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Original question:

TPS25200: [TPS25200] About programming the current-limit threshold

TPS25200: TPS25200

PRASHANT KURREY
Prodigy 70 points
Part Number: TPS25200

I have used TPS25200 for the purpose of the current limitation in my circuit. I have used 60.4Kohm for Rlim, and using equation 1 of the datasheet, Ios nominal will be approximately 1.5A. For testing, I used the potentiometer to reduce the load and thus increasing the current. But after testing the circuit I found out that for the value of 60.4k, TPS25200 limited the current in the range of 500mA. Then I tried to reduce the resistance of Rlim and found that for the value of Rlim in between 15k to 20k, threshold current changes drastically from 500mA to 5A. And the above tested has been done on different TPS25200 IC. Also as per the datasheet Rlim value should be between 33kohm and 1100kohm with the maximum current threshold of 3.1A. 

Concluding, TPS25200 is limiting the current but not with the desired value calculated through the equation from the datasheet.

What should I do to get the desirable threshold current?

  • 0
    TI__Guru** 105361 points

    Hi Prashant,

    Welcome to E2E!

    I will get back by Monday. 

    BR, Rakesh

  • 0 in reply to Rakesh
    TI__Guru** 105361 points

    Hi Prashant,

    The current limit value (Ios) should follow Equation-1. Can you use resistors (60.4kOhm or 33kOhm) at Rlim instead of potentiometer and check.

    BR, Rakesh

  • 0 in reply to Rakesh
    Prodigy 70 points

    Dear Sir,

    When I tested TPS25200, I used 60.4KOhm of resistor then I changed to 47KOhm. And when both of them did not work, I used a potentiometer to change the Rlim,  to see the graph of threshold current wrt Rlim and found that for the value of Rlim in between 15k to 20k, threshold current changes drastically from 500mA to 5A. Also, I used a potentiometer to change the load resistance. 

    Below is the schematic of TPS25200.

  • 0 in reply to PRASHANT KURREY
    TI__Guru** 105361 points

    Hi Prashant,

    Can you provide a test waveform of Vin, Vout, EN and input current for ILIM setting of 60.4KOhm resistor.

    BR, Rakesh

  • 0 in reply to Rakesh
    TI__Guru** 105361 points

    Hi Prashant,

    Any update on the test waveform

    BR, Rakesh

  • 0 in reply to Rakesh
    Prodigy 70 points

    Dear sir, 

    I am attaching the image, in which I have noted down the Vin, Vout, Iout, EN, and R_load. In this experiment, I have decreased the R_load to increase the current through the load to reach the current threshold. And for the Rlim of 60.4K Ohm, the nominal current threshold is approximately 1.6A. 

    In the graph, you can see that when I cross the current threshold limit; the output current gets saturated to 0.5A. But in the datasheet, it has given that after the overcurrent the saturation current should be the threshold current programmed by the Rlim.

  • 0 in reply to PRASHANT KURREY
    TI__Guru** 105361 points

    Hi Prashant,

    Lets take the last case, for Rload of 2Ohm and with 4.85V at output -> you should observe 2.425A  right ?

    Can you use oscilloscope and capture.

    BR, Rakesh

  • 0 in reply to Rakesh
    Prodigy 70 points

    Dear sir,

    The previous value for Rload of 2Ohm got misprint. Sorry, for the wrong data. 

    I again checked with some different parameters due to the unavailability of components. I changed my Rlim to 100kOhm which corresponds to nom Ios 0.96A. 

    Below are the graphs for different values of Rload.  Data on the multimeter represents current and waveform on the oscilloscope represents voltage across Rload

    Case1: Rload=2.6Ohm(approx)

    Case2: Rload=0.6Ohm(approx)

  • 0 in reply to PRASHANT KURREY
    TI__Guru** 105361 points

    Hi Prashant,

    It is doing its job well. The thing is... the device goes into thermal shutdown after operating a while in current limit mode. This is due to the power stress across the device (Vin-Vout)*Ilimit. So, use current probe to measure the real events.

    Please refer http://www.ti.com/lit/an/slva862a/slva862a.pdf for some more details.

    As you see,

    • when Rload=2.6Ohm, the vout = 2.6 * 0.96A = 2.5V
    • when Rload=0.6Ohm, the vout = 0.6 * 0.96A = 0.576 V

    BR, Rakesh