ltc4006egn-6-trpbf Linear Technology Corporation, ltc4006egn-6-trpbf Datasheet - Page 11

• Manufacturer Part Number: ltc4006egn-6-trpbf
• Description: 4a, High Efficiency, Standalone Li-ion Battery Charger
• Manufacturer: Linear Technology Corporation

OPERATIO
be inhibited if it is not already active. 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.
As the voltage at BAT increases to near the input voltage
at DCIN, the converter will attempt to turn on the top
MOSFET continuously (“dropout’’). A watchdog timer
detects this condition and forces the top MOSFET to turn
off for about 300ns at 40µs intervals. This is done to
prevent audible noise when using ceramic capacitors at
the input and output.
Charger Startup
When the charger is enabled, it will not begin switching
until the I
current will be positive. This threshold is 5% to 15% of the
maximum programmed current. 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.
The thermistor detector performs a sample-and-hold func-
tion. An internal clock, whose frequency is determined by
the timing resistor connected to R
closed to sample the thermistor:
The external RC network is driven to approximately 4.5V
and settles to a final value across the thermistor of:
t
for R
V
SAMPLE
RTH FINAL
RT
(
TH
= 309k
= 127.5 • 20 • R
voltage exceeds a threshold that assures initial
)
=
U
4 5
R
.
TH
V R
•
+
R
TH
9
RT
• 17.5pF = 13.8ms,
T
, keeps switch S1
This voltage is stored by C7. Then the switch is opened for a
short period of time to read the voltage across the thermistor.
When the t
testing is stored in the D flip-flop (DFF). If the voltage at
NTC is within the limits provided by the resistor divider
feeding the comparators, then the NOR gate output will be
low and the DFF will set T
continue. If the voltage at NTC is outside of the resistor
divider limits, then the DFF will set T
will be shut down, and the timer will be suspended until
T
BAD
(NOT TO
SCALE)
NTC
R
t
for R
10k
HOLD
TH
V
CLK
NTC
returns to zero (see Figure 4).
RT
VOLTAGE ACROSS THERMISTOR
= 10 • R
= 309k
HOLD
32.4k
C7
0.47µF
R9
interval ends the result of the thermistor
RT
6
LTC4006
NTC
• 17.5pF = 54µs,
Figure 4
Figure 3
S1
BAD
D
C
COMPARATOR LOW LIMIT
COMPARATOR HIGH LIMIT
Q
to zero and charging will
t
HOLD
BAD
t
+
+
SAMPLE
–
–
–
+
to one, the charger
LTC4006
~4.5V
60k
45k
15k
4006 F03
CLK
T
11
4006 F04
BAD
4006fa