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TPS2121: Query Regarding Design For Power Muxing

JIGAR PATEL2
Intellectual 615 points
Part Number: TPS2121

Dear Sir,

We are going to use TPS2121 power mux IC for power switching purpose.
Schematic Diagram is attached.

TPS2121 Design.pdf

Our system is Battery operated system.
Battery voltage = (4 x AA Cell Battery) = 6V

Normally system will run on Battery.
But in case of Low Battery or Battery is removed, then system should operate on 5V USB power supply.

If Battery is present with good health & USB power is applied then
system should operate on USB power supply to save Battery Life.

So, our first priority power supply is USB power supply.

Design detail is as follow.
1st Priority power supply = 5V USB power = IN1
2nd Priority power supply = 6V Battery = IN2
Over voltage protection = disabled for both power supply

My questions regarding this design are as below.

1) During power on the system, if IN1 supply is not present(USB power supply is not applied),
will system start directly from IN2 power supply (Battery), or
It is compulsary to start the system first from IN1 & then it could switch to IN2.

2) I am clear about Status Pin (ST) functionality.
But I am not clear about Status Pin (ST) output voltage specification.
If this pin is open drained? So, that I need to apply pull up resistor.
Or
It gives high or low logic output just as simple GPIO.
If yes, then what will be the output voltage at logic high? is it 5V or 3.3V?

3) What are the maximum values of resistor (R1 & R2) I can apply to save power?
(i.e. (1M & 1M range),(100K & 100K range),(10K & 10K range) )

  • 0
    TI__Mastermind 32650 points

    Hello,

    1. The device does not require IN1 to be present to work. If IN1 is not present and IN2 is valid, then IN2 will be passed to the output.

    2. The ST pin is open drain and thus you need a pull up resistor to whatever value works for you. This pin is rated to be pulled up to 5V so 3.3V is no issue. 

    3. TI recommends pull up resistors of 6k to 20k on the ST pin.

    I took a look at your schematic. By grounding the OV pins, you will not have an overvoltage protection. I hope this is okay for your application. By grounding CP2, the device will always prioritize IN1 whenever IN1 is valid. The switchover will happen when PR1>Vref so I calculate that the switchover to IN1 will happen when IN1 is around 2.7V during ramp up. Please check if this is acceptable in your application. Your current limit will be typically around 1.5A and I assume this is correct.

    Finally, I noticed that you have no input or output capacitance. This is application dependent but output capacitance can help the output voltage stabilizing during switchover and input capacitance will protect your supply during inrush current.

     

  • 0 in reply to Shreyas Dmello
    Intellectual 615 points

    hi shreyash,

    thanks for the guidance.

    this resolved my doubt.

    i have no further question.

    regards,

    jigar