MP9486A's output oscillating when Vin is just above Uvlo

DC-DC Power Converters
1

Hi,

We have an issue on a product which is using MP9486A to generate 5VDC:

The output is oscillating at a period of 50ms approx when the input voltage is between UVLO (4V approx) and 5V (The voltage set on the output). Is there any way to prevent these oscillations?

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We would like to get a stable voltage on the output as soon as UVLO is reached, is it possible with this component?

Thank you.

Best regards,

Victor G.

PS: I noticed that I sent you the wrong screenshot of the oscilloscope, it is the 3V3 output of the LDO that is powered by the MP9486A, but the overall shape is the same. I can send you a screeshot of the 5VDC on Monday.

2

On the graph, CH2 is the output of the MP9486A and CH3 is the input.

And here is the schematic:

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3

Have you looked at what the Vin is doing? Maybe the Vin has poor transient response or high output impedance and is being pulled down by the startup of the 9486. In which case the chip is validly reacting to an input undervoltage condition.

4

Thanks for your reply. Vin is on CH3 on the first picture I sent and seems OK to me. These oscillations are stopping when Vin is above 5.5V.

5

So you are trying to generate 5V from 5V the chip has some maximum duty cycle that will prevent this. Maybe what your are seeing is that BST cap is being discharged and so the Fet is being turned off. After a while the BST cap is recharged and it tries again. You may need to consider a 4 switch buck boost if you really need 5 to 5

6

Thanks, that makes sense. We don’t really need to have 5V when Vin is < 5V, in this case we only need Vout = Vin, so maybe we need to consider another components that have a bypass function when Vin < Vout.

7

Hi Victor,

Thank you for contacting the MPS Forum. Can you tell me the Vin range that you are looking to meet? It’s possible we have a different device that would be better for meeting your duty cycle requirements below 5V.

Thanks,
Brendan Schoemehl
Field Applications Engineer
Monolithic Power Systems | MPS Now

8

Hi Brendan,

We are looking for a device that will allow us to have an output voltage equal to the input when Vin is under Vout_set (5V). The lower input voltage possible, the better it will be for us, 4V can be acceptable. The maximum voltage on the input is 100V.

Best regards,

Victor