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Original question:

TPS7B7701-Q1: Reverse Current detection in Switch Mode

TPS7B7701-Q1: about the the short to battery and reverse current detection of TPS7B7701

zhihong lin
Prodigy 60 points
Part Number: TPS7B7701-Q1

Dear TI Teams:

 

We have a trouble about reverse current detection when we check ISO16750 , the waveform as below:

The IC TPS7B7701 detect the short-to-BAT error now and then.

We add a diode before Vin, It can avoid error detection of the short to battery and reverse current. and this trouble is eliminated.

But I have two questions, please help to confirm :

Question 1, As the description of datasheet 6.6 and 7.3.3. there need a 200mA(typical) 5us to trigger the short-to-bat and reverse current detection function.

If we add a diode before TPS7B7701, I think there will not be reverse current through TPS7B7701, if so, please confirm whether the short-to-bat and reverse current function is going or not?

I think the current of cap can't supper the current (200mA*5uS). How this IC detect the reverse current ?  Can you share it to us. 

Question 2, if the short-to-bat is detected, we hope to check one more time to avoid error detection,(在检测到异常之后，对EN进行拉低后重新拉高，再次检测，以2次的检测结果来作为判断。)

How long time we need to set  EN pin Lo and then set Hi to restart TPS7B7701? 

I can’t find the description in datasheet.

 

  • 0 in reply to zhihong lin
    TI__Mastermind 23361 points

    Hi Zhilong, 

    Will you use the "insert pictures" from the toolbox instead of pasting them? The ones you pasted cannot be viewed. 

    Regards, 
    Jason Song

  • 0 in reply to Jason Song
    Prodigy 60 points

    add the picture ,can you see it now ?

  • 0 in reply to zhihong lin
    TI__Mastermind 23361 points

    If you open the thread after posting, we can see that the pictures for Question 2 are visible now. But there are no pictures for Q1. Will you repeat what you did for Q2 and reupload the ones for Q1? 

  • 0 in reply to Jason Song
    Prodigy 60 points

    I have edit it one more time, please check it.

  • 0 in reply to zhihong lin
    TI__Mastermind 23361 points

    HI Zhihong, 

    The pictures are all visible now. 

    1. For STB, the circuit for the detection is similar to a comparator circuit that is comparing Vin and Vout during the startup. For RC, it will be trigger by high than 1mA(typically 50mA) of current flowing into the output of the LDO, and as you have noticed, the current will need to last around 5uS. The RC is protecting the LDO from damage due to the current flowing through the body diode of the pass-FET and it only becomes necessary when the reverse current reaches a certain value. 

    2. This is a very good question and it's not discussed in the datasheet. Please allow some time for me to check from design and I will get back to you probably early next week. 

    Regards, 
    Jason Song

  • 0 in reply to Jason Song
    Prodigy 60 points

    1, as you word ,  we add a diode before TPS7B7701, the STB also active .but For RC detection function cannot active ,is it right ?

    2,waiting for you update.

  • 0 in reply to zhihong lin
    TI__Mastermind 23361 points

    Hi Zhihong, 

    1, as you word ,  we add a diode before TPS7B7701, the STB also active .but For RC detection function cannot active ,is it right ?

    As the output may have long cable connecting to the load, the inductive could excessive voltage spikes that may trigger the RC. 

    2. I got the response from the design team, and to clear the error state, the EN needs to stay low for at least 10uS. 

    Regards, 
    Jason Song

  • 0 in reply to Jason Song
    Prodigy 60 points

    1. For STB, the circuit for the detection is similar to a comparator circuit that is comparing Vin and Vout during the startup. For RC, it will be trigger by high than 1mA(typically 50mA) of current flowing into the output of the LDO, and as you have noticed, the current will need to last around 5uS. The RC is protecting the LDO from damage due to the current flowing through the body diode of the pass-FET and it only becomes necessary when the reverse current reaches a certain value. 

    -》Is STB  only detect during the at startup? 

         we found the error when we check the  waveform of  ISO16750  is RC ,is it right ? 

         a diode add before TPS7B7701 is not recommend ，because it may hinder the RC detection ?

  • 0 in reply to zhihong lin
    TI__Mastermind 23361 points

    Is STB  only detect during the at startup? 

    Yes, the STB is only checked during startup. If the part is already powered-on, and an RC/STB error shows up, it is RC. 

    The RC is designed to help the LDO and having the diode before the input is optional.