We are currenting using TPS2121 in our design with the following parameters:
VIN1 = VBUS = expected Voltage range 4.75 - 5.2V
VIN2 = V_POE_5V = Regulated voltage range 5V
The idea is to prioritize on VIN1 due to better efficiency. In our calculations, PR1 resistors set so VIN1 would switch over to V_POE_5V when it goes lower than 4.25V.
When prototypes come in, we see instability when V_POE_5V is powered on, and VBUS is at a range between 4.27V - 4.69V
See pdf for schematic and measurements.
1. This is to simulate user plugging in a bad voltage source. But if V_POE_5V is in place, it shouldn't affect the overall voltage output of TPS2121 right?
2. What may be the cause of the instability? Please advise on solutions.
Leona TPS2121_Instability Issue.pdf