self biased LDO's opamp

May 13, 2006 1 Replies
J

hello



i need some help with self biased opamp's LDO



the circuit is here:


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    circuit

it is response on load current step below. as you can see, when load drops, the regulator doesn't regulate.


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    load response

it's retaled to no bias current in M0 so in opamp as you can see there:


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    M0 current

the problem is how to size transistors to have right current also for low load conditions. how the bias current in M0 is determined in this circuit? how to ensure non-zero current during low load?



regards


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K

So, why did you design the circuit so that M0's bias current goes to zero?

Wouldn't you be better off making the M0 current have some minimum value?

You have an awful lot of overshoot and ringing in the servo loop. You should fix this too. Part of the unregulated problem is that the overshoot voltage gets "trapped" when the gain drops to zero.

When the M0 current goes to zero, your whole circuit is shut down. There is nothing you can do about it without making something pass current in that case. The easiest thing, I think, would be to add something in parallel with M0 to make the idle current flow there.

If your V1 is a fixed voltage, you may want to bring that into this added circuit to make the idle current constant.

-- kensmith@rahul.net forging knowledge
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