spread spectrum dc-dc converter question

[conal]

spread spectrum for pwm

I am desiging a spread spectrum dc-dc converter with a +-3% spread around a programmable centre frequency of 500K to 2.4MHz. I am planning to change the frequency every cycle using a pseudo-random bit sequence generator.

My question is, what should the cycle repition time of the pseudo-random generator be? If the cycle repitition time is in the audio range 20Hz to 20KHz, will I get a buzzing sound from dc-dc due to inductor magnetostriction? I am finding that if I want to keep my cycle repition time below the audio range I will have to use a very long and area inefficient shift register - hence the question.

Your help is much appreciated.

Cheers
conal

 

[conal]

spectrum dc converter

help anyone??

 

[FvM]

As the PRBS involves a spectrum up to the half PWM cycle, you'll get audible components anyway. A sufficient FM bandwidth is required to achieve a noticable spread spectrum effect in the PWM noise spectrum. If you want to avoid audible sub harmonics, spread spectrum isn't an option, I think.