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TPS4811-Q1: Comprehensive Problems with Gate-control at Pre-Charge and turn on under load condition

Thomas Leipfinger
Prodigy 95 points
Part Number: TPS4811-Q1

Hi TI Team,

i use the TPS48111LQDGXRQ1 in my design. I use the pre charge feature according the schematic in data sheet, page 33. Both, precharge supply and Vbatt are the source. Should'nt be a problem i think.

Well after applying the Vbat to the system i developed i little logic, that first, for 327ms, controls the INP-G pin to be high and the INP pin to be low at that time. After that the logic levels are vice versa in the way that INP is high now, while INP-G is low.

In my opinion during precharge the gate called G should be "high" while the gate PU/PD should be "low". Please have a look at the scope screenshot.

Here you can see yellow is gate PU/PD, green is gate G, data D0 is the control pin INP-G and D1 the one for INP. Vbat =18V.

As you can see during the time where D0 (INP-G) is high (12V over Vbat) the precharge gate G controls Q3 (in your example) conductive. But what happens to the gate INP at the same time? It looks like it follows the gate INP-G during precharge.

After precharge has stopped by D0 low D1 (INP) is high and you can see the gate control PU/PD with round about 12V above Vbat=18V. Normal? I think it could be this way, but i'm not sure.

Second aspect is turn on under load condition. The TPS48111 could turn on Vout with a load current up to 2.4A. I give you the file of calculation tool. What do i have to do turning on with load current up to 5A?

Thanks for help and kind regards

Thomas E1320-TPS4811-Q1_Design_Calculator_RevC.xlsm

  • 0
    TI__Guru** 105361 points

    Hi Thomas,

    When INP=Low, PU/PD shorted to SRC so PU/PD follows the output voltage. It is the expected behavior.

    Can you turn-on the load (2.4A or 5A) once pre-charge sequence is completed ? otherwise, the precharge resistor needs to handle lot of stress 

    Best Regards,

    Rakesh

  • 0
    Prodigy 95 points

    Hi Rakesh,

    thanks for quick reply. Ok, now i understand that behavior.

    The second challenge, turn on under load condition, could also be solved. I used an electronic load for testing. This unit obviously doesn't use real resistors for load purposes. I suppose it has some greater imaginary parts, like a capacitor and, or inductor.

    If i use a normal power resistor the efuse has absolutely no problems. Turn on with 10A or even more is no problem.

    Question:

    By the way: i calculated the design with the calculation tool. I must have an overcurrent condition at 20A (calculated 19.83A its ok) and a short circuit protection at 21A.

    Testing the overcurrent condition results in 18A. Any idea?

    I go forward testing 20A with power resistor due to the problems with electronic load.

    Thanks and kind regards

    Thomas

  • 0 in reply to Thomas Leipfinger
    TI__Guru** 105361 points

    Hi Thomas,

    Current shunt might be showing higher value due to improper kelvin sensing or solder impedance. Please check  

    Best Regards,

    Rakesh

  • 0 in reply to Rakesh
    Prodigy 95 points

    Hi Rakesh,

    i prooved the Layout at Rsns, measured the trace resistance and via resistance  in CAD-Software (Altium Designer).

    You are right! In deed  the resulting Rsns is slightly higher. Well, i change the Layout.

    Best Regards,

    Thomas