Spread Spectrum LED driver

January 13, 2015 | 01:26
Spread Spectrum LED driver
Spread Spectrum LED driver

It’s standard practice to use Pulse Width Modulated (PWM) signals to control electrical energy delivered to a load. Special precautions are however necessary to mitigate the effects of electromagnetic interference (EMI) generated when high-energy loads are regulated in this way. Spectral analyses of the interference generated show strong peaks at the fundamental and harmonics of the PWM frequency. For high-power switch-mode drivers used to control LED car headlights it is important to ensure the interference cannot disrupt radio or GPS reception. EMI filters and gate resistors can be used to slow down switching edges but have the effect of increasing energy losses.

A more efficient method is to allow the switch mode power supply operating frequency to cover a range of values. This so-called spread-spectrum method has the effect of suppressing the EMI peaks and spreading the EMI energy over a range of frequencies. LED driver SMPSs also require the frequency spreading to be synchronized with the PWM dimming so that the LED does not flicker.

The LT3795 from Linear Technology uses the same PWM dimming scheme used in their LT3756/LT3796 family of devices but incorporates an internal spread spectrum ramp to reduce EMI.

LT3795 Key features:

  • 3000:1 true color PWM dimming
  • Input voltage range: 4.5 to 110 V
  • Input and output current reporting
  • PMOS switch driver for PWM and output disconnect
  • Internal spread spectrum frequency modulation
  • ±2% Constant voltage regulation
  • ±3% Constant current regulation: 0 V ≤ VOUT ≤ 110 V
  • Programmable input current limit
  • CTRL inputs linearly adjust LED current
  • Adjustable frequency: 100 kHz to 1 MHz
  • Programmable open LED protection with OPENLED flag
  • Short-circuit protection and SHORTLED flag
  • Programmable under-voltage lockout with hysteresis
  • Soft-start with programmable fault restart timer
  • C/10 detection for battery charging
  • Available in 28-lead TSSOP package

The LT3795 data sheet is available here.

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