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Need to set initial (at first power-up) current limit and duration of a HotSwap controller

Other Parts Discussed in Thread: TPS25921A

We have a system comprising a single SM power supply that is connected to up to 6 peripheral modules each employing several local LDOs (e.g., TPS76933, and its relatives).     The power for each LDO comes from this SMPS, through a current limiting resistor placed in series with the LDO's  incoming power net, with a 1000uF charge storage capacitor  located at the junction of the in-board end of this resistor and the input pin of the LDO.  At startup,  the maximum inrush current to the capacitor is primarily  limited by the value of the input series resistor (about 60 ohms in most cases), with a duration defined by that RC time constant.   In a fully populated system as many as 16 capacitor+LDO circuits (configured as above) may be connected to the single SMPS output (9.7v nominal, to a Vmax of 11.55V during SMPS fault conditions).   Under normal operating conditions, the overall current draw of these peripherals will be a few hundred mA---however, due to the relatively low value of the resistors (as determined by other design requirements), the total inrush current to all these 1000uF capacitors is several amps which is over-budget for the SMPS such that the whole system promptly shuts down.  So…can the hotswap chip be configured to act as an 800mA current limiter during power up for a sufficient duration (up to 1 second, to be safe) to allow all  the capacitors to charge gracefully, and then revert to its normal mode? Please let me know if this is possible, and if so, how I should specifically configure the chip to achieve it.  Except for this one circuit, we are ready to release the SMPS module for production, so a prompt answer would be much appreciated.  NOTE: under certain conditions, we may need to briefly pause the SMPS for a full system reset,, and therefore we will also need to  "reset" the HotSwap to be ready as a limiter again when the SMPS is re-activated: a suggestion about how best to achieve this would also be helpful.

  • Hi Bill,

    If I understand the requirements correctly, the system draws a few hundred mA and you are looking for a device with around a 800mA - 1A current limit to manage inrush current into a large set of bulk capacitors, is that correct?

    In that case, the overall power is pretty small and so you may be able to use what we call an "eFuse" device.

    I will transfer this thread to the Load Switches forum (which covers eFuse products). You can check out the portfolio of device here:
    www.ti.com/.../integrated-fet-devices-products.page

    I do not support eFuse products but at a first glance, TPS25921A may be a good starting point. It would have the basic features you are asking for such as an accurate, adjustable current limit which can work in the 800mA to 1A range.
    www.ti.com/.../tps25921a

    Thanks!
    Alex
  • Hi Bill,

    I understand your system requirements. I have put together a solution block diagram based on the TPS25921A. Soft start time is programmed to ~1sec for safe charging of the bulk capacitors. Enable can be used to reset or disable the device when the SMPS is in full system reset mode. Current limit is programmed to 0.8A for steady state current limit. 

    Charging current during start-up is limited to (16000u*9.7/0.939) = 0.16A. With eFuse circuit, 60ohm charging resistor can be eliminated. Few questions:

    What is the maximum ambient temperature of the application? Is there any load current during soft-start? 

    Regards,

    Venkat