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TPS7A49: Soft-start length and high power for tens of ms.

Elder Costa17
Expert 2825 points
Part Number: TPS7A49
Other Parts Discussed in Thread: TPS61085, TPS7A30,

Hello.

Two questions about this  part:

1- May I set the SS for say 50ms?

2- Can it stand a few watts during this interval?

To put this into context, I am going to use this LDO (and the  TPS7A30 too for that matter) to post-regulate the output of a +15.5V/50mA boost supply (ADP5073 buck/boost inverter, -15.5V/50mA in TPS7A30 case). However, the capacitance at the LDO output is about 400uF. The inverter does not start with such a high capacitance (on simulator). I did not simulate the boost inverter yet (TPS61085) but I suspect something similar will happen. I am thinking of controlling the LDOs with the power-good outputs of the converters and set them to a reasonably slow (around 50ms) time. So, during a short time the LDO will sink a relatively high power, starting with a few watts and reducing as the capacitor voltages reaches the operating voltages (+/- 14.0V). Can the LDOs stand this power for this time?

Is the scheme described above OK? Would you suggest another  approach?

  • 0
    TI__Expert 4435 points
    Hi Elder,

    Your design scheme for the LDOs is OK.

    Yes, you can set the soft-start time to 50 msec. TPS7A49 will need Css = 33 nF (46 msec) or 39 nF (55 msec).
    TPS7A30 will need Css = 47 nF (42 msec) or 68 nF (61 msec).

    Due to the large Cout = 400 uF, the current limit could engage during startup. The soft-start and current limit will limit the current through the LDO, thereby limiting the power dissipation in the LDO. This will prevent damage during startup.

    With your 50 msec startup, 14 V output, and 400 uF Cout, and using I = C * dV/dt the average current through the regulator during startup is 112 mA. This 50 msec startup reduces the average current through the LDO during startup and helps to reduce power dissipation. At the beginning of startup, the LDO will dissipate 112 mA * 15.5 V = 1.736 W. Both of these LDOs have thermal pads and can dissipate significant power into the PCB to limit the temperature rise in the IC.

    What is the output current for the +14 V output (TPS7A49) and -14 V output (TPS7A30)? What is your max operating temperature?
  • 0 in reply to Eric.Preiss
    Expert 2825 points
    Hi, Eric, thank you for the quick response.

    The current is low, a few mA. The capacitance is high to handle short transient current peaks of the circuitry supplied by these rails.

    Regards,
    Elder.