ISL6297 Datasheet, PDF (13/16 Pages) Intersil Corporation – Li-ion/Li Polymer Battery Charger

ISL6297
Ambient Temperature Sensing
The TEMP pin sets the allowable ambient temperature
range for charging the battery. Typically, an NTC (negative
temperature coefficient) resistor is mounted on the printed
circuit board (PCB) to monitor the ambient temperature. Due
to the self-heating of the PCB during charging, the ISL6297
provides the DT pin to set a higher temperature threshold
during the charge operation.
Figure 9 shows the internal circuit for the ambient
temperature sensing function. Two comparators form a
window comparator whose high-threshold is V_TMIN and low-
threshold is V_TMAX. These two thresholds are given in the
Electrical Specifications. The two MOSFETs (Q1 and Q2)
create a hysteresis for each comparator, respectively. The
DT pin is shorted to GND via the internal Q3 MOSFET when
the charger is not charging, resulting in the equivalent circuit
shown in Figure 10 (A). The on-resistance of Q3 is typically
50Ω and is negligible compared to the external resistors.
When the charger starts to charge, Q3 is turned off to set a
higher temperature range determined by the external
resistor R_D. The equivalent circuit is shown in Figure 10 (B).
The DT pin provides a higher shut down ambient
temperature during the charger operation.
Under
Temp ^CP1 _-
+
Over
Temp
CP2
-
+
2.9V
V2P9
R1
V_TMIN
100K
To TEMP Pin
R2
75K
TEMP
R_U
Q1
V_TMAX
R3
25K
R_T
Q2 _R4
4K
CHG
DT
Q3
R_D
GND
FIGURE 9. THE INTERNAL AND EXTERNAL CIRCUIT FOR
THE NTC INTERFACE
V2P9
R_U
TEMP
R_T
DT
R_Q3
GND
V2P9
R_U
TEMP
R_T
R_D
GND
(A)
(B)
FIGURE 10. EQUIVALENT CIRCUITS FOR THE NTC DIVIDER
(A) BEFORE CHARGING STARTS
(B) DURING CHARGING
When the TEMP pin voltage is “out of the window,” as
determined by the V_TMIN and V_TMAX, the ISL6297 stops
charging and indicates a fault condition. When the
temperature returns to the set range, the charger continues
the charge cycle.
2.9V
V_TMIN (2.0V)
V_TMIN- (1.83V)
TEMP
Pin
Voltage
V_TMAX+ (0.788V)
V_TMAX (0.714V)
0V
Under
Temp
Over
Temp
FIGURE 11. CRITICAL VOLTAGE LEVELS FOR TEMP PIN
As the temperature falls, the TEMP pin voltage rises. When
it exceeds the 2.0V V_TMIN threshold, an under temperature
condition exists. This condition does not clear until the TEMP
pin voltage falls back below the threshold minus the
hysteresis voltage (V_TMIN-). Similarly, an over-temperature
condition exists when the TEMP pin voltage falls below the
0.714V V_TMAX threshold and does return to normal
temperature operation until the voltage rises above the
threshold plus the hysteresis voltage (V_TMAX+). The actual
accuracy of the 2.9V supply voltage is not important
because all the thresholds and the TEMP pin voltage are
ratios determined by the resistor dividers, as shown in
Figure 9.
13
FN9215.0
September 27, 2005