MCP16311 does not reach intended voltage

[M@rcel]

Hello,

(My first topic here. Please, be gentle )

I have setup a circuit using a Microchip MCP16311. The circuit is dimensioned for 5V output, the input is 8V (also tested with 12V, no difference).

The output voltage is not the intended 5V, but no higher than 3.2V. The scope tells me that PWM duty cycle is at his max (PWM freq. is the expected 500kHz). The feedback voltage (FB pin) is 0.5V instead of the expected 0.8V.

0.5V FB tells me that the reg will attempt to rise the output voltage and increases the PWM duty cycle to accomplish this. It, however, gets to it's max duty cycle (long) before the desired 0.8V on FB has been reached.

Increasing capacitance on input and/or output makes no difference.

I am using this Murata inductor in the setup (size is an issue).

What might be the problem?

Thanks!


Edit: Current draw is about 50mA.

[M@rcel]

It appears that the calculated output voltage is maintained by the circuit down to a minimum load of 1k. So, output voltage starts to drop when the output current rises above 5mA.

Does this give anyone a clue?

[JoeO]

What is your 8V input and how much current is the 8V input able to supply?

[M@rcel]

To keep it safe, I am currently using a 9V alkaline battery (through a Schottky). I tried with a 12V car battery, too. No difference.