BQ24600_1 Datasheet, PDF (16/31 Pages) Texas Instruments – Stand-Alone Synchronous Switch-Mode Li-Ion or Li-Polymer Battery Charger with Low Iq

bq24600
SLUS891 – FEBRUARY 2010
www.ti.com
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, 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, safety timer 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 14 summarizes the operation.
VREF
VREF
CHARGE SUSPENDED
V_LTF
V_LTFH
TEMPERATURE RANGE
TO INITIATE CHARGE
CHARGE SUSPENDED
V_LTF
V_LTFH
TEMPERATURE RANGE
DURING A CHARGE
CYCLE
V_HTF
CHARGE SUSPENDED
CHARGE SUSPENDED
V_TCO
GND
GND
Figure 14. 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 Equation 6 and Equation 7:
V_{VR EF}
´ RTH_COLD
´ RTH_HOT
´
æ1
ç
è ^VLTF
-
1ö
÷
V_{HTF ø}
RT2 =
RTH
HOT
´
æV
ç ^VREF
è ^VHTF
ö
-
1_÷
-
RTH
COLD
ø
æV
ç ^VREF
ö
- 1_÷
è ^VLTF
ø
(6)
RT1 =
V_VREF
V_LTF
-1
1
1
+
RT2 RTH_COLD
(7)
For example, a 103AT NTC thermestor is used to monitor the battery pack temperature. Select T_COLD = 0ºC and
T_HOT = 40ºC. Then we get R_T2 = 430 kΩ, R_T1 = 9.311Ω and T_{CUT_OFF} = 45ºC. A small RC filter is suggested to
use for system-level ESD protection.
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