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 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 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 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 µA-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.
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.
These current signals on the PWM pin are small enough so they can be easily overdriven by a digital signal from a microcontroller to obtain very high dimming performance. The practical minimum duty cycle using the internal signal generator is about 4 percent if the DIM/SS pin is used to adjust the dimming ratio. For 100 percent duty cycle operation, the PWM pin can be tied to INTVCC.
Figure 4: Given a high speed PWM input signal, the LT3761 still provides a high speed PWMOUT signal.
The high power and high performance LT3761 LED driver has its own on-board PWM dimming signal generator that is both accurate and easy to use. About the author
Keith Szolusha is senior applications engineer at Linear Technology Corp. Courtesy of EETimes Europe See related links: