ISL6291 Datasheet, PDF (9/11 Pages) Intersil Corporation – Li-ion/Li Polymer Linear Battery Charger

Charge Termination Voltage Regulation
As the battery voltage rises to the 4.1V or 4.2V termination
voltage, the voltage amplifier starts to output positive voltage
and to source current. This current partially cancels the
current of the reference current I_R to reduce the charge
current. If the battery voltage increases further due to the
charging, the voltage amplifier increases its output current to
reduce the equivalent reference current. As a result, after the
battery voltage reaches the termination voltage, the charge
current starts to decrease.
As the charging current drops to the end-of-charge current
level programmed by the IMIN pin, the charge stops. The
large voltage control loop gain guarantees that the battery
voltage is regulated within the 1% error specification.
Internal Thermal Management
The temperature rise of a linear charger is always a concern
in real applications. The temperature rise is caused by the
power dissipation of the charger. Maximum power
dissipation occurs when the battery is charged in the
constant current mode. The advanced thermal management
function of the ISL6291 frees users from the temperature
rise concern. The ISL6291 adopts a current-foldback
technique against the temperature rise. Under normal
operation, the ISL6291 charges the battery with the
programmed I_REF. If the internal thermal monitoring circuit
detects 100°C temperature in the IC, it starts to reduce the
charge current to prevent further temperature rise. The gain
for the current-foldback is 40mA/°C (or 0.4µA°C for the
reference current I_R) after the internal temperature reaches
100°C; therefore, for a charger with the constant charge
current set at 1A, the charge current is reduced to zero when
the internal temperature rises to 125°C. The actual internal
temperature should settle between 100°C to 125°C,
depending on the operating conditions, if the temperature
does rise above 100°C.
Battery Pack Temperature Monitoring
The ISL6291 uses two comparators to form a window
comparator. Figure 3 shows the internal circuit. When the
TEMP pin voltage is “out of the window,” as determined by
the V_TMIN and V_TMAX, the charging is stopped. The two
MOSFETs, Q1 and Q2, produce hysteresis for both upper
and lower threshold. Figure 4 shows all the critical voltage
levels and the output of the two comparators versus the
TEMP pin voltage.
The external thermistor circuit is shown in Figure 3. The NTC
thermistor R_T requires a pull-up resistor R_U to form a
resistive divider. R_U should be pulled up to the 2.8V V2P8
pin. Assume the resistance of the NTC thermistor is R_TH at
the high temperature limit, and is R_TL at the low limit. It can
be shown that:
R_TL = 9 ⋅ R_TH
9
Select an NTC thermistor whose resistance value satisfies
the above equation. A curve-1 NTC thermistor from Vishay is
a good candidate for this application. If a thermistor does not
meet this requirement, using a resistor in parallel or series
with the thermistor may solve the problem. Once the
thermistor is selected, the pull-up resistor should be chosen
as:
R_U = R_TL
ISL6291
2.8V
V2P8
Under
Temp
CP1 _-
+
V_TMIN
To TEMP Pin
R1
100K
R_U
R2
75K
TEMP
Q1
R3
5K
Over
Temp
CP2 _-
+
V_TMAX
R4
R_T
20K
Q2
R5
4K
GND
FIGURE 3. THE INTERNAL AND EXTERNAL CIRCUIT FOR THE
BATTERY PACK TEMPERATURE MONITORING
2.8V
V_TMIN (1.4V)
V_TMIN- (1.2V)
TEMP
Pin
Voltage
V_TMAX+ (0.33V)
V_TMAX (0.28V)
0V
Under
Temp
Over
Temp
FIGURE 4. CRITICAL VOLTAGE LEVELS FOR TEMP PIN
If a parallel or series resistor is used, the R_TL value is the
combined value at the low temperature limit. The
temperature hysteresis can be calculated once the thermistor
is selected. The typical hysteresis is about 3°C to 5°C.
FN9102.2
May 2, 2005