ISL78692 Datasheet, PDF (9/18 Pages) Intersil Corporation – Li-ion/Li-Polymer Battery Charger

ISL78692
Typical Operating Performance The test conditions for the Typical Operating Performance are: V_IN = 5V,
T_A = +25°C, R_IREF = 160kΩ, V_BAT = 3.7V, Unless Otherwise Noted. (Continued)
500
450
400
+70°C
350
+75°C
300
250
200
150
100
+85°C
50
0
2.2
2.7
3.2
3.7
4.2
V_BAT (V)
100
95
90
85
80
75
70
65
60 _2.2
+70°C
+85°C
+75°C
2.7
3.2
3.7
4.2
V_BAT (V)
FIGURE 16. V_BAT vs I_BAT vs AMBIENT TEMPERATURE,
R_IREF = 200k, V_IN = 5.5V, AIR FLOW = 0 LFM,
MEASURED ON THE ISL78692EVAL1Z BOARD
FIGURE 17. JUNCTION TEMPERATURE vs V_BAT vs AMBIENT
TEMPERATURE, R_IREF = 200k, V_IN = 5.5V,
AIR FLOW = 0 LFM, MEASURED ON THE
ISL78692EVAL1Z BOARD
Theory of Operation
The ISL78692 is an integrated charger for single-cell Lithium
chemistry batteries. The ISL78692 functions as a traditional
linear charger when powered with a voltage source adapter.
When powered with a current-limited adapter, the charger
minimizes the thermal dissipation commonly seen in traditional
linear chargers.
As a linear charger, the ISL78692 charges a battery in the popular
constant current (CC) and constant voltage (CV) profile. The
constant charge current I_REF is programmable up to 1A with an
external resistor or a logic input. The charge voltage V_CH has 1%
accuracy over the entire recommended operating condition range.
The charger preconditions the battery with a 10% typical of the
programmed current at the beginning of a charge cycle until the
battery voltage is verified to be above the minimum fast charge
voltage, V_TRICKLE. This low current preconditioning charge mode is
named trickle mode. The verification takes 15 cycles of an internal
oscillator whose period is programmable with a timing capacitor
on the time pin. A thermal-foldback feature protects the device
from the thermal concern typically seen in linear chargers. The
charger reduces the charge current automatically as the IC
internal temperature rises above +100°C to prevent further
temperature rise. The thermal-foldback feature guarantees safe
operation when the printed circuit board (PCB) is space limited for
thermal dissipation.
A TEMP pin monitors the battery temperature to ensure a safe
charging temperature range. The temperature range is
programmable with an external negative temperature coefficient
(NTC) thermistor. The TEMP pin is also used to detect the removal
of the battery.
The charger offers a safety timer for setting the fast charge time
(TIMEOUT) limit to prevent charging a dead battery for an extensively
long time. The trickle mode is limited to 1/8 of TIMEOUT.
The charger automatically recharges the battery when the
battery voltage drops below a recharge threshold of 3.9V (typ).
When the input supply is not present, the ISL78692 draws less
than 1µA current from the battery.
Three indication pins are available from the charger to indicate
the charge status. The V2P8 outputs a 2.8VDC voltage when the
input voltage is above the power-on reset (POR) level and can be
used as the power-present indication. This pin is capable of
sourcing a 2mA current, so it can also be used to bias external
circuits. The STATUS pin is an open-drain logic output that turns
LOW at the beginning of a charge cycle until the end-of-charge
(EOC) condition is qualified. The EOC condition is when the
battery voltage rises above the recharge threshold and the
charge current falls below a preset of a tenth of the programmed
charge current. Once the EOC condition is qualified, the STATUS
output rises to HIGH and is latched. The latch is released at the
beginning of a charge or recharge cycle. The open-drain FAULT
pin turns low when any fault conditions occur. The fault
conditions include the external battery temperature fault, a
charge time fault, or the battery removal.
Figure 18 shows the typical charge curves in a traditional linear
charger powered with a constant voltage adapter. From top to
bottom, the curves represent the constant input voltage, the
battery voltage, the charge current and the power dissipation in
the charger. The power dissipation P_CH is given by Equation 1:
P_CH = V_IN-V_BAT  I_CHARGE
(EQ. 1)
where I_CHARGE is the charge current. The maximum power
dissipation occurs during the beginning of the CC mode. The
maximum power the IC is capable of dissipating is dependent on
the thermal impedance of the printed circuit board (PCB).
Figure 18 shows (with dotted lines) two cases that the charge
currents are limited by the maximum power dissipation
capability due to the thermal foldback.
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FN8692.0
September 10, 2014