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TPS2120: cannot swap over seamlessly

Poh Kok Keng
Genius 5302 points
Part Number: TPS2120
Other Parts Discussed in Thread: TPS2121

Dear Sir/Madam,

We found that TPS2120 does not seem to be working correctly.  From below graph, Both source 1 &2 are connected to USB-C 5V, when source 2 is unplug, the output dip and cause the system to reset instead of seamless swapping over of power. But the output remains constant when source 1 is unplug.

And, if plug in source 1 first, then plug in source 2, the output will drop like below. Can you advise?

The output is OK when either source 1 or source 2 is plug in.

Best  Regards,
felix

  • 0
    TI__Genius 15625 points

    Hi Felix, 

    As the device is configured with your external circuitry, the device is in VCOMP mode where the device will select the larger input to power the output as shown below:

    Due to both inputs being at DC 5V, the device will struggle to select the 'larger' input as the input rails fall and rise as they are selected and deselected. 

    For two inputs with the same DC voltage potential, I would recommend the TPS2121 where a supply can be prioritized. 

    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 VIN1 and VIN2 comparator is quite sensitive and you can see the bouncing in the Source 2 supply line. This will change the comparison over and over again. This results in a situation where the device keeps switching between the inputs. 

    Since this device is in a break-before-make configuration each time the device changes input supply selection, the output will not be powered during the switchover time. After several switchovers, the output voltage will drop, potentially lower than the system reset. 

    This issue is further described in the following FAQ: https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1023453/faq-tps2121-how-do-i-achieve-a-seamless-power-switchover-using-ti-s-power-mux-devices

    Best Regards, 

    Elizabeth