type of feedback in LDO regulator

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cmos_ajay

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Hello,

Attached are LDO regulators with NMOS and PMOS pass device .
What is the type of feedback used in each case ? Kindly explain this.

Is it voltage series, voltage shunt , current series, current shunt type feedback ?

Thanks.
 

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  • pmos_ldo.JPG
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In both cases a portion of the output voltage is fed back - hence, we have voltage-controlled voltage feedback.
 

I agree the output voltage is being sampled in shunt by the resistors.
How is it being fed back to the input ? Is it fed back to input in series or in shunt ?
 

That's kind a bad question.

It is voltage divider between R1 and R2. On PMOS it is connected to +, on NMOS to - terminal of OP-amp.

In case NMOS (I think more often used and more stable) you have non inverting feedback:
Vout = Vref(1 + R1 / R2) where Vref is voltage connected to + terminal, in this case zener diode.
 

I guess you are asking in terms of the two port analysis of feedback systems.

The LDO in both cases is a Input Voltage Series; Output Voltage Shunt Feedback.

The output voltage is measured/sensed in shunt using a resistor divider.
The feedback voltage is fed in 'series' to the input of the amplifier.

There is no difference between the 2 LDO types other than the pass device and associated minor changes in the configurations.
 

tilz0R said:
In case NMOS (I think more often used and more stable) you have non inverting feedback:
Click to expand...
No - it`s inverting. NMOS is confugured as a source follower (non-inverting) and the feedback signal, therefore, goes to the opamps inverting terminal.
 

LvW, you are right. It was my mistake! But equation is correct.
Thanks for posting that.
 

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