WEBENCH® Tools/LM51551-Q1: Shutdown output power when connect the load

Hello,

Thank you for reaching out with your question and for considering the LM5155 in your design. Sorry to hear that you are having trouble with circuit. 

It looks like the value of the C4 should be 56pF not  56nF.  If it is populated with 56nF it is possible that the loop is not stable. Please change C4 to a value close to 56pF and see if that solves the issue.

Please let me know if you have any questions.

Thanks,

Garrett

Hi,

Thank for your reply.

C4 was replaced, but nothing changed.I am also testing the lm5121q circuit (it works correctly)and its compensation circuit is: C4 = 330p, C5 = 33n, R2 = 13k.

I used the same combination of parts for lm 51551-Q1, but without change.

edit: 

when I increase the output current.
Figure 1.

When I increase the input voltage.
Figure 2.

When the input current exceeds 6.6A, the output voltage drops to the input voltage.


Peaks at gate pin: Mosfet tries to start.

Figure 1.

1-Output voltage

2- Input voltage

3-Gate

Figure 2.

1-Output voltage

2- Input voltage

3-Gate

 Michael 

Hi Michael,

In your application is the LM5155 or the LM51551 being used? It looks like the device is running into peak current limit. I suggest measuring from CS to GND to see if the peak current limit is being exceeded.

If that is the case please decrease the current sense resistor. Note the LM5155 quick-start calculator can be used to double check the other component values.

Thanks,

Garrett

Hi Garrett,

I used LM51551 (with hiccup) mode and the pulses are similar to this protection. I reduced the sensing resistance to 1.5 Ohm, but nothing happened. I will try to use LM5155 without hiccup mode In the meantime.

I'm trying LM5121 and the results are better (thinking about LM5122). What will be better for my application? (10-30V to 40V, 6A).

Thank you for your time

Best regards

Michael

Hello Michael,

Please excuse the late response.

When you mention that you reduce the sensing resistance to 1.5 ohm, do you mean Ohm or Milliohm?

Hello Brigitte,

I apologize for the mistake. It´s 1.5 mOhm. 

Michael 

Hi Garrent,

Yes, lm5122 will be better for my application.

Thank you for your time and patience.

Best regards

Michael