colinwang
Member level 3
stability with a margin
Hello everyone,
I've designed a traditional ldo. And I added the parasitic resistors and inductors of the bound wire and pad. The values of them is around 200mOhm and 1nH. The ac simulation shows the loop is stalbe and I also performed the stb analysis under several load conditions and the phase margins are all above 70deg. But when I did the transient simulation of the circuit, the problem happened. After the circuit started up, i changed the load current and the output began to oscillation. The oscillation frequency is around 8MHz and it's regular. The loop gain bandwidth is around several kHz. I use the conservative accuracy with 10ns maxstep. If I change the maxstep to 20ns, the oscillation disappeared. I know the smaller the step is, the more accurate the result will be.
But why the loop analysis shows the circuit is stable but finds oscillation the transient simulation? Is that to say the phase margin is necessary but not sufficinet to the stability? Or there are some other reasons? Need your help!
I did the ac simulation using a huge inductor in the feedback loop and a huge capacitor to couple the ac signal to the positive node of the op. and I did the stb analysis by putting an ac source in the feedback loop.
The transient simulation is that after the ldo started up, the load current changed 20mA in 1us.
Need your help!
Regards,
Colin
Hello everyone,
I've designed a traditional ldo. And I added the parasitic resistors and inductors of the bound wire and pad. The values of them is around 200mOhm and 1nH. The ac simulation shows the loop is stalbe and I also performed the stb analysis under several load conditions and the phase margins are all above 70deg. But when I did the transient simulation of the circuit, the problem happened. After the circuit started up, i changed the load current and the output began to oscillation. The oscillation frequency is around 8MHz and it's regular. The loop gain bandwidth is around several kHz. I use the conservative accuracy with 10ns maxstep. If I change the maxstep to 20ns, the oscillation disappeared. I know the smaller the step is, the more accurate the result will be.
But why the loop analysis shows the circuit is stable but finds oscillation the transient simulation? Is that to say the phase margin is necessary but not sufficinet to the stability? Or there are some other reasons? Need your help!
I did the ac simulation using a huge inductor in the feedback loop and a huge capacitor to couple the ac signal to the positive node of the op. and I did the stb analysis by putting an ac source in the feedback loop.
The transient simulation is that after the ldo started up, the load current changed 20mA in 1us.
Need your help!
Regards,
Colin