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BQ76PL455A: bq76PL455A / Use VP for other devices

S.Satoshi
Guru 21685 points
Part Number: BQ76PL455A

Hi,

My customer is considering to use VP voltage for other devices. They think that it can be used within the power rating of NPN transistor. Can you recommend it?

Best Regards,

Satoshi / Japan Disty

  • 0
    TI__Genius 11480 points
    Hello Satoshi,

    Could you clarify what kind of load they are looking at? Typical current, base device or stack, what kind of devices, etc....

    It is really tough to give a blanket statement for "yes" or "no" without more detail.
  • 0 in reply to David Wiest Jr
    Guru 21685 points

    Hi David,

    Here is the additional information. Can you give your comment for these information?

    • They will use VP output at only base 455A
    • Load is MCU which is approx. 10mA maximum load, and also switching load which is 6mA peak with few 100 KHz switching.
    • The maximum load will be less than 20mA.

    Best Regards,

    Satoshi

  • 0 in reply to S.Satoshi
    TI__Genius 11480 points

    Satoshi,

    Thank you for the clarification.

    For this use, it should be okay to load VP within the power limits of the FET/resistor. I will try to put a small load on our EVM on Monday to see if anything weird happens with the dynamic load, but it should be okay.

    My concern now will be thermal performance and the voltage drop across the TOP resistor. I just ask that things are sized appropriately. Last I knew, there is a small possibility that there is a 40mA transient at startup (a random event with a very small chance of happening). This can prevent the part from starting up in some cases, but sizing the TOP resistor for this transient will eliminate that concern as well.

  • 0 in reply to David Wiest Jr
    Guru 21685 points

    Hi David,

    I really appreciate for your support. I'm waiting for your evaluating result.

    Regarding the voltage drop across the TOP resistor what you are concerning. First, please let me know which is TOP resistor in attached schematic that is referred from Fig.36 in datasheet. I guess that you mean R104 and R105 as TOP resistor, then do you mean that there is possibility of 40mA transient pass through these resistor at start up? It will be appreciated if you will explain this again for clarify my understanding.

    Best Regards,

    Satoshi

  • 0 in reply to S.Satoshi
    TI__Genius 11480 points

    Hi Satoshi,

    I haven't had the opportunity to look at the switching load, but i think it will be okay. I will still look at it and  reply if i get the chance today.

    You are  correct, i meant R104 and  R105. I apologize  for the misunderstanding.

    Upon POR, there is a small probability that the  device  will pull 40mA during its start  up, This spike should be very brief i believe, so if the  resistor values are  such that the voltage drop does  not put  the regulator in dropout it will  be okay.

    For example, say R104 + R105  = 1000 Ohms, and  this event occurs. Then for a very brief instance, there  will be 40V across these resistors. If the module voltage  is low  enough, it may not  leave enough headroom for  the VP regulator  to work, causing the IC to lock up.

    As long as  the  resistor  values are chosen with this in mind, it should be okay.