BQ25172: BQ25172 VIN vs Battery Count – Clarification Needed

Hi Samidha

Samidha Buriwar said:

For the BQ25172, could you please clarify the relationship between VIN and VOUT?

Minimum Vin-Vout of 80mV (VSLEEP) is required for unit to operate.  

As Vin-Vout increase the output current is increased, see figure 6-3 (Dropout Voltage vs Output Current). 

Couple of numbers, Vin - Vout 200mV for 250mA, Vin - Vout 400mV for 475mA.

Samidha Buriwar said:

• 3×NiMH (3.6 V nominal)

• 4×NiMH (4.8 V nominal)

3XNiMH, 3.6V --- should work from a 5V source.

4XNiMH 4.8V -- Will probably have difficulty.

How large are the cells?

Hi Bill,

Thanks for the details!

I’ll be using AAA NiMH cells, around 800–1100 mAh.

Hi Samidha

Note that the BQ25172 has a Recharge Voltage ( V-RECHG) of 1.33V / cell.  At the end of a charge cycle the battery voltage must be above this point or a fault will be set.  3 cell configuration, 3 X 1.33V = 3.99V  and  4 cell configuration, 4 X 1.33V = 5.32V  

For 1100mA battery - If we use 250mA charge current the drop out is about 200mV.  The charge time option would be 6 or 8 hr.

This would put the min input voltage at:

3 cell configuration, 3 X 1.33V = 3.99V + 200mV = 4.199V

4 cell configuration, 4 X 1.33V = 5.32V + 200mV = 5.52V