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I used the Webench tool to create a LM5118 circuit for 10V-15V input and 12V output at maximal 4A.
The problem is that my output is only 2.7V with no load. The UVLO pin is roughly 2.2V and is never pulled low. It takes about 5 seconds for the output to reach 2.7V after that it stays at 2.7V.
Does anybody have an idea why this is happening?
BOM:
Name | Value | Part |
L2 | 4.7uH | XAL7070-472MEB |
Q2,Q3 | SI4840BDY-T1-GE3 | |
D1,D2 | RBR20BM40AFHTL |
Hi Julius,
Sorry to hear you are having issues with the LM5118.
I have a few questions that should help narrow down the issue.
1. What is voltage on the UVLO pin and the VCC pin?
-- Does this happen if the UVLO pin is pulled low?
2. What is the voltage on the source of the high-side FET and the drain of the low-side FET?
Thanks,
Garrett
Thank you for your reply.
1. Voltage on UVLO is 2.2V, the Voltage on VCC is 6.95V. When UVLO is pulled low VCC is 6.95V and no output at all, as expected.
2. The voltage on source of the high-side FET is 2.5V and on the drain of the low-side FET 2.5V as well.
Hi Julius,
What is the state of the EN pin when this test is done? I recommend checking this with a oscilloscope to see if this value is toggling.
Thanks,
Garrett
Hi Julius,
Based on what you are seeing I think that the LM5118 might be damaged.
You can order some samples at the link below.
http://www.ti.com/product/LM5118/samplebuy
Please let me know if you have any questions.
Thanks,
Garrett
Hi Garrett,
That was my first guess as well, but I highly doubt that. I have made 5 boards and all behave the same. The gate Pin for the FETs is actually driven by the LM5118 for a few milliseconds and then stops.
Thanks,
Julius
Hi Julius,
Can you please post a picture of the gate drive waveform?
Also what is the total output capacitance on this design?
-Garrett