Is it possible to charge a 7.4V battery with 8.8V input to BQ24123

Yes, it should work fine.  The only compromise is that it may go in to drop out as it transitions from CC to CV meaning that the drop across the top FET, depending on the charge current, may exceed what overhead is needed and thus not get the full CC current as the battery approaches full.  This is no problem, it will look like it starting tapering the current a little sooner.

Hi Charles,

Thanks for the reply.

Attached is our battery charger circuit. We are following "Topology 3" among the 4 path selection topologies suggested for BQ24123 in "Implementations of Battery Charger and Power-Path Management System Using bq2410x/11x/12x (bqSWITCHER™)" (SLUA376 - June2006). The mains (VCC_IN) is 9V@2A and the Li-Ion battery spec is 7.4V @ 2.2A. Please review the implemented circuit and provide your valuable inputs.

Also, we have few queries as follows:

1) Considering the case where the VCC_IN (see attached circuit) is not available, we expect the VCC_BATT to come up. For this to happen, the PG needs to be high to switchon the N channel MOSFET. But since PG is a open drain signal and VCC_IN is not available to pull this signal, will this floating PG signal drive the gate of N-channel MOSFET adequately to drive the battery supply?

2) During the switchover from Main supply to battery supply or vis versa, will there be any glitch in the power which is being supplied to the system load ? We are expecting a smooth transition without any disturbance in final main power supplied to the load.

3) We have used IRLML6401 for P-channel and BSS138 for N-Channel. Are the MOSFETs selected in this circuit OK ? or are there any other recommended ones ?

Regards,

Prajwal Kini A

Hi All,

Please provide your valuable inputs for the above query. We are in the schematics completion stage and looking forward for review inputs.

Thanks in advance.

Regards,

Prajwal Kini A.

A1) Yes the PG pin should be pulled high via R6, Q2 boady diode and the battery.  Note that the battery is practically fully discharged at around 3V so one would get around 2V at the PG pin so one should use a logic gate N-CH FET for Q122 so it will turn on when the battery is >2.8V.

A2)  It should switch over very fast, <10us, after Vin drops to the battery voltage.  There should be enough system capacitance to hold up the system during this transition.

A3) FETs should be good

Hi,

Thank you very much for your reply.

For A1) :

Our understanding is that , when there will be no VCC_IN power, PG will be floating. Hence N channel MoSFET (Q122) will not be switched ON and in turn, P channel MoSFET's  gate (Q3) will not get ground. Thus the battery power from pin 1 of J2 will not be reaching VCC_BATT. So how will the logic work when VCC_IN is not present ?

Note: VCC_BATT is the final power going to system. Battery power is at pin 1 of J2.

Regards,

Prajwal Kini A

Hi Prajwal,

I have analysed the posted circuit. The below circuit should work for your above mentioned query.

Regards,

Rajendra

hi----------------;

please help to suggest power path management li-ion battery charge controller ic and reference design.

1) Battery voltage (VBAT)->7.4V

2)Input current (IIN) and voltage (VBUS)->9v/3A or 12V/3A

3)System voltage (VSYS) and current (ISYS)->7V/3A

4)Charge current (ICHG)-1A

Thanks&regards;

krishna pratap