BQ24250C_15 Datasheet, PDF (23/46 Pages) Texas Instruments – bq24250C 2A Single Input I2C, Standalone Switch-Mode Li-Ion Battery Charger with Power-Path Management

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LDO
TS
R₂
NTC ^R₃
bq24250C
SLUSBY7 – JULY 2014
Figure 20. Voltage Based NTC circuit
The use of R3 is only necessary when the NTC does not have a beta near 3500K. When deviating from this
beta, error will be introduced in the actual temperature trip thresholds. The trip thresholds are summarized below
which are typical values provided in the specification table. Note that the T_WARM threshold is just a warning for the
warm temperature, the device will generate an interrupt but it will not affect the charging process.
Table 3. Ratiometric TS Trip Thresholds
V_HOT
V_WARM
V_COOL
V_COLD
30.0%
38.3%
48.5%
60%
When sizing for R2 and R3, it is best to solve two simultaneous equations that ensure the temperature profile of
the NTC network will cross the V_HOT and V_COLD thresholds. The accuracy of the V_WARM and V_COOL threshold will
depend on the beta of the chosen NTC resistor. The two simultaneous equations are shown below:
%V_COLD
=
æ
ççè
R₃ R_{NTC TCOLD}
R₃ + R_{NTC TCOLD}
ö
÷÷ø
æ
ççè
R₃ R_{NTC TCOLD}
R₃ + R_{NTC TCOLD}
ö
÷÷ø
+ R2
´ 100
%V_HOT
=
æ
ççè
R₃ R_{NTC THOT}
R₃ + R_{NTC THOT}
ö
÷÷ø
´ 100
æ
ççè
R₃ R_{NTC THOT}
R₃ + R_{NTC THOT}
ö
÷÷ø
+
R2
(4)
Where the NTC resistance at the V_HOT and V_COLD temperatures must be resolved as follows:
( ) _RNTC
TCOLD
=
b
R_oe
1TCOLD_-
1
To
( ) _RNTC
THOT
β
=R_oe
1THOT-
1
To
(5)
To be JEITA compliant, T_COLD must be 0°C and T_HOT must be 60°C. If an NTC resistor is chosen such that the
beta is 4000K and the nominal resistance is 10kΩ, the following R2 and R3 values result from the above
equations:
R₂ = 5 kΩ
R₃ = 9.82 kΩ
Figure 21 illustrates the temperature profile of the NTC network with R2 and R3 set to the above values.
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