DCDC from battery makes 2Vpp noise

[buenos]

hi

I designed a digital board with 2 main DCDC converters: 3.3V and 5V. They are working from a 20V adapter input or from a 16V battery pack.

If i use a battery pack, both DCDCs are producing a few 100 Hz noise, instead of a DC level. From the adapter input, they work fine.
In the case of the battery, if i measure the DCDC input voltage by oscilloscope, its DC, nut jumping. Only the DCDC outputs are jumping.

Why?

The 2 DCDCs: TPS40057, LM3489MM. the system power selector is with the ISL6255 battery charger.

 

[bobcat1]

Hi

Upload circuit schematic so I can see more

All the best

Bobi

 

[buenos]

hi.

i solved it:
extra 220uF elko is needed at the input. its not in the reference design.
strange, that the input on the scope looks clean without the elko.