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TPS745-Q1: Start up time tuning

Kohsuke Hamasaki
Intellectual 1180 points
Part Number: TPS745-Q1

Hi, Team,

Are there any ways to make the startup time longer?

We are seeing about 300usec startup time onTPS74533-Q1 with LDO Univ EVM. but customer expects to reach more than 1msec.

Thanks, 

  • +1
    TI__Guru 50025 points

    Hi,

    There aren't any easy ways to slow the startup time for parts without an NR/SS pin that don't come with drawbacks. 

    With an adjustable device (I see they are using a 3.3V fixed-output device) a feedforward capacitor can slow the output rise time, but a large Cff can cause other problems that can potentially damage the FB pin. With a fixed-output device this isn't an option because there is no external connection to the error amplifier input (FB pin).

    Adding more output capacitance can slow the startup time, but will introduce more inrush current. Furthermore, with large Cout, the device could enter current limit during startup, which can cause overshoot on startup. 

    Putting a resistor in series at the input of the LDO can limit the amount of inrush current and so slow down the output rise time, but at steady state the resistor will burn extra power and will reduce the available headroom from the supply. In addition to these drawbacks, if the resistor is too large the LDO cannot turn on because the voltage that the LDO sees at its input can be below the UVLO (rising) threshold or may fall below the UVLO (falling) threshold once current is allowed to pass through to the output. 

    Regards,

    Nick