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TPS548D22: Current above OCP level set by ILIM does not shut down device when Vout = 0.675

Syasya Nik
Prodigy 150 points
Part Number: TPS548D22

Hi, 

I am implementing Vout = 0.675 Iout= 27A with feedback resistors on TPS548D22 evaluation board. I have set Vref (VSEL) to 0.5996V to provide the desired output voltage, as shown in the schematic below.

To limit the current to 28A, I have set ILIM = 95.3Kohms, as per datasheet.

However, when I am drawing current above the current limit set (33A max.) using an electronic load, it does not trigger the device to shut down. Even when I am drawing 40A from the electronic load, the device does not shut down.

I have probed out the current (Iout) and Vout = 0.675V signal and it seems that above 28A, the Vout decreases to 0.4V but does not shut down completely.  Can you suggest why this might be happening?

On the other hand, if I set ILIM = 63.4Kohms (22A max.) when Vout = 0.675, the device shuts down as expected.

And 

When Vout = 0.75V (with the same setup), with ILIM = 95.3Kohms (33A max.) , the device shuts down as expected. 

Regards, 

Sya

  • 0
    TI__Mastermind 32350 points

    Hello Nik, 

    The current limit is Valley based, so your average should be higher than your setting, you need to check if the valley is above the set point, depends on the inductor current ripple, the average current will be Delta_IL/2.  I can not tell from your plots what is that. 

    Can you please probe VFB and see if the UVP level is achieved.  Because when the feedback voltage becomes lower than 68% of the target voltage, the UVP comparator output goes high and an internal UVP delay counter begins counting. After 1 ms, the device latches OFF both high-side and low-side MOSFETs drivers. The UVP function enables after soft-start is complete.

    Can you please check that? 

    Thanks 

    Tahar

  • 0 in reply to Tahar Allag
    Prodigy 150 points

    Hi Tahar,

    I probed Vrsp (across Rfbb) when ILIM = 95.3K :

    T

    When current is drawn above its limit, Vrsp stays at 0.47V, where it is still above the UVP threshold (68% of target voltage = 0.5996 * 0.68 = 0.401V). It does not shut down as expected. 

    To compare, I then probed Vrsp (across Rfbb) when ILIM = 63.4K, where it did shut down as expected: 


    The Vrsp is pulled down below the UVP threshold which caused the device to shut down. 

    It seems that it is only affected when Vout = 0.675V, ILIM = 95.3K, that is causing Vrsp to not be pulled below the UVP threshold. 

  • 0 in reply to Syasya Nik
    TI__Genius 11155 points

    Hello Nik, 

    Tahar is out of office, he will provide a response next week.

    Thank you,

    Calan

  • 0 in reply to Calan Elaya
    Prodigy 150 points

    Hi, 

    Can I get an update on this issue?

    Sya

  • 0 in reply to Syasya Nik
    TI__Genius 11155 points

    Hi Sya,

    Please allow a few days for a response as U.S. was on a holiday weekend.

    Thank you,

    Calan

  • 0 in reply to Calan Elaya
    TI__Mastermind 32350 points

    Hello NIK, 

    From an UVP prospective, the device is working as expected. It will not shut down if it is not below the 68% threshold. 

    now, why VOUT does not drop below that, i am not sure. 

    Can you measure the inductor current?

    The device is valley current based, the current limit will be current limit + 1/2*(Inductor current). I am guessing, as the VOUT drops, the current ripple increases and threshold moves with it. 

    Thanks 

    Tahar