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TPS754: how long is the time needs to turn off Vo after EN became high level?

Hung Ching Hsu
Expert 3086 points
Part Number: TPS754
Other Parts Discussed in Thread: TPS752

Hi Team, 

My customer wants to know following info, could you kindly help? Thanks.

1. I think "EN" pin controls output voltage, how long is the time needs to turn off Vo after EN became high level(higher than 2.0V)?

Regards,

Jo

  • 0
    TI__Mastermind 40825 points

    Hi Jo,

    Firstly, there is a newer version of this part, the TPS752 that is recommended over the TPS754.

    If I'm understanding this correctly, they are asking for the turn off time of the part (i.e. how long the output takes to fall to near 0V) once the part has been enabled and its output voltage is at its target. Is that correct? 

    If so, the answer is application dependent. The reason for this is that for the output to collapse the output capacitor needs to discharge, and the rate at which the output capacitor discharges is dependent on the operating load current. Some LDOs have an active discharge circuit to discharge the output during turn off that removes much of the dependence on load current. I do not think TPS754 has active discharging, but TPS752 does have it. The TPS752 datasheet mentions that the active discharge circuit connects a several hundred ohm resistor to the output to discharge so the time constant for the discharge could be approximated, but this should not be the only way the output is discharged because it introduces the possibility of reverse current when the condition VOUT > VIN + 0.3V occurs. 

    Regards,

    Nick

  • 0 in reply to Nick Butts
    TI__Expert 3086 points

    Hi Nick, 

    Thank you for the explanation. 

    Customer is using TPS75433Q and he wants to confirm following. Could you kindly help? Thanks.

    1. So is it correct that 3.3V Vout will starts to drop soon after EN became high(more than 2.0V) without any delay? 

    2. Is it correct that the time needed for 3.3V to drops to 0V depends on the output capacitor connected to output and load current? If yes, is there any formula that can estimate the time to discharge?

    Thanks.

    Regards,

    Jo

  • +1 in reply to Hung Ching Hsu
    TI__Mastermind 40825 points

    Hi Jo,

    1. The datasheet does not imply that there is any delay between raising EN above 2V and the output starting to shut off. 
    2. That is correct. If the load current is resistive then the formula is a basic exponential RC decay, but if the load is not resistive then the time to shut off is not so easily calculated. The reason for this is that some devices will pull a similar amount of current until their input voltage falls to a level where the device stops pulling any current, and that level and the current profile under this scenario is dependent on the device in question. So, we do not have a general equation that can be used to calculate the shutdown time. This is something that would likely need to be tested on-board with the load devices connected.

    Regards,

    Nick