Apparatus and method for driving LED display

A method for driving LED display, the method comprising:

• receiving a power-on-reset signal to operate a plurality of phase locked loop circuits;
• changing a spread spectrum modulation depth of each of the phase locked loop circuits after a predetermined period of time;
• transmitting a synchronization start signal from a synchronization signal generator on a system control unit to the plurality of the spread spectrum modulation depth controllers so as to start a spread spectrum modulation process;
• wherein each of the spread spectrum modulation depth controllers is coupled to each of the phase locked loop circuits,
• wherein the predetermined period of time is less than 200 milliseconds.

An apparatus for driving LED display comprising:

• a system control unit comprising a synchronization signal generator configured to generate a synchronization start signal;
• and
• a plurality of phase locked loop circuits, each of the phase locked loop circuits comprising:
• a phase frequency detector configured to generate a phase difference signal according to a frequency divided signal and a reference frequency signal;
• a charge pump configured to receive the phase difference signal and to generate an output current according to the phase difference signal to adjust phase alignment;
• a loop filter configured to receive the output current and to convert the output current to a voltage controlled signal;
• a voltage controlled oscillator configured to receive the voltage controlled signal and to generate a voltage controlled output signal;
• a divider coupled to the voltage controlled oscillator and configured to change the dividing ratio over a modulation period;
• a sigma delta modulator configured to generate random numbers to the divider;
• and
• a spread spectrum modulation depth controller coupled to the sigma delta modulator and configured to receive the synchronization start signal from the synchronization signal generator,
• wherein upon receipt of the synchronization start signal the spread spectrum modulation depth controller starts a spread spectrum modulation, and the synchronization signal generator transmits the synchronization signal to the spread spectrum modulation depth controller after the system control unit is configured to change the spread spectrum modulation depth of the spread spectrum modulation depth controller, and the system control unit changes the spread spectrum modulation depth of the spread spectrum modulation depth controller after a predetermined period of time from when the phase locked loop circuits are locked up, and
• wherein the plurality of phase locked loop circuits are divided into a plurality of groups in accordance with locations, and the synchronization signal generator transmits a plurality of synchronization start signals to the plurality of groups respectively with a time difference.