I am using mp2617a with input voltage of 5v using power path management. With a 3.7v battery attached, I am getting a vsys voltage of 3.9V, which is expected. However, with the battery disconnected, I am only getting a voltage of 1.5V instead of the expected 3.6V volts expected and necessary for system function.
This looks to be expected behavior given how you have your schematic set up currently. Please take a look at the Setting the System Voltage section of the datasheet page 27. You are missing a resistor network on the SYSFB pin that can set the System output.
It looks like from the data sheet that sys voltage should be regulated to 3.6V when battery V is less than 3.4V. Is that not true when a battery is not connected?
I looked at the block diagram, it looks like there should be a resistor divider network internal to the device that keeps the voltage at 3.6V minimum. I don’t see anything for a disconnected battery situation but Figure 2 in the datasheet shows the System should be at 3.6V in the case of a battery voltage below 3.5V like the datasheet claims. It’s possible that no battery presence would change things, but the block diagram shows this shouldn’t be the case. Have you tried this in our evaluation board? Do you have something connected down stream of the System?
I have this IC on a daughterboard with only a smd board to wire connector on it, and the voltage was the same whether it was connected to the main board or not. I have not tried it on the evaluation board. I didn’t see anything on the block diagram either that would change things.