Datasheet

LTC4008
11
4008fb
OPERATION
If the charging current decreases below 10% to 15%
of programmed current, while engaged in input current
limiting, BGATE will be forced low to prevent the charger
from discharging the battery. Audible noise can occur in
this mode of operation.
An overvoltage comparator guards against voltage transient
overshoots (>7% of programmed value). In this case, both
MOSFETs are turned off until the overvoltage condition
is cleared. This feature is useful for batteries which “load
dump” themselves by opening their protection switch
to perform functions such as calibration or pulse mode
charging.
PWM Watchdog Timer
There is a watchdog timer that observes the activity on
the BGATE and TGATE pins. If TGATE stops switching for
more than 40μs, the watchdog activates and turns off the
top MOSFET for about 400ns. The watchdog engages to
prevent very low frequency operation in dropout which
is a potential source of audible noise when using ceramic
input and output capacitors.
Charger Startup
When the charger is enabled, it will not begin switching
until the I
TH
voltage exceeds a threshold that assures ini-
tial current will be positive. This threshold is 5% to 15%
of the maximum programmed current (100mV/R
SENSE
).
After the charger begins switching, the various loops will
control the current at a level that is higher or lower than
the initial current. The duration of this transient condition
depends upon the loop compensation but is typically less
than 100μs.
Thermistor Detection
The thermistor detection circuit is shown in Figure 3. It
requires an external resistor and capacitor in order to
function properly.
I
CHARGE
(% OF MAXIMUM CURRENT)
0
0
V
PROG
(V)
0.2
0.4
0.6
0.8
4008 F02
1.0
1.2
20 40 60 80 100
1.19V
0.309V
Figure 2. V
PROG
vs I
CHARGE
8
NTC
LTC4008
S1
R10
32.4k
C7
0.47μF
R
TH
10k
NTC
–
+
–
+
–
+
60k
~4.5V
CLK
45k
15k
TBAD
4008 F03
D
C
Q
Figure 3