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TPS53319 ripple immunity from the power source

Fabio Boschi
Intellectual 300 points
Other Parts Discussed in Thread: TPS53319

Hi,

I'm working with the EVB of the TPS53319 to check the performance for my new application.
My power source is a transformer powered by 230VAC @ 50Hz and the output is 12VAC rectified by a diode bridge and filtered by a capacitor 4700uF; basically I have available an unregulated voltage of approximately 15VDC with a slight ripple 100Hz.

I I've modified the EVB for outgoing 5,1VDC 8Amax.
The changes to the EVB are:
L1 = 4,7uH 13A Wuerth 7443551470
R11 = 0R
R10 = 73k2
R2 = 100k
Fsw = 500kHz, and MODE = SS = 5,6ms FCCM.

So, if the turn on the EVB without loading the TPS53319 is stable his 5,1VDC output it's OK; it's only present a little 500kHz ripple about 4-5mVpp.
When apply an important resistive load, for example 3A, his 5,1VDC output presents a 100Hz ripple coming from the unregulated power supply; the amplitude of this ripple at 100Hz is about 18mVpp and is perfectly in phase with the ripple of the unregulated power supply.
I tried to change the values ​​of the Vinj_sw network  but the ripple at 100Hz remains.
It seems that TPS53319 is sensitive to the ripple voltage coming from the main power supply.
Do you have any suggestions on how to solve this problem of immunity?

thanks,

Fabio