LTC4008 Linear Technology, LTC4008 Datasheet - Page 10

• Manufacturer Part Number: LTC4008
• Description: Multi-Chemistry Battery Charger
• Manufacturer: Linear Technology

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OPERATIO
LTC4008
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 tran-
sient 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 protec-
tion 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.
10
1.2
0.8
0.6
0.2
1.0
0.4
0
0
I
CHARGE
Figure 2. V
U
0.309V
20
(% OF MAXIMUM CURRENT)
40
PROG
60
vs I
1.19V
CHARGE
80
4008 F02
100
Charger Startup
When the charger is enabled, it will not begin switching
until the I
initial current will be positive. This threshold is 5% to 15%
of the maximum programmed current (100mV/R
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.
NTC
R
10k
TH
TH
C7
0.47µF
32.4k
R10
voltage exceeds a threshold that assures
8
LTC4008
NTC
Figure 3
S1
D
C
Q
+
–
–
+
–
+
60k
45k
15k
SENSE
~4.5V
4008fa
CLK
TBAD
4008 F03
).