I am using the TPS62560 in an application where I switch a resistor in parallel with the lower feedback resistor in order to dynamically change the output from 1.2 to 1.35V. 

During the transition from 1.2V to 1.35V the voltage drops from 1.2 to 1.12V  then overshoots a little to 1.38V before settling at 1.35V. 

I would prefer zero overshoot and undershoot, and doesn't care about transition time.

The inductor value is either 4.7uH.

The output capacitance is  3x47u + 41.5u downstream = 182.5uF.

Top feedback is 226K || 82pF

Bottom feedback is 182K, with a FET to switch an additional 931K resistor to ground in parallel.

MODE pin is tied to Vin through a 100K resistor.

Any suggestions?

Thanks,

John

John,

In your current configuration (a total of 182.5uF output capacitance), the part is unstable. The recommended range for the output capacitance on this part is in the range of 4.7u to 22u (from the applications section of the datasheet, page 18).

Just to double check, I built up the customer's CCT. The switch node shows instability with 182.5uF output capacitance. At 10uF (the recommended output capacitance), the part demonstrates stability.

As a further check, I tested the FET switch to add the 931KOhm resistor to ground in parallel with R2, 182KOhm. Changing the output capacitance to 10uF and leaving all other parts to the polycom schematic, adding the 931K to ground does not cause overshoot or undershoot.

Please let me know if you have further questions.

Tyler Pate


Q: Re: TPS62560 undershoot & overshoot when switching feedback network by 1574398

John,

Just to let you know, the datasheet suggests a range from 4.7uF to 22uF. The part is optimized at 10uF, so this is a great starting point if engineers want to test out multiple values.

Here is a scope shot showing the change in output voltage without undershoot or overshoot. This capture was taken using a 10uF output capacitor.

 

Pink trace is output voltage, Blue trace is gate control voltage.

Tyler