
8
FN9139 .1
April 19, 2005
Applications Information
Power-On Reset (POR)
The ISL6292B has a 3.4V rising POR threshold. Before the
input voltage reaches the POR threshold, the 2.9V V2P9 pin
outputs 0V and the charger is disabled. Once the POR
threshold is reached, the V2P9 pin outputs 2.9V, the open-
drain MOSFET on the DT pin is turned on, and the ambient
temperature monitoring circuit starts to function. After a
delay and after the input voltage rises above the battery
voltage, the charger control circuit starts to work and the DT
pin impedance becomes high. Figure 3 shows the sequence
of the events at power on. The POR has a falling threshold
of 2.4V typically. At power on, all counters are reset to zero.
Charge Current and R
IREF
Selection
When the ISL6292B is used as a traditional linear charger,
the R
IREF
sets the constant charge current. The trickle
charge current is 10% of the CC current I
REF
. When working
with a current-limited supply, the CC current is determined
by the supply limited current I
LIM
. I
REF
needs to be
programmed higher than I
LIM
and is used as an overcurrent
protection. Taking into account the tolerance of both the I
LIM
and I
REF
, it is recommended the I
REF
be programmed at
least 30% higher than the I
LIM
. I
REF
can be found by the
following equation:
(EQ. 1)
The trickle charge current is 10% of IREF, that is,
(EQ. 2)
The ISL6292B has a comparator with 100mV offset voltage
to ensure the input voltage is higher than the battery voltage
before charging starts (see the Block Diagram). This
condition requires I
LIM
be higher than 400mA. The upper
limit for I
LIM
is 1.5A.
EOC Current and R
IMIN
Selection
The EOC current is programmed by the IMIN pin and can be
calculated by the following equation:
(EQ. 3)
The EOC current has a programming range up to 400mA.
To qualify as an EOC condition, the charge current needs to
drop below the I
MIN
level and stay below the IMIN level for
more than three to four cycles of the internal oscillator;
additionally, the battery voltage rises above the recharge
threshold given in the Electrical Specification table.
Internal Oscillator
The internal oscillator establishes a timing reference. The
oscillation period is programmable with an external timing
capacitor, C
TIME
, as shown in Typical Applications. The
oscillator charges the timing capacitor to 1.5V and then
discharges it to 0.5V in one period, both with 10
μ
A current.
The period T
OSC
is:
(EQ. 4)
A 1nF capacitor results in a 0.2ms oscillation period. The
accuracy of the period is mainly dependent on the accuracy
of the capacitance and the internal current source.
Total Charge Time and C
TIME
Selection
The total charge time for the CC mode and CV mode is
limited to a length of TIMEOUT, which can be found by:
(EQ. 5)
FIGURE 3. EVENT SEQUENCE AT POWER UP
VIN
V2P9
DT
Imped
Charger
Start
POR
I
REF
IREF
-----------------
10
5
×
A
( )
=
I
Trickle
IREF
-----------------
10
4
×
A
( )
=
I
MIN
R
IMIN
----------------
10
4
×
A
( )
=
T
OSC
0.2 10
6
C
TIME
=
onds
sec
(
)
FIGURE 4. OPERATION WAVEFORMS
VIN
V2P9
RED
GRN
VBAT
ICHG
15 Cycles to
1/8 TIMEOUT
15
Cycles
POR
Threshold
t0
t1t2t3
t4
t5
t6t7
t8
Charge
Cycle
Charge
Cycle
IMIN
DT
Imped.
MIN
RECHRG
V
2.8V VMIN
TIMEOUT
14
C
-----------------
=
minutes
(
)
ISL6292B