Help me decide whether I need to add another buffer to a LDO regulator

[alicia8283]

hi all,

i need to design a LDO regulator right now. the error amp for the regulator is only a simple one stage op-amp. Do i need to add another buffer for it?

 

[pseudockb]

Re: LDO question

It really depends on how large your power transistor is. If the power MOS is large, you might need to add a buffer stage to push the non-dominant pole further. And also, adding buffer will help to improve your power supply ripple rejection.

 

[cinch]

Re: LDO question

Try one stage folded cascode opamp.... Advantage with this architecture is, folded cascode opamp's stability improves as u increase load cap. Since power mosfet is usually big, it offers huge capacitive load to error amplifier. So u dont have much stability issues.

 

[xxf_86]

LDO question

I think it just decided by the pass device, if the load current is much large, so the pass device will be very large and the unstable will be a big problem, so the buffer will be added!

 

[renwl]

Re: LDO question

It really depends on how large your power transistor is. If the power MOS is large, you might need to add a buffer stage to push the non-dominant pole further. And also, adding buffer will help to improve your power supply ripple rejection.

why adding buffer will help to improve your power supply ripple rejection ?
thanks.

 

[pseudockb]

Re: LDO question

The ability to suppress the power supply noise depends on the bandwidth of the loop gain. Once the loop gain starts to drop, the loop can no longer rejects power supply noise effectively. The 3dB bandwidth of the loop is determined by the pole at the output of the error amp. Adding a buffer after the error amp will push the 3dB bandwidth of the loop further.

 

[holddreams]

Re: LDO question

Try one stage folded cascode opamp.... Advantage with this architecture is, folded cascode opamp's stability improves as u increase load cap. Since power mosfet is usually big, it offers huge capacitive load to error amplifier. So u dont have much stability issues.

How should the dc gain of the folded cascode opamp?
25dB to 45dB?

thanks.

 

[liuyonggen_1]

Re: LDO question

The ability to suppress the power supply noise depends on the bandwidth of the loop gain. Once the loop gain starts to drop, the loop can no longer rejects power supply noise effectively. The 3dB bandwidth of the loop is determined by the pole at the output of the error amp. Adding a buffer after the error amp will push the 3dB bandwidth of the loop further.

what's the dominant pole in the loop? if the output of error amplifier contribute a non-dominant pole?

 

[holddreams]

Re: LDO question

The ability to suppress the power supply noise depends on the bandwidth of the loop gain. Once the loop gain starts to drop, the loop can no longer rejects power supply noise effectively. The 3dB bandwidth of the loop is determined by the pole at the output of the error amp. Adding a buffer after the error amp will push the 3dB bandwidth of the loop further.

what's the dominant pole in the loop? if the output of error amplifier contribute a non-dominant pole?

 

[jonneylu]

LDO question

if the op drive R,buffer is needed.

 

[steve_guo]

LDO question

I think your design relys on you specs.
Sometime,one stage op-amp is sufficient.

 

[niezimei]

LDO question

If you have a huge Cout say at the order of uF, obviously the output pole is the dominate.

 

[edajason]

Re: LDO question

The simple one stage op-amp will do it. You need to choose the right type of the input transistor in order to get a good PSRR. Say, you are gonna use PMOS pass transistor to provide the current, use n-input pair for your amplifier. A good choice for the amplifier can be a mirror amplifier (better output swing).

hi all,

i need to design a LDO regulator right now. the error amp for the regulator is only a simple one stage op-amp. Do i need to add another buffer for it?

 

[fanrong]

Re: LDO question

Why use a n mos as input stage transistor ?

 

[mists]

LDO question

what is the mean of mirror amplifier ? thanks!

 

[liuyonggen_1]

Re: LDO question

Why use a n mos as input stage transistor ?

in order to force the PMOS pass transistor operated in saturation using the larger output voltage of N-input amplifier.

Added after 2 minutes:

what's the mirror amplifer? the complemetary output amplifier?

 

[baniliu]

LDO question

sometimes

Added after 1 minutes:

sometime the buffer may form another non-dominant pole in the gate of the buffer with first stage, if the loop gain is high enough and within the unity gain bandwidth