Question about LDO and big capacitance between bandgap and gnd

[Analog_starter]

Hi all,

I have some questions about LDO and bandgap for it.
Why there is always added big capacitance between bandgap output (Vbg)
and gnd?
And for what level LDO output current it must need a current limit circuit?
Thanks.

Best Regards
Analog_starter

 

[sunking]

Question about LDO

1. For low noise
2. For big current (300mA or more)

 

[surianova]

Re: Question about LDO

Hi all,

I have some questions about LDO and bandgap for it.
Why there is always added big capacitance between bandgap output (Vbg)
and gnd?
And for what level LDO output current it must need a current limit circuit?
Thanks.

Best Regards
Analog_starter

the capacitor at the bandgap output is to reduce ripple.

 

[schwang1970]

Re: Question about LDO

The capacitor put in Vbg is to reduce noise and ripple. In high speed circuit this capacitor will reduce the recovery time.

 

[dulmedaboard]

Question about LDO

Another more important consideration is bias stabilization.you can simu AC analyse

 

[nandu_r]

Re: Question about LDO

current limit circuit activation level should be function of power dissipation in the pass transiistor of ldo

 

[allan_guo]

Re: Question about LDO

the cap is to provide a low freq pole to filter the noise,and reduce the time for stablizing output.
but can anybody explain: with the low freq introduced , How to maintain stable?when we dc AC simulaition,we always inject AC signal from the vref point,and see the AC response at the feedback path(middle of two feedback resistors).then in the signal path , the added cap with introduce a low freq pole, I think this will cause instable.

 

[pillar_chen]

Question about LDO

However, The large cap will introduce a low freq pole, and reduce the hase margin of the OTA.

 

[pillar_chen]

Question about LDO

To reduce the high Fre ripple and stabilize the AC operation

 

[baniliu]

Question about LDO

Mainly use to stablize the LDO

 

[niezimei]

Question about LDO

1. stablize the loop.
2. smother the load transient.
3. short the ac ripple to ground.

 

[hung_wai_ming@hotmail.com]

Question about LDO

Hi all,

I think there's misunderstanding the cap for bandgap output.
Cap at bandgap output is JUST use for bandgap purposes. There's nothing deal with LDO. The bandgap output connects to LDO -ve input of the error amplifier and the resistive divider output will connect back to +ve input of error amplifier and completes the LDO operation. The closed loop path does not deal with -ve input of error amplifier as the -ve input JUST provides a stable reference. Anything not inside the loop will not contribute stability concern. So, pls clear your mind that this cap will not cause any stability issue in LDO.

OK, the cap is just for bandgap. It creates a pole in the bandgap transfer curve and depends on the size, it forms a low pass filter with the output impedance to cut off the high frequency noise and AC couples to VSS when there is AC source higher than the bandwidth. Nothing more. Ppl always use cap to stabilize sth especially the output or current mirror. It is just a habit and ppl may just arbitrarily add the cap value depending on the layout area.