US Patent No. 10,368,410

PWM CONTROL FOR LEDS WITH REDUCED FLICKER WHEN USING SPREAD SPECTRUM SWITCHING FREQUENCIES


Patent No. 10,368,410
Issue Date July 30, 2019
Title Pwm Control For Leds With Reduced Flicker When Using Spread Spectrum Switching Frequencies
Inventorship Xin Qi, Cary, NC (US)
Eric S. Young, Apex, NC (US)
Keith D. Szolusha, Saratoga, CA (US)
Assignee Linear Technology Corporation, Milpitas, CA (US)

Claim of US Patent No. 10,368,410

1. A light emitting diode (LED) driver comprising:a PWM dimming circuit for receiving a dimming control signal, wherein the dimming circuit outputs a string of PWM pulses having pulse widths corresponding to the perceived brightness of at least one LED, where the PWM pulses control a first switch in series with the at least one LED using pulse width modulation;
a switching converter for supplying a regulated current or regulated voltage, the converter controlling at least a second switch at a switching frequency to supply the regulated current or regulated voltage to the at least one LED, the converter comprising a first oscillator for generating a first signal that controls the switching frequency;
a spread spectrum control (SSC) circuit configured to generate a varying second signal level, the varying second signal level being coupled to the first oscillator to control the first oscillator to vary the switching frequency during operation of the converter; and
a synchronizing circuit coupled to the SSC circuit for controlling the SSC circuit to generate substantially the same second signal level at a start of each PWM pulse in the string of PWM pulses while the pulse widths are constant, such that the switching frequency of the converter is forced to be substantially the same at the start of each PWM pulse while the pulse widths are constant.