Dimming LEDs with PWM Generator

nlike incandescent bulbs, dimming Light Emitting Diodes (LEDs) is not an easy task. Incandescent bulbs operate on alternating voltage supply, whereby using Triacs, one can control the effective RMS voltage applied to the bulb. Moreover, since the incandescent bulbs are resistive elements, a simple reduction is voltage is sufficient to reduce the current through it, thereby reducing its light and heat output.

LED operation is different, as they work on direct voltage. Each LED requires an optimum load current to produce light, while dropping a fixed voltage across its terminals. Therefore, it is impossible to dim the LED light output by decreasing the voltage across it or by limiting its current load.

However, an LED responds much faster, switching on and off at a much higher speed than an incandescent bulb does. This feature allows switching an LED on/off rapidly to change its light output. For instance, if the LED is repeatedly switched on for the same amount of time that it is switched off, the resultant average intensity from the LED is halved. By continuously changing the ratio of the on-to-off period, the LED can be made to traverse from zero output to its maximum light output. Engineers call this technique the Pulse Width Modulation (PWM), and this has become the de facto mechanism for dimming LEDs.

Linear Technology makes different types of PWM controllers for LEDs, and they have designed the LT3932 for dimming a string of LEDs efficiently. A monolithic, synchronous, step-down DC/DC converter, the LT3932 utilizes peak current control and fixed-frequency PWM dimming for a number of LEDs connected serially.

The user can program the LED current of the LT3932 using an analog voltage, or control its duty cycle of the pulses from the CTRL pin. A resistor divider on the FB pin of the LT3932 sets its output voltage limit.

One can use an external clock at the SYNC/SPRD pin of the LT3932 to control the switching frequency, which is programmable from 200 KHz to 2 MHz. Alternatively, an external resistor connected to the RT pin can also serve the same purpose. To reduce EMI generated by the switching frequency, the LT3932 features an optional function of frequency modulation involving spread spectrum that varies the frequency from 100 to 125%.

The LT3932 features an external high-side transistor rated for 3.6-36 V, 2 A, and a synchronous step-down PWM LED driver for dimming an LED string. This uses an internal signal generator for controlling the analog PWM dimming in the absence of an external PWM signal. LT3932 regulates the LED current to ±1.5%, while regulating the output voltage to ±1.2%. The IC achieves a 5000:1 PWM dimming at 100 Hz, and the internal PWM achieves a 128:1 dimming ratio with a maximum duty cycle of 99.9%.

The LT3932 protects the LED string from open/shorts while offering fault indication, as it has an accurate LED current sensor with a monitor output. Along with thermal shutdown, the IC features an accurate under voltage lockout threshold and an open-drain fault reporting for open circuit and short-circuit load conditions. With its silent switcher topology, the LE3932 is well suited for several applications including automotive, industrial, and architectural lighting.