TPS54360: Why does it have overshoot ?

Hi Went,

It could have something to do with the soft-start time? What frequency are you running at?
The internal soft-start time is

Soft-start time fSW = 500 kHz, 10% to 90% 2.1 ms

Soft-start time fSW = 2.5 MHz, 10% to 90% 0.42 ms

 

Try to reduce frequency and see if it helps.

 

Thanks

-Arief

Hello Went,
It is likely your converter is hitting current limit trying to charge the load capacitance on startup.
With highly capacitive loads such as yours, a chip with adjustable soft-start time would be better controlled.
However the soft-start of the TPS54360 is internal, so the only other option is to tune your compensation network.

In the TPS54360 product page click on the "Tools&Software" tab and scroll all the way to the bottom. In the "Software" section download the "TPS54360 and TPS54361 Family Design Excel Tool." Input your design in the excel tool, and use the combined regulator and load capacitance in place of the output capacitance. This tool will automatically recalculate your Rcomp, Ccomp and Chf values, and the new network should help with your overshoot.

If you have any problems please let us know.
Regards,
-Orlando

Went,
I assume in #1 you mean capacitance instead of voltage.
A higher capacitor will need more charge than a small capacitor to reach the same voltage.
In typical use, the TPS54360 has output capacitance ~100uF and our control loop is tuned for that.
When you add almost 2000uF, it will require more charge to reach the output voltage and more charge = more current.
The inductor current increases to charge the output into regulation, but inductors do not like change in current.
What happens is when your capacitive load reaches regulation, your inductor is still trying to supply current to the load.
This current is basically "forced" into the capacitor, and that leads to the output voltage overshoot problem you have.

Soft start controls how fast the chip increases the output voltage, and if you can slow down the voltage rise time you can limit the inductor current during startup to reduce overshoot problems.

Hope this helps,
-Orlando

Went,

We usually dont specify a maximum output capacitance.

A well controlled soft start eliminates overshoot on startup.

However in your case you cannot tune soft start, so you will have to tune part of the control loop on the COMP pin.

Please use the "TPS54360 and TPS54361 Family Design Excel Tool" as described in my first post to recalculate the compensation network.

-Orlando