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TLV733P-Q1: inrush current during startup

Akihiro Hirata
Expert 6200 points
Part Number: TLV733P-Q1
Other Parts Discussed in Thread: TPS745-Q1

Hello team,

To consider the current rating of the buck converter in front of the TLV733P-Q1, we would like to know the inrush current of the TLV733P-Q1 during startup.

How to calculate/estimate this?

Do you have a waveform of the input current during startup?

Customer schematic is not decided yet.

Vin = 3.3V, Vout = 1.8V, Cout=TBD

Thank you.

Regards,

Hirata

  • +1
    TI__Genius 12511 points

    Hi Hirata,

    The inrush current of an LDO is heavily dependent upon the ramp rate of the output voltage, combined with the output capacitance the LDO has to charge up. Estimating it you will need to know the COUT in order to find what the device will see in terms of peak inrush current. This will be dominated by the input voltage, capacitance charge rate C*DV/DT = I for the capacitors, plus the load current as well. I've linked an app note on inrush current explaining more and detailing ways to minimize inrush current peaks.

    App note on reducing inrush current in LDO

    App note

    Regards,

    John

  • 0 in reply to John Schneider
    TI__Expert 6200 points

    Hi John-san,

    Thanks for your answer.

    Regarding TLV733P-Q1, D/S specifies the startup time.

    Does this mean the slew rate of the Vout(= dV/dt) is controlled in the TLV733P-Q1?

    There is waveform so I believe so but let me doublecheck.

    Also, if the capacitor is very big, inrush becomes big.

    In this case, the device limits the inrush current to the below foldback current limit level during the startup?

    In any case the inrush current does not exceed the 700mA?

    I also want to ask same question to TPS745-Q1 as well.

    Thank you.

    Regards,

    Hirata

  • +1 in reply to Akihiro Hirata
    TI__Genius 12511 points

    Hi Hirata,

    Yes, the datasheet spec for startup time means that the bandgap is a controlled rise. However for the TLV733-Q1 if Input is applied and then En is asserted at a later time, the bandgap charge begins when the device is powered on, not enabled. So you can see in the second graph how the output rises quickly and overshoots the bandgap before coming back down to match the rise of the devices internal reference. If En is asserted after the bandgap has been fully charged, given by the startup time in the datasheet, then this overshoot may also occur above VOUT despite the internal softstart of the device. The TPS745-Q1 will always have a consistent ramp rate so VOUT should not overshoot, however there are still concerns with COUT charging as you've stated, and the current limit response time.

    For the second question, Yes the device has internal current limit protection, however it is a control loop and with that, there is a delay between when the device sees the current exceed the limit and when current limit actually kicks in. Once it kicks in, you will see the current continue to decrease as foldback is enacted. However for a very brief time you will see the current exceed 700mA.

    Regards,

    John

  • 0 in reply to John Schneider
    TI__Expert 6200 points

    Hi John-san,

    I understand. Thanks for your answer.

    If I have further question in the future, I will post here.

    I appreciate your support.

    Regards,

    Hirata