Accurate PWM LED Dimming without External Signal Generators, Clocks or µControllers Keith Szolusha LEDs can be dimmed in two ways: analog and pulse-width modulation (PWM) dimming. Analog dimming changes LED light output by simply adjusting the DC current in the string, while PWM dimming acheives the same effect by varying the duty cycle of a constant current in the string to effectively change the average current in the string. Despite its attractive simplicity, analog dimming is inappropriate for many applications because it loses dimming accuracy by about 25%+ at only 10:1 brightness levels, and it skews the color of the LEDs. In contrast, PWM dimming can produce 3000:1 and higher dimming ratios (at 100Hz) without any significant loss of accuracy, and no change in LED color. The LT3761 combines the simplicity of analog dimming with the accuracy of PWM dimming by generating its own PWM signal. High dimming ratios are possible by adjusting a simple DC signal at its dimming input—no additional PWM-generating microcontrollers, oscillators or signal generators are required. The LT3761’s internal PWM signal can produce 25:1 dimming, while it can still deliver up to 3000:1 dimming with an external PWM signal. HIGH POWER LED DRIVER The LT3761 is a high power LED driver similar to the LT3755-2 and LT3756-2 family. It is a 4.5V-to-60V input to 0V-to-80V output single-switch controller IC that can be configured as a boost, SEPIC, buck-boost mode or buck mode LED driver. It has a 100kHz to 1MHz switching frequency range, open LED protection, extra internal logic to provide short-circuit protection, and can be operated as a constant voltage regulator with current limit or as a constantcurrent SLA battery or supercap charger. Figure 1 shows a 94% high efficiency 60V, 1A (60W) 350k Hz automotive headlamp 34 | October 2012 : LT Journal of Analog Innovation application with PWM dimming. The LT3761 uses the same high performance PWM dimming scheme as the LT3755/LT3756 family, but with the additional feature of the internally generated PWM dimming signal and no additional pins. INTERNAL PWM DIMMING GENERATOR Unlike other high power LED drivers, the LT3761 can generate its own PWM dimming signal to produce up to 25:1 dimming. This enables it to produce accurate PWM dimming without the need for external PWM-generating components. The Figure 1. 94% efficient boost LED driver for automotive headlamp with 25:1 internal PWM dimming L1 10µH VIN 8V TO 60V CIN 2.2µF ×2 100V 499k 90.9k EN/UVLO VREF 1M VIN 140k RSENSE 10mΩ 1M RLED 0.25Ω 1A 16.9k FB 60W LED STRING ISP OPENLED ISN DIM/SS PWM PWMOUT VC RT INTVCC RDIM 124k CSS 0.01µF COUT 2.2µF ×4 GND 100k DIM M1 SENSE LT3761 CTRL INTVCC GATE D1 CPWM 47nF 300Hz RC 5.1k CC 4.7nF RT 28.7k 350kHz M1: INFINEON BSC123N08NS3-G D1: DIODES INC PDS5100 L1: COILTRONICS HC9-100-R M2: VISHAY SILICONIX Si2328DS COUT, CIN: MURATA GRM42-2X7R225K100R INTVCC CVCC 1µF M2 (CURRENT DERATED FOR VIN < 10V) design ideas The LT3761 generates its own PWM signal to achieve accurate PWM dimming, but with the simple control of analog dimming. High dimming ratios are possible by adjusting a simple DC signal at its dimming input—no additional PWMgenerating microcontrollers, oscillators or signal generators are required. Figure 2. Internally generated PWM signal and LED current for the application in Figure 1 VDIM = 7.7V DCPWM = 96% VDIM = 4V DCPWM = 50% ILED 1A/DIV VDIM = 1.5V DCPWM = 10% VDIM = 0.4V DCPWM = 4.3% 0.5ms/DIV LT3761 requires only an external DC voltage, much like analog dimming control, for high performance PWM dimming at a chosen frequency. It can still receive a PWM input signal to drive the LED string with that signal in standard fashion. the PWM pin to GND according to the equation: fPWM = 14kHz • nF/CPWM. The duty cycle of the signal at PWMOUT is set by a µ A-scale current into the DIM/SS pin as shown in Figure 3. Internally generated pull-up and pull-down currents on the PWM pin are used to charge and discharge its capacitor between the high and low thresholds to generate the duty cycle signal. These current signals on the PWM pin are small enough so they can be easily overdriven by a digital signal The internal PWM dimming signal generator features programmable frequency and duty cycle. The frequency of the square wave signal at PWMOUT is set by a capacitor CPWM from Figure 3. Setting the duty cycle at the DIM/SS pin takes a µA-scale signal. This pin can also be used with an external PWM signal for very high dimming ratios. CONCLUSION The high power and high performance LT3761 LED driver has its own onboard PWM dimming signal generator that is both accurate and easy to use. n Figure 4. Given a high speed PWM input signal, the LT3761 still provides a high speed PWMOUT signal. 100 PWMOUT DUTY RATIO (%) from a microcontroller to obtain very high dimming performance. The practical minimum duty cycle using the internal signal generator is about 4% if the DIM/SS pin is used to adjust the dimming ratio. For 100% duty cycle operation, the PWM pin can be tied to INTVCC . CPWMOUT = 2.2nF 80 PWM INPUT 60 PWMOUT 5V/DIV 40 20 0 –10 0 10 20 30 DIM/SS CURRENT (µA) 40 50 200ns/DIV October 2012 : LT Journal of Analog Innovation | 35