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a problem with the small LDO circuit?

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huashizng

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now I am designing a small LDO circuit, which is used to provides power supply to low voltage blocks in the chip. In this LDO circuit, power supply ranges from 6v---25v. the output voltage is 5.7v. the output voltage's tolerance may be 10%. the output stage is PMOS common source structure.if power supply>6.5v, the gain can reach 70db. when supply is 6v, because Vds of the regulator is very small, so it will operate in the linear region. at this time, the loop gain is very smll,for example -100db. but the output voltage is ok( still larger than 5.3v). what's the shortcoming? I don't know whether I can use this circuit???
because the size of the regulator transistor is large, i don't want to continue to increase the size. there is a outer 10u capacitor connected the ouput of LDO.
 

sunking

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when the supply voltage is 6v, the gain must be bigger than -100dB

Of course you can use this circuit for the supply.
 

huashizng

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Why the gain must be bigger than -100db when the supply voltage is 6v??
In the 6v supply voltage, the gain is so small. what's the disadvantages it will cause? does the circuit also work well in the 6v power supply?
 

Fom

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You regulator is already not regulating output voltage. The output is just open PMOS transistor in linear region. The output voltage will depend on loading current and VCC voltage (or PMOS Vgs). If it is acceptable for your application you can use this circuit.
 

scottieman

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huashizng said:
now I am designing a small LDO circuit, which is used to provides power supply to low voltage blocks in the chip. In this LDO circuit, power supply ranges from 6v---25v. the output voltage is 5.7v. the output voltage's tolerance may be 10%. the output stage is PMOS common source structure.if power supply>6.5v, the gain can reach 70db. when supply is 6v, because Vds of the regulator is very small, so it will operate in the linear region. at this time, the loop gain is very smll,for example -100db. but the output voltage is ok( still larger than 5.3v). what's the shortcoming? I don't know whether I can use this circuit???
because the size of the regulator transistor is large, i don't want to continue to increase the size. there is a outer 10u capacitor connected the ouput of LDO.

Just Curious.... the loop gain is minus 100dB (-100dB) when Vin is 6V?

Scottie
 

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