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Which Error amplifeir in DC-DC converter?

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sudheer_vlsi

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Hi all,

To compensate the PWM mode buck converter,which topology of the Error amplifier is better? weather Gm-based(folded cascode) or voltage amplifier(two stage opamp)? what difference do they make in compensation? what are the advantages and disadvantages using those topologies?

Thanks.
 

paulux

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In general, OTA is sufficient if the output does only have capacitive load. If the driving requires resistive load, opamp should be recommended.
 

Chethan

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a two stage opamp is sufficient as the output of the error amplifier goes to the PWM block. Three stage opamps are generally used when a large resistive load is to be driven. In this case the load is primarily capacitive. I think a two stage opamp would be fine.

chethan
 

scottieman

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sudheer_vlsi said:
Hi all,

To compensate the PWM mode buck converter,which topology of the Error amplifier is better? weather Gm-based(folded cascode) or voltage amplifier(two stage opamp)? what difference do they make in compensation? what are the advantages and disadvantages using those topologies?

Thanks.

Just want to clarify:

What do you mean by two-stage amplifier?
A gain stage with a output buffer, OR
Two cascaded gain stage?

Scottie
 

Chethan

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scottieman said:
sudheer_vlsi said:
Hi all,

To compensate the PWM mode buck converter,which topology of the Error amplifier is better? weather Gm-based(folded cascode) or voltage amplifier(two stage opamp)? what difference do they make in compensation? what are the advantages and disadvantages using those topologies?

Thanks.

Just want to clarify:

What do you mean by two-stage amplifier?
A gain stage with a output buffer, OR
Two cascaded gain stage?

Scottie
i think

two gain stages cascaded.primarily a diff amplifier and an common source amplifier
 

scottieman

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Chethan said:
scottieman said:
sudheer_vlsi said:
Hi all,

To compensate the PWM mode buck converter,which topology of the Error amplifier is better? weather Gm-based(folded cascode) or voltage amplifier(two stage opamp)? what difference do they make in compensation? what are the advantages and disadvantages using those topologies?

Thanks.

Just want to clarify:

What do you mean by two-stage amplifier?
A gain stage with a output buffer, OR
Two cascaded gain stage?

Scottie
i think

two gain stages cascaded.primarily a diff amplifier and an common source amplifier

If then, the error amplifier will introduce two poles to the whole switching-regulator. Stability may hence be affected. Or, it does complicated the compensation.

Scottie
 

leebluer

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i think maybe we should use an amplifier with the dominant pole at the ouput stage. then we could use outside capacitor to compensate the whole loop. And the speed of charging or discharging the compensating capacitor is also a consideration.
 

sudheer_vlsi

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Hi all,
i am doing PI compensation for the buck converter,in which a resistor=50k and capacitor=2nF are connected in series with the output of the error amplifier.the converter is operating at 1MHz.how to determine SR and howmuch current to push to drive such a big resistor and capacitor?what topology would be better to drive such a load on Error Amplifier?
plz help me.
 

hoonsk

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Using OTA --> dominant pole depends on the output impedance of OTA which can vary according to process but we can build on-chip.
Using Opamp--> dominant pole compensation typically rely on huge resistor or capacitor which are usually external, since internal R or C for on-chip is unfeasible.
 

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