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TPS25924: eFuse as supercapacitor charger

matianfu
Intellectual 500 points
Part Number: TPS25924
Other Parts Discussed in Thread: TPS25940, , TPS2595

TI has documented how to use eFuse as a supercapacitor charger. (http://www.ti.com/lit/an/slva920/slva920.pdfHowever, TPS25940A has a reverse-blocking feature. A separate PMOSFET is required if we want to discharge the supercapacitor back to the VSYS.

The forward resistence is 28mΩ according to the datasheet.

The accuracy is not required here. 100% error is OK. For example, if we set it to 200mA according to calculation, 100mA min and 400mA max is accepted.

  • 0
    TI__Guru** 105361 points
    Hi Matianfu,

    You are correct. For the application covered in the app note - separate FET or DC-DC converter is required to connect to the VSYS.

    Can you please share your requirement/concern, end equipment and the application use case.

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    Intellectual 500 points

    This is a simple diagram of our design.

    It is an arm/linux system. The only input power is from USB connector. PMU is a bunch of DC-DC buck converters and LDOs with input voltage from 3.3V to 5V.

    We are going to put a 5-10F 5.5V supercapacitor into the system. When the user removes usb power, the supercapacitor can power the system for around 10 seconds, ensuring the data are written back to storage device (ssd) before the  system shutdown.

    The supercap is charged from Vmain and also discharged back to Vmain.

    Here's our concerns:

    1. The charging current should be small, since the usb power input is 3A max and the system consumes 2.0-2.5A in peak usage. In datasheets, the minimal current limit is 600mA for TPS25940 and 1A for TPS25924. We hope the value could be set to less than 500mA. The charging speed is not a big concern.

    2. TPS259241 has no built-in reverse blocking. Can we use this part as both charging and discharging, eliminating a separate MOSFET? If so, we hope the discharge should drain to 3.3V with no more than 50mΩ  resistance.

  • 0 in reply to matianfu
    TI__Guru** 105361 points
    Hi Matianfu,

    TPS2595 eFuse can be good option to set current limit around 500mA. You can use it for both charging and discharging however make sure that the device is in enable state while powering Vmain from supercap so that you can avoid conduction through the body diode.

    Best Regards,
    Rakesh