ISL6292_14 Datasheet, PDF (14/20 Pages) Intersil Corporation – Li-ion/Li Polymer Battery Charger

ISL6292
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, which is shown in Equation 10:
--^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 estimated in
Equation 11:
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 estimated in Equation 12:
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
2.4V
V2P8
FIGURE 23. THE V2P8 PIN OUTPUT vs THE INPUT VOLTAGE
AT THE VIN PIN. VERTICAL: 1V/DIV,
HORIZONTAL: 100ms/DIV
Shutdown
The ISL6292 can be shutdown by pulling the EN pin to
ground. When shut down, the charger draws typically less
than 30µA current from the input power and the 2.8V output
at the V2P8 pin is also turned off. The EN pin needs to be
driven with an open-drain or open-collector logic output, so
that the EN pin is floating when the charger is enabled.
TABLE 2. STATUS INDICATIONS
FAULT STATUS
INDICATION
High High
Charge completed with no fault (Inhibit) or
Standby
*Both outputs are pulled up with external resistors.
14
FN9105.9
December 17, 2007