No generator, clock or microcontroller required for PWM LED dimming

LED  dimming can be done in two ways: analog dimming and pulse width modulation (PWM)  dimming . Analog dimming simply adjusts the DC current in  the LED  string to change the LED's light output, while PWM dimming changes the duty cycle of the constant current in the LED string to effectively change the average current in the LED string to achieve dimming. Although the simplicity of analog dimming is attractive, this method is not suitable for many applications because analog dimming loses more than 25% accuracy at only 10:1 brightness adjustment, and this dimming method distorts the color of the LED. In contrast, PWM  dimming can produce dimming ratios of 3000:1 and higher (at 100Hz) without any significant loss of accuracy, and the LED color does not change.

The LT3761 provides both the simplicity of analog dimming and the accuracy of PWM dimming by generating its own PWM signal. High dimming ratios can be achieved by adjusting a simple DC signal at the dimming input of the LT3761 without the need for an additional microcontroller, oscillator or signal generator to generate the PWM signal. The LT3761's internal PWM signal can produce 25:1 dimming, while the device can provide up to 3000:1 dimming when using an external PWM signal.

High Power LED Driver

The LT3761 is a high power LED driver similar to the LT3755-2 and LT3756-2 series. The LT3761 has an input voltage range of 4.5V to 60V and an output voltage range of 0V to 80V. It is a single switch controller  IC that can be configured as a boost, SEPIC, buck-boost mode or buck mode  LED  driver. The device offers a switching frequency range of 100kHz to 1MHz, LED open circuit protection, and additional internal logic circuitry to provide short circuit protection. It can work as a constant voltage regulator with current limiting, or as a constant current SLA battery or supercapacitor charger.

Figure 1 shows a high efficiency 94% 60V, 1A (60W) 350kHz automotive headlight application with PWM  dimming . The LT3761 uses the same high performance PWM dimming circuit as the LT3755/LT3756 family, but adds the ability to internally generate a PWM dimming signal without the additional pins.

Figure 1: Boost-mode LED  driver with 25:1 internal PWM dimming and 94% efficiency for automotive headlights  Internal PWM dimming signal generator

Unlike other high-power LED drivers, the LT3761 can generate its own  PWM  dimming signal for dimming ratios up to 25:1. This allows the device to generate accurate PWM dimming without the need for external components to generate PWM signals. The LT3761 requires only an external DC voltage to achieve high-performance PWM dimming at a specified frequency, much like analog dimming control. The device can also receive a PWM input signal to drive the LED string using a standard mode signal.

The internal PWM dimming signal generator provides programmable frequency and duty cycle. The frequency of the square wave signal at PWM OUT is set by the capacitor CPWM between the PWM pin and GND, according to the equation fPWM = 14kHz • nF/CPWM. The duty cycle of the signal at PWMOUT is set by the µA current into the DIM/SS pin, as shown in Figure 3. The internally generated pull-up and pull-down currents of the PWM pin are used to charge and discharge its capacitor between high and low thresholds to generate the duty cycle signal. The current signals at these PWM pins are small enough to be easily overdriven by digital signals from a microcontroller, thereby achieving very high dimming performance. When using the internal signal generator, the practical minimum duty cycle is about 4% if the dimming ratio is adjusted using the DIM/SS pin. For 100% duty cycle operation, the PWM pin can be connected to INTVCC.

Figure 3: The DIM/SS pin sets the duty cycle with a µA signal. This pin can also be used for an external PWM signal to achieve very high dimming ratios.

Figure 4: In the presence of a high-speed PWM input signal, the LT3761 still provides a high-speed PWMOUT signal

in conclusion

The high power, high performance LT3761  LED  driver has its own built-in PWM signal generator that is both accurate and easy to use.