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too many ripples from TP76633 output

 

I have both TPS76650 and TPS76633 connect to 6.0V DC power supply.

When I only connect TPS76650 to power supply, the 5.0V output is OK.

When I only connect TPS76633 to power supply, the 3.3V output is OK.

 

But when I connect both TPS76650 and TPS76633 to power supply, then the output from TPS76633 become unstable.

If I remove all the load from TPS76633, only  100 ohm resistor left, the output is still unstable.

The output from TPS76633 become triangle, the highest output is about 3.3V, and the lowest is about 2.5V and period is about 4ms.

Any help?