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The function of the clamp circuit in DC/DC convert (Read 4042 times)

dandelion

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The function of the clamp circuit in DC/DC convert
Apr 20^th, 2008, 8:35pm

Hi,
I noticed a lot of DC/DC converter(Buck and booset converter) have a clamp voltage at the output of the error amplifier.

I wonder why this clamp circuit is necessary? Is there any speciall considerations?

I found when I added this clamp block, the gain of my error amplifier is decrased a lot(from 55dB down to 25dB). What the priciples should I set the the clamp output?

Thanks

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Monkeybad

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Re: The function of the clamp circuit in DC/DC con
Reply #1 - Apr 29^th, 2008, 10:31pm

Hi,dandelion
I guess this clamp circuit is used to limit the maximum voltage at the output of the error amp(Vcomp).
In the initial time the VFB is low and is compared to the VBG (bandgap voltage, 1.25V). Because the high gain of the error amp the Vcomp is pulled up to VDD. This high voltage will cause the converter saturated, which means no matter what the current sensing voltage(VCS) is the output of the following comparator is always low. Therefore a clamp circuit is added to prevent it.
The following figure is chopped from LM3430 datasheet of NS.

ErrorAmp_output.JPG

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