Simulate the starup of the bandap

[jasonhu]

I simulated the starup of bandgap reference with a PWL source which ramp from 0 to vdd, and get a ripple(spike) about 400mv at the stable point.(stabe Vref=1.25v, the spike is 1.65v).
1) Why the ripple happen and does it mean any problem in my starup circuit?
How can i eliminate it if i can?
2)The rise time may be several ns ,seneral us, even several ms, Which one is the best?

Thanks

 

[scottieman]

I simulated the starup of bandgap reference with a PWL source which ramp from 0 to vdd, and get a ripple(spike) about 400mv at the stable point.(stabe Vref=1.25v, the spike is 1.65v).
1) Why the ripple happen and does it mean any problem in my starup circuit? How can i eliminate it if i can?

It is a bit complicated. However, I do believe the spike is due to your loop-response (I assume you are working on a bandgap needing a amplifier for voltage clamping). In order to feel what's that mean, you can try over-compensate the ampifier in your bandgap and then redo the simulation again, the spike should be smaller. However, the start-up time is increased.

2)The rise time may be several ns ,seneral us, even several ms, Which one is the best?

Thanks

To exam the stability of your bandgap, please use as small as possible rise time

Hope this help
Scottie

 

[jasonhu]

It is a bit complicated. However, I do believe the spike is due to your loop-response (I assume you are working on a bandgap needing a amplifier for voltage clamping). In order to feel what's that mean, you can try over-compensate the ampifier in your bandgap and then redo the simulation again, the spike should be smaller. However, the start-up time is increased.

Yes. if i reduce the phase margin of the OP-AMP, the spike will be smaller.


To exam the stability of your bandgap, please use as small as possible rise time Can you tell me why the rise time should be very small?

Thanks very much.
Regards!

jason

 

[ambreesh]

I have misundestood please fogive me, but are you saying that decreased phase margin is redusing overshoot . I hope you mean the spike is same as the overshoot.

 

[faninglee]

As for startup circuit ,shotest rise time is best,but don`t influence bandgap performance. you may change statup circuit ,maybe solve this question.

 

[longstar]

i think it is a tradoff of overshoot and rise time.

 

[liuyonggen_1]

could you tell me why the rise time maybe increase if i make the phase margin of the amplifier larger? what parameters are the rise time of the bandgap depends on? thank you!

 

[JT]

You cannot just play with the phase margin of the amplifier itself, you have to look at the overall bandgap circuit stability.
The bandgap is in fact a combination of a feedforward and feedback circuit, where you try to equalize the Vbe to the Vbe_x + Vt node.

Make sure you do a correct assessment of the AC loop stability. Most convenient is cutting at the amplifier output, injecting to the bandgap and see what's coming back from the amplifier.

It could also be that it's just your startup circuit giving you the spike, but a spike could also point to a marginally stable bandgap circuit.

 

[eezou]

Start-up circuit is closed when bandgap works.It doesn't affect bandgap.