This kind of PMOS driver must juggle L1 4.3µH VIN 8V TO 16V SHDN VIN TGEN Figure 6. PWM dimming waveform for Figure 5 at 120Hz PWM frequency and VIN = 10V Linear Technology Magazine • October 2007 C5 0.22µF FB 300mA LT3518 ISP VREF RSENSE 330mΩ CTRL C1 2.2µF ISN SYNC C4 0.1µF 2µs/DIV R1 3.92M SW PWM PWM 5V/DIV IL 1A/DIV D1 R2 124k PWM VC ILED 200mA/ DIV the tradeoffs between fast transient response and high power consumption. The diverse input voltage and LED voltage combinations also make the level shifter design difficult. In contrast, the LT3518 incorporates a PMOS driver inside, which can transition a 1nF gate capacitance PMOS switch in 200ns with a small holding current, typically 600µA. In this way, the LT3518 simplifies board layout, reduces the bill of material, and avoids the dilemma of trading off the power consumption for a fast transient response. Additionally, the LT3518 includes an internal level shifter to ensure the that the TG pin TG RT SS RT 6.04k 2MHz GND M1 C2 4.7µF C3 0.1µF C1: KEMET C0806C225K4RAC C2: KEMET C1206C475K3RAC C3, C4: MURATA GRM21BR71H104KA01B C5: MURATA GRM21BR71H224KA01B D1: ZETEX ZLLS2000TA L1: TOKO B992AS-4R3N LEDS: LUXEON I (WHITE) M1: ZETEX ZXMP6A13GTA Figure 7. Buck-boost LED driver for automotive applications 21 L DESIGN FEATURES PWM 5V/DIV ILED 200mA/ DIV VIN 3.3V IL 1A/DIV 500ns/DIV Figure 8. 3000:1 PWM dimming waveform of application circuit of Figure 7 at 120Hz PWM frequency and VIN = 12V. Applications 1-Wire High PWM Dimming Boost LED Driver Many LED drivers feature high side current sensing that enables the parts to function as a 1-wire current source. To improve PWM dimming ratio in boost configuration, those LED drivers typically rely on a low side NMOS disconnect switch, unfortunately limiting the 1-wire operation. On the contrary, the unique internal PMOS driver of the LT3518 makes 1-wire operation feasible in boost configuration while keeping a high PWM dimming ratio. Figure 5 shows the LT3518 driving eight 300mA LEDs in boost configuration. This setup only needs to provide 1-wire for the top side of the LED string, while the other side of the LED string can be returned to ground anywhere. Figure 6 shows a 1000:1 PWM dimming waveform captured by using this setup. Buck-Boost PWM LED Driver For an application in which the VIN and VOUT ranges overlap, a buckboost topology is preferred. To make the LT3518 with a low side switch function as a buck-boost converter, the LED current should be returned to VIN. Thus, the LEDs see a voltage of VOUT -VIN. Figure 7 depicts a buckboost PWM LED driver for automotive applications. In this setup, the single C3 10µF M1 C2 2.2µF C1 2.2µF L1 15µH 1.5A ISP VIN D1 ISN TG SW SHDN LT3518 VREF CTRL FB PWM PWM SS SYNC RT TGEN VREF VC GND C4 0.1µF is 7V or less below ISP pin. The internal PMOS driver can also be used to implement fault protection. When a fault is detected (e.g., an input surge), the LED array will be disconnected and protected by pulling down the PWM input. 22 RSENSE 68mΩ PVIN 24V RT 16.9k 1MHz C5 0.1µF C1: KEMET C0805C225K4RAC C2: MURATA GRM31MR71E225KA93 C3: MURATA GRM32DR71E106KA12B C4, C5: MURATA GRM21BR71H104KA01B D1: ZETEX ZLLS2000TA L1: TOKO B992AS-150M LEDS: LUXEON K2 (WHITE) M1: ZETEX ZXMP6A13GTA Figure 9. Buck mode 1.5A LED driver battery input voltage is able to vary from 8V to 16V. The 6.04kΩ R T resistor sets the system up for 2MHz switching, which permits a higher PWM dimming ratio than the standard 1MHz switching frequency. The 3000:1 PWM dimming ratio shown in Figure 8 is achieved at 120Hz PWM frequency. High Current Buck PWM LED Driver The LT3518 features a 2.3A switch, which makes it capable of driving 1.5A LEDs in buck configuration. Special attention should be paid to the internal power consumption when driving high current LEDs. Both high switching frequency and high power input voltage (PVIN) tend to cause high power consumption and heat up the silicon. With 1MHz switching frequency and 24V PVIN, the circuit shown in Figure 9 can provide 1000:1 PWM dimming ratio as shown in the waveforms in Figure 10. When a high power input voltage drives a few LEDs in buck configuration, open LED protection should be considered. Unlike the boost configuration, the output voltage needs to be level-shifted to a signal with respect to ground as illustrated in Figure 11. In this manner, the unique constant voltage loop of the LT3518 can regulate the output voltage of the buck configuration at the predefined value, thus protect LEDs. Conclusion The LT3518 is a high current, high voltage and high accuracy LED driver offering high PWM dimming ratios a variety of topologies. Its versatility, simplicity and reliability make it very attractive in most LED applications. The LT3518 is available in the tiny footprint QFN UF16 package and leaded FE16 package. It provides a complete solution for both constant-voltage and constant-current applications. L PWM 5V/DIV R1 + VOUT ILED 1A/DIV LT3518 – Q1 RSEN(EXT) LED ARRAY FB LI 1A/DIV R2 2µs/DIV Figure 10. 1000:1 PWM dimming waveform of the application circuit of Figure 9 at 120Hz PWM frequency. Figure 11. Open LED protection setup for buck configuration Linear Technology Magazine • October 2007