• State TI Thinks Resolved
  • Locked Locked
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 64 subscribers
  • Views 303 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ77904: charger shutdown, common ground

user2012342
Prodigy 70 points
Part Number: BQ77904

Hi,

considering typical application (figure 27) in the datasheet...I need common ground in the entire circuit, thus I can't use N-FETs. My charger circuit has a shutdown pin (log 0 = shutdown), so my idea is:

  • charger circuit connect directly to the akupack
  • CHG output connect to the shutdown pin of the charger circuit
  • place PFET between AKUPACK+ and LOAD+; use DSG output to connect its gate to the ground via NPN
  • between AKUPACK- and LOAD- there will be only the current sense resistor, so what should I do with the LD pin?

I don't mind that there won't be current protection whilst charging (I have it handled in the charger circuit).

Is this a good idea? Will it work? Many thanks

  • 0
    TI__Guru 54715 points

    Hi,

    You must determine if it is a good idea and meets the requirements for your system or product. It sounds like a separate current path design with charge protection switching in the charger and high side discharge protection switching.  I think it will work with some areas to be cautious:

    • Avoid excessive load on CHG and DSG.
    • Your Load P-channel will turn on quickly.  When a fault occurs and it needs to turn off be sure it will turn off fast enough to avoid damage from heating.  With a P-channel turn off is often accomplished by a gate resistor with a bias current.  With a battery it is typically desired to have the bias current small for long battery life which means slower turn off.  See www.ti.com/lit/sluu474 page 43 Q5 as one method to speed up turn off, other methods are possible.
    • A signal FET may be preferred for DSG over the NPN since there is little gate current
    • Although you have 4 cells and may not need a zener diode on the P-ch gate for static DC levels, be sure to check for system transients.