ISL6292_06 Datasheet, PDF (14/20 Pages) Intersil Corporation – Li-ion/Li Polymer Battery Charger

ISL6292
As the TEMP pin voltage rises from low and exceeds the
1.4V threshold, the under temperature signal rises and does
not clear until the TEMP pin voltage falls below the 1.2V
falling threshold. Similarly, the over-temperature signal is
given when the TEMP pin voltage falls below the 0.35V
threshold and does not clear until the voltage rises above
0.406V. The actual accuracy of the 2.8V is not important
because all the thresholds and the TEMP pin voltage are
ratios determined by the resistor dividers, as shown in
Figure 22.
The NTC thermistor is required to have a resistance ratio of
7:1 at the low and the high temperature limits, that is,
R_{-----C----O-----L---D--}
R_HOT
=
7
(EQ. 7)
This is because at the low temperature limit, the TEMP pin
voltage is 1.4V, which is 1/2 of the 2.8V bias. Thus,
R_COLD = R_U
(EQ. 8)
where R_U is the pull-up resistor as shown in Figure 22. On
the other hand, at the high temperature limit the TEMP pin
voltage is 0.35V, 1/8 of the 2.8V bias. Therefore,
R_HOT
=
-^R----^U--
7
(EQ. 9)
Various NTC thermistors are available for this application.
Table 1 shows the resistance ratio and the negative
temperature coefficient of the curve-1 NTC thermistor from
Vishay (http://www.vishay.com) at various temperatures. The
resistance at +3°C is approximately seven times the
resistance at +47°C, that is:
--^R-----³---^°--^C----
R _{47 ° C}
=
7
(EQ. 10)
Therefore, if +3°C is the low temperature limit, then the high
temperature limit is approximately +47°C. The pull-up
resistor R_U can choose the same value as the resistance at
+3°C.
TABLE 1. RESISTANCE RATIO OF VISHAY’S CURVE-1 NTC
TEMPERATURE (°C)
0
R_T/R_25°C
3.266
NTC (%/°C)
5.1
3
2.806
5.1
5
2.540
5.0
25
1.000
4.4
45
0.4368
4.0
47
0.4041
3.9
50
0.3602
3.9
The temperature hysteresis can be estimated. At the low
temperature, the hysteresis is approximately,
T_hysLOW ≈ ₁--¹--_.--_4.-⁴--_V--^V----_â‹-^---₀₁---_.-^.-₀₂---₅-^V---₁- ≈ 3
(°_C)
(EQ. 11)
where 0.051 is the NTC at +3°C. Similarly, the high
temperature hysteresis is,
T_hysHIGH ≈ ⁰--₀--^.-_.4--₃--⁰-₅--⁶--_V--^V----_â‹^----₀₀---_..--₀₃---₃₅---₉-^V-- ≈ 4
(°_C)
(EQ. 12)
where the 0.039 is the NTC at +47°C.
For applications that do not need to monitor the battery
temperature, the NTC thermistor can be replaced with a
regular resistor of a half value of the pull up resistor R_U.
Another option is to connect the TEMP pin to the IREF pin
that has a 0.8V output. With such connection, the IREF pin
can no longer be programmed with logic inputs.
Battery Removal Detection
The ISL6292 assumes that the thermistor is co-packed with
the battery and is removed together with the battery. When
the charger senses a TEMP pin voltage that is 2.1V or
higher, it assumes that the battery is removed. The battery
removal detection circuit is also shown in Figure 22. When a
battery is removed, a FAULT signal is indicated and charging
is halted. When a battery is inserted again, a new charge
cycle starts.
Indications
The ISL6292 has three indications: the input presence, the
charge status, and the fault indication. The input presence is
indicated by the V2P8 pin while the other two indications are
presented by the STATUS pin and FAULT pin respectively.
Figure 23 shows the V2P8 pin voltage vs. the input voltage.
Table 2 summarizes the other two pins.
3.4V
2.8V
V_IN
V2P8
2.4V
FIGURE 23. THE V2P8 PIN OUTPUT vs THE INPUT VOLTAGE
AT THE VIN PIN. VERTICAL: 1V/DIV,
HORIZONTAL: 100ms/DIV.
14
FN9105.8
November 14, 2006