MP8007 current Limit problem

DC-DC Power Converters
1

Hi!

I’m debugging a PCB with MP8007GZ-Z. And have some trouble with output current limit.
I’m using the MP8007GZ-Z with Buck application referencing the Fig.16 in datasheet , page 26.
I set the output voltage to 5V. The R3 is 30Kohm. L1 is 22uH.

I got 5V output with 0-1.5A current load.
But when the load exceeds about 1.7A, the output voltage downs.
I changed the L1 to 33uH. The result is the same. It downs with about 1.7A load.

According to the equation (8) in page 18. I think the Ilim should be as follows.
Ilim = 100/R3 + Vl*0.18/L = 100/30Kohm + (-48V)*0.18/22uH = 2.94A
Am I wrong? Or maybe this equation Is not correct with Buck application?

I need at least 2.2A/5V output.
Please help.

Tsukasa Kobayashi

2

Hi!

Probably, I found the reason…
In Buck application , Ipk can be maximum 3A. And Iout is Ipk/2 = maximum 1.5A.
MP8007 can handle only 1.5A with the buck application.
Is this right?

Thanks.
Tsukasa Kobayashi.

3

I think you are correct. The part operates in DCM up until the boundary of CCM which makes the current triangular, and the Iout at 1/2 of the I peak.

If you can use a step down transformer, you can beat this limitation. Assuming your Vin is high enough.

4

Thank you for your replying.
I’m changing my design using step down method.