MPQ8633B OCP function work incorrectly

DC-DC Power Converters
1

Hi Expert,

My design used MPQ8633B with below information:
1/. 12V input, 5V output, 5A output max
2/. 1uH output inductor, Fsw = 1.1Mhz, 400uF output capacitor
3/. OCP trigger point is set to 6.9A though 22kR Rcs which calculated by equation (4) in datasheet

I try to measure to find the actual OCP trigger point. VR triggered OCP function when the load current (generated by E-load) is only 5.2A while I expected it should be around 6.9A.
I read the OCP feature in datasheet on page 15. Controller monitor the LS-FET current to trigger the OCP while I_LIM in equation [4] is load current, it’s not the inductor current. It cause the confusion for me.
Could you help to clarify how OCP feature work on MPQ8633B? and how to find the OCP trigger point by measurement? Should I measure inductor current or output current?

Thank you.

2

Hello,

Apologies for the delayed response.

Yes, I_lim is setting the limit for the output current. You can find the trigger point by slowly increasing Iout. Probing switch node, inductor current, Vcs, Vout could be helpful to look at. The graph at the bottom of page 9 shows entry in hiccup mode and top of page 10 shows recovery for OCP which may help in understanding how it works.
If this is still happening, please let us know your test set up and provide scope captures.

Best Regards,
Yu