US 10,368,410 B2
PWM control for LEDs with reduced flicker when using spread spectrum switching frequencies
Xin Qi, Cary, NC (US); Eric S. Young, Apex, NC (US); and Keith D. Szolusha, Saratoga, CA (US)
Assigned to Linear Technology Corporation, Milpitas, CA (US)
Filed by Linear Technology Corporation, Milpitas, CA (US)
Filed on Oct. 19, 2012, as Appl. No. 13/656,559.
Prior Publication US 2014/0111110 A1, Apr. 24, 2014
Int. Cl. H05B 33/00 (2006.01); H05B 33/08 (2006.01)
CPC H05B 33/0845 (2013.01) [H05B 33/0818 (2013.01); Y02B 20/347 (2013.01)] 22 Claims
OG exemplary drawing
 
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.