DESIGN IDEAS L 3A Converter Drives LEDs with 500:1 Dimming by Jaino Parasseril Introduction DESIGN IDEAS 3A Converter Drives LEDs with 500:1 Dimming .........................37 Jaino Parasseril 40nVP–P Noise, 0.05µV/°C Drift, Chopped FET Amplifier .....................39 Jim Williams 100 VIN = 5V 4 LEDs fPWM = 100Hz 10 1 0.1 0.1 10 1 PWM DUTY CYCLE (%) Save Board Space with a High Efficiency Dual Synchronous, 400mA/800mA, 2.25MHz Step-Down DC/DC Regulator ..............44 Damon Lee Linear Technology Magazine • March 2006 Figure 3), resulting in a wide dimming range of 500:1 at a PWM frequency of 100Hz. The LED current can be controlled by feeding a PWM signal with a broad range of frequencies. Dimming below 80Hz is possible but not desirable due to perceptible flashing of LEDs at lower frequencies. The LED current can be controlled at higher frequencies, but the dimming range decreases with increasing PWM frequency. In high temperature applications, the leakage of the Schottky diode D1 increases, which in turn, discharges the output capacitor during the PWM “off” time. This results in a smaller with the addition of a few external components results in a wider dimming range of 500:1. The technique requires a PWM logic signal applied to the gate of both NMOS transistors (refer to Figure 2). When the PWM signal is taken high the part runs in normal operation and ILED of 100mV/ RSENSE runs through the LEDs. When the PWM input is taken low the LEDs are disconnected and turn off. This unique external circuitry produces a fast rise time for the LED current (see L1 2.0µH VIN 5V C1 3.3µF C1: TAIYO YUDEN EMK316BJ335ML C2: TAIYO YUDEN UDK325BJ106MM D1: ZETEX ZLLS1000 D2: DIODES INC. 1N4148 L1: TOKO D53LC (PN A915AY-2ROM) NMOS1: ZETEX 2N7002 NMOS2: FAIRCHILD FDG327N LED1 TO LED4: LUMILEDS LXHL-BW02 ISN1 ISP1 D1 VIN IADJ1 IADJ2 SHDN FBN 75k LT3477 ISP2 5V 0V PWM 100Hz FBP NMOS1 100k RC 2.49k CC 10nF LED1 RT GND 300mA RSENSE 0.33Ω ISN2 VREF D2 C2 10µF 1M SW VC Mayur Kenia Philip Karantzalis 100 Figure 1. The average LED current changes proportionally with PWM duty cycle. Many LED manufacturers specify PWM dimming because it offers wider dimming ranges and better LED performance than analog dimming. SC70 LED Driver Drives Six White LEDs from a Single Li-Ion Battery ..............40 A Simple Digitally Tunable Active RC Filter .................................42 PWM dimming offers several advantages over analog dimming and is the method preferred by LED manufacturers. With PWM dimming, the chromaticity of the LEDs remains unchanged and a wider dimming range is possible. ILED (mA) For LED applications where a wide dimming range is required, two competing methods are available: analog dimming and PWM dimming. The easiest method is to simply vary the DC current through the LED—analog dimming—but changing LED current also changes its chromaticity (color shift), undesirable in many applications (such as LCD backlights). The better method is PWM dimming, which switches the LED on and off, using the duty cycle to control the average current. PWM dimming offers several advantages over analog dimming and is the method preferred by LED manufacturers. By modulating the duty cycle of the PWM signal, the average LED current changes proportionally as illustrated in Figure 1. The chromaticity of the LEDs remains unchanged in this scheme since the LED current is either zero or at programmed current. Another advantage of PWM dimming over analog dimming is that a wider dimming range is possible. The LT3477 is a 3A DC/DC converter that is ideally suited for LED applications. For the LT3477, analog dimming offers a dimming ratio of about 10:1; whereas, PWM dimming SS LED2 CSS 33nF 6.81k LED3 LED4 NMOS2 Figure 2. A 5V input drives four white LEDs in boost mode with PWM dimming 37 L DESIGN IDEAS PVIN 32V PWM 5V/DIV C1 2.2µF RSENSE 0.33Ω IL 1A/DIV LED1 ILED 200mA/DIV VIN = 5V 4LEDs, 300mA fPWM = 100Hz BOOST MODE C1: NIPPON UNITED CHEMICON NTS40X5R1H225M C2: TAIYO YUDEN GMK316BJ105ML C3: TAIYO YUDEN LMK316BJ335KL L1: TOKO D53LC (PN A915AY-100M) D1: ZETEX ZLLS400 D2: DIODES INC 1N4148 NMOS1, NMOS2: ZETEX 2N7002 PMOS: SILICONIX SI2303BDS LED1 TO LED6: LUMILEDS LXHL-BW02 10µs/DIV Figure 3. Rising LED current for the circuit in Figure 2 settles in under 20µs, thus allowing short pulse widths, and high dimming ratios. effective LED dimming ratio. Consequently, the dimming range decreases to about 200:1 at 85°C. PWM dimming can be used in boost mode (Figure 2), buck mode (Figure 4) and buck-boost mode (Figure 5). For the typical boost topology, efficiency exceeds 80%. Buck mode can be used to increase the power handling capability for higher current LED applications, A buck-boost LED driver works best in applications where the input voltage fluctuates to higher or lower than the total LED voltage drop. L LED6 • • • 6-LED 300mA STRING NMOS2 ISP1 D1 VIN IADJ1 IADJ2 C3 3.3µF FBN 10k LT3477 SHDN ISP2 ISN2 VC VREF D2 5V 0V RT FBP PWM GND SS CSS 33nF NMOS1 100k 100Hz 280k SW ISN1 PWM C2 1µF PMOS L1 10µH VIN 3.3V 1k 1k 6.81k CC 0.1µF Figure 4. Buck mode converter drives six white LEDs with PWM dimming from a 32V input 1k NMOS2 PWM 1k 300mA LED1 LED2 PMOS VIN 10V C1 3.3µF ISP1 L1 4.7µH ISN1 FBN 49.9k LT3477 ISP2 VC 5V 0V FBP PWM 100Hz ISN2 VREF D2 NMOS1 100k 1M SW1 VIN IADJ1 IADJ2 SHDN RSENSE 0.33Ω D1 RT GND SS CSS 33nF 6.81k C2 10µF RC 1.5k CC 10nF C1: TAIYO YUDEN LMK316BJ335ML C2: TAIYO YUDEN UDK325BJ106MM D1: ZETEX ZLLS1000 D2: DIODES INC. 1N4148 L1: TOKO D53LC (PN A915AY-2ROM) NMOS1, NMOS2: ZETEX 2N7002 PMOS: SILICONIX Si2303BDS LED1, LED2: LUMILEDS LXHL-BW02 Figure 5. Buck-boost mode converter drives two white LEDs with PWM dimming from a 10V input 38 Linear Technology Magazine • March 2006