Bandgap startup+stability

[ccw27]

Hi,

For startup circuit below why is PMOS MS3 connected to the right branch of the bandgap core not to the left? Same for PMOS MSB3? Does it has to do with stability? Can someone explain the thought process as to why connecting to one branch instead of the other leads to instability?

Thanks

 

[sunking]

MS3 is for startup. If the bandgap is not on startup, the gate of MS3 will be high, so MS3 will be on and the current will come to bandgap.

The same for MBS3.

 

[Btrend]

there exist loops in the startup circuits, and bandgap core.
to talking about stability , u should first identify which loop.
startup loop ? or bandgap loop ?

 

[xwcwc1234]

How to identify the startup loop and bandgap loop ?

 

[Btrend]

How to identify the startup loop and bandgap loop ?

e.g. startup loops: MS3--MA2--MA4--MS1 (negtive feedback)
bandgap loops: MA2--MA4--M2 (positive feedback),
MA1--MA3--MA4--M1 (negtive feedback)

 

[ccw27]

e.g. startup loops: MS3--MA2--MA4--MS1 (negtive feedback)
bandgap loops: MA2--MA4--M2 (positive feedback),
MA1--MA3--MA4--M1 (negtive feedback)

How do i determine whether it is positive or negative feedback?

Thanks

 

[Btrend]

this is simple.
apply a small signal at some node in the loop, and break the loop at that node, than go though the loop, u will get a signal feedback to that node, if it is in phase
this is positive feedback, otherwise it is negative feedback.

 

[kamalakkannan]

hi btrend.
what will happen if we swap connection MA1,MA2 gates in bandgap..

 

[Btrend]

in doing this, u swap the + & - of ur OP, so u will find the gain of positive loop (left part) > gain of negative loop (right part), and then gate of M1 & M2 will go up until it finally turn of M1 & M2, so ur bandgap will not work.

 

[ccw27]

Does it matter whether I connect MS3 to the gate of MA2 or MA1? I know one is negative and the other is positive feedback.

Thanks

 

[Btrend]

if u doing this, the drain node of MS2 will always in 0, so ur startup loop will not stop after "startup process"

 

[leonken]

in doing this, u swap the + & - of ur OP, so u will find the gain of positive loop (left part) > gain of negative loop (right part), and then gate of M1 & M2 will go up until it finally turn of M1 & M2, so ur bandgap will not work.

Hi, Btrend.
You said that the gain of positive loop > gain of negative loop.
my questions is How to calculate the gain of the positive and negative loop.
thank you.

 

[Btrend]

it's a tedious way to do the hand calculation, but it's worthful.
anyway, u can do a AC simulation to get the answer, as I stated above.

 

[wdd]

hi betrend,

the capacitor C1 maybe used to enhance the stability, also it can increases the startup time.(from a paper)

can you explain it clearly, i am confused.

 

[Btrend]

adding C1 will increase the phase margin of the loop, so does the stability. but it also move the dominant pole more lower, so the fu is reduced, what this mean is that ur response is slower .