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does it possible get greater than 0.5 duty ratio using SG3525A

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Maloth Ramesh

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i am designing dc dc converter, is it possible to get more than 0.5 duty ratio using SG3525A

thank you........
 

Warpspeed

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Yes, you can get almost 100% but not quite, due to the dead time limitation.

The trick is to ground both the output pins 11 and 14, as well as pin 12.
Then fit a 1K pullup resistor to pin 13 that normally supplies power to the output section.

As each output tries to turn on, because it is directly shorted to ground, it pulls down pin 13 almost to ground. You get twice the output frequency doing this, but the duty cycle can then go from 0% to maybe 95%.

This is all a bit odd, but if you read the application notes for the chip, it is a recommended method.

You will need an inverting gate drive buffer to drive your mosfet gate, because connecting the 3525 like this, its output at the pullup resistor now goes logic low when you want your mosfet gate to go logic high.
 

schmitt trigger

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The reason for the 50% duty limitatation in a 3525 is because it was designed for push-pull or bridge stages. Two nonoverlapping outputs are required in those topologies.

The method that warpspeed recommends combines the two outputs to a single output. Is that what you want? Or do you want both outputs to go above 50%?
 
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