MPQ8904 abnormal output behaviour

Applications
#1

Hi,

I am using MPQ8904DD-AEC1-LF-Z in our project. I found it’s output behaviour is weird as the images shown.

mpq8904_1out5
mpq8904_1out5800×503 10.8 KB

MPQ8904’s output is worst with no loading and is better with larger load current but still has large ripple voltage. I am wondering what’s the root cause.

Thanks

#2

Hi lizhuoyang,
Thank you for utilizing MPS Technical Forum.

What is your input and output? and what is the output cap you are using?

Thank you,
Nouman

#3

Hi Nouman,

The input is 5V and output is 1.8V. The output cap is a 4.7uF ceramic cap.

mpq8904
mpq8904995×409 54.8 KB

#4

Hi Lizhuoyang,
The circuit looks correct, not exactly sure then why output would have such ripple.

Can you please try the following -

  1. confirm how is EN pin powered or is it floating,
  2. Can you test without the 1nF cap?

Thanks,
Nouman

#5

Hi Nouman,

EN pin is not floating, powered by 5V voltage. The test result was without feedforward cap. Recently I haved tried 1nF to 15nF feedforward cap. I found the ripple is smaller with larger feedforward cap. Now I am using 15nF feedforward cap. The ripple is about 20mV p-p which is acceptble. But I am still confused why a LDO has such kind of ripple. Is it oscillating? MPQ8904 internal compensation is so bad?

8904_0_out_rip_15n2
8904_0_out_rip_15n2800×512 15.1 KB

#6

Hi,
Apologies on the late response,

That is correct, typically for LDOs the ripple must be much lower than 20mV. You may try a slightly higher capacitor on the output instead of 4.7uF.

I do not think the internal compensation would be an issue, you were able to lower the ripple via the 15nF cap.

Kind Regards,
Nouman