LM25118-Q1: Incorrect output

Hi Eng,

Thanks for reaching out with your question. I'm sorry to hear that you are having issues with the board. There are a few things that can be tested to help see what is causing the voltage to be low.

1. Please probe UVLO pin and see if it is pulled low about every 256 clock cycles. If this happens the device hitting current limit. To work around this a the RAMP resistor, RAMP capacitor and current sense resistor can be changed accordingly.
2.Probe the output voltage to see how much ripple is on the output.
3. Add a bypass filter in parallel to the VIN and the VOUT pin respectively. 0.1uF is a good value.

Thanks,

Garrett

Hi Eng,

Another suggestion is add more output capacitance. 200uF should be a good starting point.

Thanks,

Garrett

Hi Garrett,

Thanks for your suggestion. We will try it out
An update for you. We changed the R408 value to 16.9K, it can output 15V.
My understanding is R408 has to be 11.1K based on the equation 43 in datasheet pg 29

Could you advise?
Thanks

Eng,
Are you sure the low side feedback resistor (R411) is correct? Perhaps it was populated incorrectly. What is the feedback pin (FB) voltage now that the output voltage is 15V?

Thanks,

Garrett

Hi Garrett,

I've checked the value of R411, it is 1K

When Vout is 15V, FB voltage is 0.96V, is this considered OK?

Anything false connection in the schematic?

Thanks

Eng,
The FB voltage should be 1.23V as specified in the datasheet with the output voltage in regulation. Assuming the resistors are the correct value, if the FB pin is 960mV then the output voltage should be ~11.5V. Please ensure that you are measuring the output voltage and FB pin voltage correctly.
Thanks,
Garrett