BQ24780S: Rsr short protection

Hi Ann,

Even if Rsr is shorted, we still have two protections that will protect the system -- ACOC (adapter overcurrent at Rac, see 7.3.9.1 in datasheet) and Inductor/MOSFET short protection (see 7.3.9.6).  Also, keep in mind that the battery pack should have a protection IC that will most likely kick in before either of these become necessary.

When Rsr is shorted, the charger IC will think that it is sensing 0A charging current.  This will drive the feedback loop to continually increase charging current into the battery, which is the concern.  Also, the charge overcurrent protection (see datasheet section 7.3.9.2) will be ineffective because it uses Rsr.  However, the increasing charging current will eventually trip either the ACOC or the Inductor/MOSFET protections (which one first depends on what value ACOC is set to as well as characteristics of the FETs), either of which will latch off the charging to protect the system.

Again, the battery should have a protection IC that will sense the charging overcurrent and block it, which will likely kick in before either of the above protections.  But if for some reason that battery protection IC were not in place, we still have these other two protections on the charger IC. 

Regards,

Steve