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TPS25921A: Setting dynamic current limit using MCU.

Part Number: TPS25921A


My application requires eight electronic fuses whose current limits can be dynamically set by an MCU to between 0.8A and 1.2A. 

Rlim @ 0.8A = 76.8kOhm

Rlim @ 1.2A = 115kOhm

My current proposal is to use a 76.8kOhm fixed resistor + 50kOhm digital potentiometer whose value can be set by the MCU over I2C. To adhere as best as I can to the datasheet recommendation of using a 1% tolerance resistor, I will use a 0.1% tolerance fixed resistor and a 1% digital potentiometer. The problem with this method is digital potentiometers with that tolerance range are prohibitively expensive for this design.

Q1. Can I tie all the ILIM pins together, as shown on page 6 of this applicaton note, keeping in mind that Vin and Vout will not be tied together in my case? 

If so, the cost of only one potentiometer shared across all eight electronic fuses will be acceptable. The downside to this method, however, is I will lose the ability to control the current limit on each power rail. 

Q2. My understanding is that the external resistor is used to set the voltage of the ILIM pin. Is this correct?

Q3. If so, am I able to use a DAC to set the voltage level instead? This will give me a cost effective way of controlling the current limit on all the electronic fuses using a MCU.

Thank you,

Ali Goname

  • Hi Ali Goname,

    Welcome to e2e!

    Before I answer your questions in detail can I know more about your application so that I can suggest you a more suitable part.

  • Hi Praveen,

    Thank you for your quick response.

    I am using a current-sink LED driver to vary the brightness of common-anode RGBW LED strips. The amount of current that can be safely sunk by the IC will be dependent on the ambient temperature and heatsink arrangement. This board will be used in a few different setups where both these variables will change, as well as in future setups which are yet to be determined. The client has insisted that there be no hardware differences, so changing physical resistor values is not an option. 

    I would like to have an electronic fuse on the cathode side of each channel (RGBW), configured to ensure the maximum current is within the right range in each setup.

    Kind Regards,
    Ali Goname

  • Hi Ali,

    Thanks for the detailed explanation.

    We don't see any issue in using a DAC to the set the voltage level at ILIM pin and change the current limit setting but we haven't tested this condition. Can you please verify this setup on an EVM before you implement this on your system.