Hello,
What is the significance of the VDD bypass capacitor. I am using the tps40200 as a DC-DC step-down with:
Vin min = 24V
Vin max = 30V
Vout = 6V
Iout max = 100mA
* I am not using the over-current function, so VDD and ISNS are shorted.
On several prototype boards I did not install the typical bypass cap of 0.1uF on the VDD pin, and did not find any problems during powering up the circuit (I tested them over min/max inputs and outputs). Now, on some (about 1/4 of a prototype-production run) of the boards I find the switcher does not power-up. It seems to operate in the hiccup mode (as mentioned in another post here). Once or if the circuit powers up (switcher starts to regulate), it works fine.
I have not seen any noise on the VDD pin before or after putting the 0.1uF bypass cap on in, but when the VDD pin is indeed bypassed it so far seems to eliminate the issue 100%!
In the data sheet the VDD pin is depicted as a positive rail to a possible op-amp, and all the layout and circuit examples I have found show the bypass cap to have the GND referenced directly to the power GND (PGND) loop, not the regulated side GND (I do not have the PGND and GND spilt via any components, just routed separately right to the main GND connection on the PCB).
If noise is the issue, than having the bypass cap referenced to the PGND would seen counter-intuitive. This bypass cap is proving to be critical. Can anyone please give more information on the VDD function?
Thanks a bunch.
