ISL6292_14 Datasheet, PDF (13/20 Pages) Intersil Corporation – Li-ion/Li Polymer Battery Charger

ISL6292
End-of-Charge (EOC) Current
The end-of-charge current I_MIN sets the level at which the
charger starts to indicate the end of the charge with the
STATUS pin, as shown in Figure 19. The charger actually
does not terminate charging until the end of the TIMEOUT,
as described in “Total Charge Time” on page 12. The I_MIN is
set in two ways, by connecting a resistor between the IMIN
pin and ground, or by connecting the IMIN pin to the V2P8
pin. When programming with the resistor, the I_MIN is set in
Equation 6.
I_MIN
=
10000
⋅
-^V----^R-----^E----^F--
R_IMIN
=
---0---.--8---V-----
R_IMIN
×10⁴ ₍
A
)
(EQ. 6)
where R_IMIN is the resistor connected between the IMIN pin
and the ground. When connected to the V2P8 pin, the I_MIN
is set to 1/10 of I_REF, except when the IREF pin is shorted to
GND. Under this exception, I_MIN is 5mA. For the ISL6292 in
the 3x3 DFN package, the IMIN pin is bonded internally to
V2P8.
Charge Current Thermal Foldback
Over-heating is always a concern in a linear charger. The
maximum power dissipation usually occurs at the beginning
of a charge cycle when the battery voltage is at its minimum
but the charge current is at its maximum. The charge current
thermal foldback function in the ISL6292 frees users from
the over-heating concern.
Figure 20 shows the current signals at the summing node of
the current error amplifier CA in the Block Diagram shown on
page 9. I_R is the reference and I_T is the current from the
Temperature Monitoring block. The I_T has no impact on the
charge current until the internal temperature reaches
approximately +100°C; then I_T rises at a rate of 1µA/°C.
When I_T rises, the current control loop forces the sensed
current I_SEN to reduce at the same rate. As a mirrored
current, the charge current is 100,000 times that of the
sensed current and reduces at a rate of 100mA/°C. For a
charger with the constant charge current set at 1A, the
charge current is reduced to zero when the internal
temperature rises to +110°C. The actual charge current
settles between +100°C to +110°C.
I_R
2.8V Bias Voltage
The ISL6292 provides a 2.8V voltage for biasing the internal
control and logic circuit. This voltage is also available for
external circuits such as the NTC thermistor circuit. The
maximum allowed external load is 2mA.
NTC Thermistor
The ISL6292 uses two comparators (CP2 and CP3) to form a
window comparator, as shown in Figure 22. When the TEMP
pin voltage is “out of the window,” determined by the V_TMIN
and V_TMAX, the ISL6292 stops charging and indicates a fault
condition. When the temperature returns to the set range, the
charger re-starts a charge cycle. The two MOSFETs, Q1 and
Q2, produce hysteresis for both upper and lower thresholds.
The temperature window is shown in Figure 21.
2.8V
V_TMIN (1.4V)
V_TMIN- (1.2V)
TEMP
Pin
Voltage
V_TMAX+ (0.406V)
V_TMAX (0.35V)
0V
Under
Temp
Over
Temp
FIGURE 21. CRITICAL VOLTAGE LEVELS FOR TEMP PIN
ISL6292
2.8V
V2P8
Battery
Removal
CP1 _-
+
Under
Temp
CP2 _-
+
R1
V_RMV
40K
R2
60K
R_U
V_TMIN
R3
To TEMP Pin
75K
TEMP
Q1
I_T
I_SEN
Over
Temp
CP3 _-
+
V_TMAX
R4
R_T
25K
Q2
R5
4K
GND
100^OC Temperature
FIGURE 20. CURRENT SIGNALS AT THE AMPLIFIER CA INPUT
Usually the charge current should not drop below I_MIN because
of the thermal foldback. For some extreme cases (if that does
happen) the charger does not indicate end-of-charge unless
the battery voltage is already above the recharge threshold.
FIGURE 22. THE INTERNAL AND EXTERNAL CIRCUIT FOR
THE NTC INTERFACE
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
13
FN9105.9
December 17, 2007