LP2951: Output voltage is dropping from 3.3 to 1.2

Hello,

We will need more information to assist you. 

You mentioned that this is happening in many boards - does it happen in every board or do you have some boards that work as expected?

What is the input voltage? Is the STM32F302RDT6 the only load for the LP2951? What is the expected load?

Is the STM32F302RDT6 or LP2951 being destroyed? Does this failure happen right when the board is powered or does it happen after it has been active for some time? Do you have any oscilloscope shots of the failure?

Regards,

Nick

Hi,

You mentioned that this is happening in many boards - does it happen in every board or do you have some boards that work as expected?

Not all boards. only some is having this issue

What is the input voltage? Is the STM32F302RDT6 the only load for the LP2951? What is the expected load?

5V/12V is converted to 3.3V and this is giving to MCU. We are having Flash and MCU is load for 3.3V.board will take 150mA as load current

Is the STM32F302RDT6 or LP2951 being destroyed? –NO

Does this failure happen right when the board is powered or does it happen after it has been active for some time? Do you have any oscilloscope shots of the failure?

Boards re working fine .suddenly it is getting failed. Some is during testing.

Please check above value

Thanks,

Madhavi.

Hi Madhavi,

I am confused by the data you have provided. You said the load is 150mA above, but in the table for the non-working board it says 250mA while the good board says 29mA. If the non-working board is operating at 250mA then it is likely in current limit - LP2951ACSDX-3.3/NOPB has foldback current limit protection. 

Unfortunately you are operating the LP2951ACSDX-3.3/NOPB outside of the guaranteed specified range (100mA) so it is difficult to troubleshoot your issue. 

Regards,

Nick

Hi Nick,

In KK board we have two ways to give power.one is USB side ,other is DC supply.

On USB side from PC it is taking 250mA.

In DC supply side It is taking 150-170mA.

one question is why LDO o/p is droping to 1.2V from 3.3.

any reason for this .in some board it is showing 0.9V also.

shorting to GND. please check below for schematic. One is USB i/P ,other one is DC i/P.both together also it should work.

but our case in USB side only failing max cases.

Please check and let us know if any more info needed. we need to solve this asap.

Hi Madhavi,

There is no obvious reason the output should go down to 1.2V or 0.9V under normal operation. With no oscilloscope traces there is no way for me to know what is happening on your board because the LDO is not isolated from the rest of the circuit. Does the whole 250mA go through the LDO? If it does then I think this would point to current limit but if it does not then there's no way for me to know the actual operating conditions the LDO is seeing because I don't know what fraction of 250mA is going through the LDO. If you cannot provide oscilloscope traces or measure the current through the isolated LDO then there is not much I can do to troubleshoot your board from my end because the schematic looks fine as far as I can tell. I can try recommending a different part as a drop in if you cannot provide this information.

Regards,

Nick