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TPS74801: Need clarification on quick discharge output functionality

Thrinesh S
Prodigy 80 points
Part Number: TPS74801

Tool/software:

Hi E2E forum,

In our design we are using TPS74801 IC for SD Card Power purpose. As per datasheet mentioned R Pulldown value is 833Ohm and external load capacitance is 10uF + 0.1uF which is about 10.1uF 

As per above formula to calculate time constant we need consider RL resistor with parallel to 833Ohm but RL is not a constant load resistance in our case. While read or write condition it will consume around 140mA to 150mA.  And San-disk SD card datasheets mentioned max current consumption is up-to 800mA. In this case RL is not fixed due to that fall time varies drastically but we want quick discharge functionality. So, Can you suggest any other LDO ICs with lesser internal Pulldown resistor value. Our expected discharge time should be less than or equal to 550us So, if internal resistor value lesser means we no need to depend on RL. Because RL is parallel with internal pulldown resistor then equivalent resistance will be less than the least. But we didn't find any LDO with lesser Pull down resistor value. Suggest a LDO for it.  

  • 0
    TI__Guru 50025 points

    Hi Thrinesh,

    If you wanted to rely only on the pulldown resistor to discharge the output capacitor, the pulldown resistance would need to be approximately 550us / 3 / 10.1uF = 18Ω to account for 3 time constants. Unfortunately, we do not have any LDOs that have a pulldown resistance this low. Most devices have closer to 100Ω ~ 150Ω.

    You mentioned that the current consumption during read/write is approximately 140mA ~ 150mA. Can that be considered the minimum current level or is the current consumption lower when the device is in more of an idle state? If the current consumption is minimum 140mA ~ 150mA, you can get pretty close to your design goal with a device that has a pulldown resistance of about 100Ω, but if the load current can be much lower then it likely won't work. I don't know the output voltage you are using in your application, but I ran a simulation assuming 3.3V output. I used a resistive load of 3.3V/140mA = 23Ω. Of course the actual load won't be resistive and would actually probably go to zero once the device is insufficiently powered, but at least you can see that it's pretty close to meeting your goal.

    If you need more margin then you will likely need to use a couple transistors and a resistor to implement an external pulldown that pulls the output down with the falling edge of EN. If you were to do this, you could use a device that doesn't have an integrated active pulldown. 

    Regards,

    Nick

  • 0 in reply to Nick Butts
    Prodigy 80 points

    Hi Nick,

    Thanks for the response

    As per you question, Our idle current is max up to 950uA as mentioned in SD card data sheet, At this point output discharge time will take more time ~ 35ms when VOUT is 3.3V.

    Any other alternate method is there which meets this requirements (Quick discharge output in LDO).

  • 0 in reply to Thrinesh S
    TI__Guru 50025 points

    Hi Thrinesh,

    In that case, I think you will necessarily need to implement an external pull down as I mentioned. 

    Regards,

    Nick