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I am designing a Li Poly charger circuitry using the BQ24170 chip. Battery charger circuit

I am connecting a bench power supply instead of an adaptor to monitor the current consumption. Also I am using a 2S Li Poly battery pack. I found that I had connected the CELL pin in the FB configuration which is wrong as I am using the BQ24170. So the CELL pin should be kept floating for 2S battery pack. Hence-forth I removed the R11 and R18. But it still does not charge. The chip datasheet states the following -

The following conditions have to be valid before charging is enabled:

  • ISET pin above 120mV

  • Device is not in Under-Voltage-Lock-Out (UVLO) mode (i.e. VAVCC > VUVLO)

  • Device is not in SLEEP mode (i.e. VAVCC > VSRN)

  • OVPSET voltage is between 0.5V and 1.6V to qualify the adapter

  • 1.5s delay is complete after initial power-up

  • REGN LDO and VREF LDO voltages are at correct levels

  • Thermal Shut down (TSHUT) is not valid

  • TS fault is not detected

  • ACFET turns on (See System Power Selector for details)

I am using a 10K resistor for the RT1 thermistor. Two conditions from the above are not met in my design -

Pin OVPSET - gives me 2.48 where as it should be between 0.5 and 1.6. and ACFET should be turned on which is not.

ACFET is not turned on. This means that ACDRV pin voltage is not enough and therefore the MOSFET Q2 is not ON. I see that R165 and C62 should be connected between the Source and the GATE of the MOSFET which is not so.

Is there any other issue which is not spotted by me please. ANt pointers will be welcome. The MOSFET used are CSD17313Q2.

One of the application engineers state that-

" The bq24170 only supports 4.2V/cell, but you can use the bq24172 if you would like an adjustable cell voltage such as for Li-Po battery. "

Does it mean that once charging using the BQ24170 starts , it stops only when the voltage in each battery is 4.2V/cell ? And that if I use BQ24172 I can stop charging it when the voltage reaches 3.7V/batter, right ? BTW , I am using a 2S battery which is giving me 5.6V.

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You are right. According to manufacturers website the BQ24170 chip has a fixed 4.2 charging voltage. You cannot adjust it. This site http://www.ti.com/product/bq24170 has a comparison table for these chargers. BQ24171 and BQ24172 are adjustable.

(bq24172) Charge voltage analog feedback adjustment. Connect the output of a resistor divider powered from the battery terminals to VFB to AGND. Output voltage > is regulated to 2.1 V on FB pin during constant-voltage mode.

So make a divider to pin 14 ( FB ) as suggested :

The bq24172 uses external resistor divider for voltage feedback and regulate to >internal 2.1-V voltage reference on FB pin. Use the following equation for the >regulation voltage for bq24172: V =2.1V where • R2 is connected from FB to the battery. • R1 is connected from FB to GND.

For 2x 3.7 = 7.4V R2/R1 = 2,52380952380952.

Using bq24170 you can adjust charge voltage for multiple cells but in 4.2 increments. If you want 3.7 / cell you have to use adjusted version.

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