TPS560430: Connect buck in series

Hello,

thanks for your reply.

Then the problem must be elswhere, the current should be okay, but after a curtain time the 3.3v buck turns off and output of this drops to arround 1.5-2v.

Regards

Hello,

Can you share your schematic? 

Do you have waveforms as the voltage drops?

Regards, Jason

Hello,

here my schematic.

Thanks for sharing the schematics. The values seem to be OK.

Are you able to send the waveforms of the +VPOW, +5V, +3V3, & the SW pin of the 3.3V supply?

Also, can you share your layout? Do you have input capacitors placed near each of the VIN pins? Are you FB resistors placed near the FB pin?

Thanks, Jason

Find attached the routing.

I will make the waveform bictures the next few days.

Regards

Hi,

The first thing I notice is that there is no input capacitor at the input to the 3.3V supply. Also, the +5V trace is very long and thin.

Try placing an input capacitor near the VIN pin and see if this improves the 3.3V output.

The layout guidelines in the datasheet should be followed for PCB layout.

Regards, Jason

Hello Jason,

i placed an input capacitor close to the input of the 3,3V Buck, changed nothing.

Then i made a seperate flying lead from 5V to the input of the 3,3V Buck to get bigger "trace", changed nothing.

For me it looks like (i have the PFM version) when the buck gos into light load operation mode this beginn.

But what is strange the 5Vout Buck the top most in the pcb picture works most of the time even if there is near to zero load, only 1 led with arround 5mA.

Sometimes also the 5VOut Buck turns off.

So far i have no idea why this happens. Next step are the scope pictures for you.

Thanks in advance.

Hi,

Ok, thanks for checking. The waveforms will be very helpful. If it's easier to contact me directly to share multiple waveforms, please send me an email (jarrigo@ti.com).

Please send the waveforms mentioned above and also, please include SW waveforms of the 5V and 3.3V regulators. Please monitor the 5V input directly at the 3.3V VIN pin.

Regards, Jason