# LDO input-output characteristic problem

Discussion in 'Mathematics and Physics' started by anhnha, Apr 21, 2014.

1. ### anhnhaMember

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Hi.
Please help me solve the problem with my LDO in the picture below. Thank you.

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• ###### LDO problem 15.png
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2. ### ronsimpsonWell-Known MemberMost Helpful Member

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Part numbers.
What is Vbg? How is this voltage made?
Power supply pins on op-amp?
Resistor values?
Where are the capacitors?

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3. ### anhnhaMember

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Thank you.
I simulated the circuit in Cadence using TSMC 180nm.
I used a bangap circuit with Vbg = 0.8V.
It is relatively good.
Opamp is powered from Vin (2 to 3.3V)
Top resistor: 100K
Bottom resistor: 80K.
I didn't measure it. But Rload = 180Ohms and therefore, Iload = 0.01A.

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5. ### ronsimpsonWell-Known MemberMost Helpful Member

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So the op-amp is powered from Vdd, the same as on the source of the MOSFET.
What op-amp are you using? I need to know input range. If you give a part number I can find the numbers I am looking for.
What MOSFET are you using? I need to know gate turn on voltage. N or P?
Please plot Vdd and Vout, Gate of MOSFET. I want to see the output of the opamp.