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TPS22951 Ron value after hitting current limit

Antony Lin
Genius 13480 points
Other Parts Discussed in Thread: TPS22951

Hi,

I have some questions about TPS22951 as listed below.

  1. Regarding the value of 600mA current limit value, is there a range of it?  What is the minimum value?  Is 550mA a reasonable number for our test result?
  2. Based on the datasheetm we will increase on resistance to limit the output current.  Is this on resistance the same thing as Ron shown in Figure 1 of datasheet?  If so, how will on resistance increast?  Is there any curve showing this?  Would it cause Vout drop to 0V very soon after hitting the threshold?
  3. Regarind the typical Ron value vs Vcc shown in Figure 1 of spec, do we have other chips with smaller Ron value?  Customer's application is 500mA current limit with 1 cell battery cell input.

Thanks al ot!

Antony

  • 0
    TI__Genius 15740 points

    Hi Antony,

    Please find the answers to your questions below:

    1. The range of the current limit varies between 300mA and 600mA.  The chip is designed to support a maximum continuous current of 300mA and trigger a current limit by the time the device reaches 600mA.  However, the device current limit may trigger at any point between 300mA and 600mA.

    2. This resistance is the same thing as Ron, and once the current has exceeded the current limit the device will adjust Ron to limit the current to its trip point.  For example, if the current limit trips at 600mA, then the on-resistance will be changed to keep the current going through the device at 600mA.  If the output load tries to draw 1A, the device will change its on-resistance to limit the current to 600mA.

    3. While we do have devices with smaller on-resistance, these devices are larger in solution size.  The TPS25921 may be a good start since it has an on-resistance of 90mΩ.

    Thanks,

    Alek Kaknevicius

  • 0 in reply to Aleksandras_Kaknevicius
    TI__Genius 13480 points

    Hi Alek,

    Thanks for you feedback.  We would like to know better about Vout behavior before/after Iout reach the current limit.  Please see below the test waveform from HTC. 

    Vbat=3.8V from power supply, current load is 400mA in the beginning. They increase the current gradually and see see Vout drop to 0V around 500mA.

     

    From the waveform, can you tell us what is the current limit and why it drops to 0V after 500mA?  Is it expected behavior?

     

    Thanks!

     

    Antony


     

  • 0 in reply to Antony Lin
    TI__Genius 15740 points

    Hi Antony,

    If the blue waveform is the measured current through the device, then the current limit in this case would be between 500mA and 600mA.  Once the current limit is hit, then the device limits the current to that amount.  Because the current through the device kept increasing up to 500mA, the current limit was not reached until that point.

    The reason I say the current limit may not have been hit yet is because the device also has thermal shutdown.  The current through the device may have overheated the TPS22951 until the part shut itself off.  Once the thermal shutdown was triggered, the device was turned off and the 500mA load brought VOUT down to 0V.

    A better test for the current limit would be attaching a high current load to the output (1A should do) and see what the output current is before the device shuts itself off.

    Thanks,

    Alek Kaknevicius