BQ24618 Datasheet, PDF (21/36 Pages) Texas Instruments – Stand-Alone USB-Friendly Synchronous Switch-Mode Li-Ion or Li-Polymer Battery Charger

bq24618
www.ti.com
SLUSA55 – OCTOBER 2010
Temperature Qualification
The controller continuously monitors battery temperature by measuring the voltage between the TS pin and
GND. A negative temperature coefficient thermistor (NTC) and an external voltage divider typically develop this
voltage. The controller compares this voltage against its internal thresholds to determine if charging is allowed.
To initiate a charge cycle, the battery temperature must be within the V(LTF) to V(HTF) thresholds. If battery
temperature is outside of this range, the controller suspends charge and the safety timer and waits until the
battery temperature is within the V(LTF) to V(HTF) range. During the charge cycle the battery temperature must
be within the V(LTF) to V(TCO) thresholds. If battery temperature is outside of this range, the controller suspends
charge and waits until the battery temperature is within the V(LTF) to V(HTF) range. The controller suspends
charge by turning off the PWM charge FETs. Figure below summarizes the operation.
VREF
VREF
V_LTF
V_LTFH
CHARGE SUSPENDED
TEMPERATURE RANGE
TO INITIATE CHARGE
CHARGE SUSPENDED
V_LTF
V_LTFH
TEMPERATURE RANGE
DURING A CHARGE
CYCLE
V_HTF
CHARGE SUSPENDED
V_TCO
CHARGE SUSPENDED
GND
GND
Figure 18. TS pin, Thermistor Sense Thresholds
Assuming a 103AT NTC thermistor on the battery pack as shown in Figure 1, the value RT1 and RT2 can be
determined by using the following equations:
RT2 =
V_VREF
´
RTH_COLD
´
RTH_HOT
´
æ1
ç
è ^VLTF
-
1ö
÷
V_{TCO ø}
RTH_HOT
´
æ
ç
V_VREF
è ^VTCO
ö
-1_÷ - RTH_COLD
ø
´
æ
ç
V_VREF
-
ö
1_÷
è ^VLTF ø
(8)
RT1 =
V_VREF
V_LTF
-1
1
1
+
RT2 RTH_COLD
(9)
VREF
bq24618
TS
RT1
RT2
RTH
103AT
Figure 19. TS Resistor Network
Copyright © 2010, Texas Instruments Incorporated
Product Folder Link(s): bq24618
Submit Documentation Feedback
21