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TPS2421-1: Driving Enable Input

Daniele Bruno
Prodigy 70 points
Part Number: TPS2421-1

Hello,

I have inserted in a board the TPS2421-1 component.

At page 12, the datasheet says:

EN: When this pin is pulled low, the device is enabled. The input threshold is hysteretic, allowing the user to program a startup delay with an external RC circuit.

EN is pulled to VIN with a 10-MΩ resistor and to GND with a 16.8-MΩ resistor. Because high impedance pullup and pulldown resistors are used to reduce current draw, any external FET controlling this pin must be low leakage.

In order to drive this pin from a microcontroller I thought to use directly a general purpose output pin but with this datasheet assertion I am uncertain about this solution.

Even if I try to use a MOSFET logic level, I find leakage currents of about microAmperes that with a pullup of 10-MΩ create an incompatible voltage drop at the ENABLE input pin.

How can I solve this problem?

Thank you in advance for the attention.

Best Regards,

Daniele

  • 0
    TI__Guru** 105361 points

    Hi Daniele,

    Thanks for reaching out!

    To remove dependency of FET leakage, I suggest to use external resistor divider of lower values which comes in parallel with internal 10M and 16.8M resistor divider. Since, the external resistor values are 100 times less in value, the external resistor divider network becomes effective (I mean bypasses the internal resistor divider) and releases the need of low leakage FET.

    Best Regards,

    Rakesh

  • 0 in reply to Rakesh
    Prodigy 70 points

    Hi Rakesh,

    I have two TPS2421-1 devices in the board: in the first one VIN is +5V and in the second one VIN is +12V.

    1) What are the Zener voltages that you suggest me with VIN equal to +5V and VIN equal to +12V ? Do you have some p/n ?

    2) Is the 100K resistor sufficient to bias the Zener ?

    Thank you in advance for all the attention.

    Best Regards,

    Daniele

  • 0 in reply to Daniele Bruno
    TI__Guru** 105361 points

    Hi Daniele,

    Single configuration would be sufficient for both the devices. Use MMSZ4684T1G  Zener diode and scale resistor divider to reduce the shunt current. I suggest to use 200k for pull up and 320k for pull down.

    Thanks,

    Rakesh

  • 0 in reply to Rakesh
    Prodigy 70 points

    Hi Rakesh,

    I have the MOSFET to enable the TPS2421 with a microcontroller.

    These resistor values and the Zener work with the MOSFET without low leakage current, is it true ?

    Thanks for all.

    Daniele 

  • 0 in reply to Daniele Bruno
    TI__Guru** 105361 points

    Correct Daniele.

    Best Regards,

    Rakesh