BQ24257 Datasheet, PDF (25/45 Pages) Texas Instruments – 2A Single Input I2C, Standalone Switch-Mode Li-Ion Battery Charger with Integrated Current Sense Resistor

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bq24257
bq24258
SLUSBG0B – FEBRUARY 2013 – REVISED JULY 2013
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 thresholds 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.
Example NTC Network Profile of %LDO vs. TEMP
60
Tcool
55
50
45
40
Twarm
35
30
0
10
20
30
40
50
60
Temperature (C)
Figure 21. Voltage Based NTC Circuit Temperature Profile
For JEITA compliance, the T_COOL and T_WARM levels are to be 10°C and 45°C respectively. However, there is
some error due to the variation in beta from 3500K. As shown above, the actual temperature points at which the
NTC network crosses the V_COOL and V_WARM are 13°C and 47°C respectively. This error is small but should be
considered when choosing the final NTC resistor.
Once the resistors are configured, the internal JEITA algorithm will apply the below profile at each trip point for
battery voltage regulation and charge current regulation.
Copyright © 2013, Texas Instruments Incorporated
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