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TPS2121: Instability observed with TPS2121

Part Number: TPS2121


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

  • Hi Leona,

    What seems to be happening here is that the device is bouncing between the inputs during the switchover point. I see that you have expected this by adding some hysteresis with a 1M hysteresis resistor but I think you need more. Please retry this test with a 100k resistor but note that by adding hysteresis, you may have to change the transition voltages because it will affect the voltage divider when IN2 is selected. Please look at this tech document for more information: 

    The device is always responding to its inputs. When the device switches from one supply to another, the unselected supply will slightly rise in voltage as it is unloaded and the loaded supply will slightly drop in voltage. The PR1 and CP2 comparator is quite sensitive and the bouncing of the VBUS line is quite extreme which changes the PR1 and CP2 comparison over and over again. This results in a situation where the device keeps switching between the inputs. I think you may also nee some input capacitance on the VBUS line to help maintain the voltage. This is because during fast switchover, there is no soft start or current clamping and your output capacitance is quite large. This means that an inrush will flow through the device which could cause the input voltage to dip noticeably.

  • Hi Shreyas,

    Thank you for your response.

    I tried changing the hysteresis resistor from 1M OHM to 100k. Like you said, the switchover voltage for PR1 did change, but it did not help in terms of narrowing the instability range of VBUS.

    There were two additional suggestions that I'm testing out.

    1. Css - there was no Css due to rule of maintaining a seamless voltage transition when VBUS switches to V_POE_5V and vice versa. 
       -> So far this is quite promising. I've added a 1uF Css cap. Instability was seen from 4.27 to 4.34 V. Only issue is there is approx 194ms VOUT drop when transitioning from VBUS to V_POE_5V.
    2. Balance out Cin and Cout. - Did not try yet, but will update here. 


  • Hi Leona,

    Thanks for trying out the increased hysteresis. I think you may be able to drop the resistor further but you risk making the hysteresis window too big. I think we need to focus on stabilizing the VBUS rail. I think adding SS will help but during turn on but with fast switchover, SS is not used. 

    I think adding input capacitors may help keep the voltage on the rail more stable. Please try a test with balanced input caps, 100k hysteresis and Css cap to try and give it the best chance of stabilizing.

  • Hi Shreyas,

    I've reworked my board by moving C334 and C335 over to VBUS rail, removed the previously added Css cap, and changed ST hysteresis resistor to 100k.

    This resolved my issue. Thank you for your help.