ISL6297_07 Datasheet, PDF (14/16 Pages) Intersil Corporation – Li-ion/Li Polymer Battery Charger

ISL6297
The ratio, K, of the TEMP pin voltage to the bias voltage is:
K
=
--------^R----^T---------
R_T + R_U
and
(EQ. 8)
R_T
=
-----^K-------
1–K
×
R
U
At V_TMIN,
(EQ. 9)
K
=
--²----^.-⁰----
2.88
=
0.6844
Similarly, at V_TMAX.
(EQ. 10)
K
=
0_----._-7_----1_---8_--
2.88
=
0.24^· 93
(EQ. 11)
Using these equations to calculate the ratio of the thermistor
hot to cold resistance results in:
-^R----^T----^C----^O----^L----^D--
R_THOT
=
-₀-²--_.-_3.--²--₃--⁷-₂--^R--_R---^U_·--U--
=
6.84
and
(EQ. 12)
R_U = 0.44R_TCOLD
(EQ. 13)
where R_TCOLD and R_THOT are the NTC thermistor
resistance values at the cold and hot temperature limits
respectively.
It is usually difficult to find an NTC thermistor that has the
exact ratio given in EQ. 12. A thermistor with a ratio larger
than 6.84, that is:
-^R----^T----^C----^O----^L----^D-- ≥ 6.84
R_THOT
(EQ. 14)
can be used in series with a regular resistor to form an
effective thermistor that has the right ratio, as shown in
Figure 12.
V2P8
R_U
TEMP
ISL6297
R_S
R_T
GND
FIGURE 12. EFFECTIVE NTC THERMISTOR CIRCUIT
With the series resistor R_S, EQ. 12 can be re-written as:
R_{-----S-----+-----}R_{-----T---C----O-----L---D--}
R_S + R_THOT
=
6.84
(EQ. 15)
Once the thermistor and the temperature limits are selected,
R_S and R_U can be calculated using:
R_S
=
R_{-----T----C----O----L----D-----–-----}6_---._--8_---4_----R_{----T----H----O-----T-}
5.84
and
(EQ. 16)
R_U = 0.44 × (R_S + R_TCOLD)
(EQ. 17)
To summarize, the NTC thermistor circuit design requires
three steps:
1. Find an NTC thermistor that satisfies EQ. 14. The
temperature limits are determined by the application
requirement.
2. Calculate the series resistance according to EQ. 16.
3. Calculate the pull-up resistance according to EQ. 17.
Example:
The charger is designed to charge the battery with the
temperature range from -5°C to +45°C. The 10kΩ NTC
thermistor NCP15XH103F03RC from Murata
(http://www.murata.com) satisfies EQ. 14. The resistance
values for this thermistor are given in Table 3. The typical
resistance at -5°C and +45°C are:
R_TCOLD = 33.8922kΩ and
R_THOT = 4.9169kΩ.
Using EQ. 16 and EQ. 17 result in:
R_S = 41.6Ω and R_U = 14.931kΩ.
Selecting R_S = 41.2Ω and R_U = 15kΩ gives a low
temperature threshold of -5.1°C and a high temperature
threshold of +44.9°C.
TABLE 3. RESISTANCE TABLE OF NCP15XH103F03RC
TEMP (°C) R-LOW (kΩ) R-CENTER (kΩ) R-HIGH (kΩ)
-10
41.4765
42.5062
43.5570
-5
33.1462
33.8922
34.6515
0
26.6780
27.2186
27.7675
6
20.7560
21.1230
21.4944
7
19.9227
20.2666
20.6143
40
5.7443
5.8336
5.9238
45
4.8333
4.9169
5.0015
50
4.0833
4.1609
4.2395
55
3.4634
3.5350
3.6076
14
FN9215.1
March 20, 2007