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TPS51225C ramp up enable issue

Patrick Chen
Mastermind 8875 points

Hello,

             Customer use LDO and resistor divider to 5V_EN1 signal for EN Pin control. But while input 12V, the EN Pin will be trigger EN threshold on 0.88V to  device due to ramp up voltage of resistor divider of  5V_EN1. Customer don't use GPIO to contorl EN pin. Do we have any design suggestion to solve the ramp up volatge EN issue on enable under wrong VIN ? Whie VIN is 6V, the device will be ebable. Thank you. 

Schematics

https://drive.google.com/file/d/0B4htKR16pRs8Ty1WOUdWckJyWTg/view?usp=sharing 

  • 0
    TI__Mastermind 18510 points

    Patrick,

    From your schematic, 5V_EN1 is controlled by 12Vin power, not LDO, right? if so, Actually, I think it is difficult to avoid the mosfet be turned on(EN asserted)
    at lower input voltage, and this maybe cause input has drop, even cause output voltage not monotonous if vin power is not strong enough.

    Generally, GPIO signal control EN is common application.

    I thought a lot about how to delay the EN with vin just now, two ideas for reference, but i am not 100% sure if it is ok.

    1, use resistor devider devided from VIn to control Q1 turn on time, when vin coming en will be pulled low by Q2 fisrt, then turn off when Q1 turn on.

    2, Add a real LDO componet to control, cost will up.

    3, 12Vin better before input inductor, which has smaller switching noise.

    yuchang