LTC4008 Linear Technology, LTC4008 Datasheet - Page 11

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

www.DataSheet4U.com
OPERATIO
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
(
= 150k
= 127.5 • 20 • R
)
=
U
4 5
R
.
TH
V R
+
•
R
TH
10
RT
• 17.5pF = 6.7ms,
(NOT TO
SCALE)
V
CLK
NTC
T
, keeps switch S1
VOLTAGE ACROSS THERMISTOR
Figure 4
COMPARATOR LOW LIMIT
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, FAULT pin is set low and the timer will
be suspended until T
COMPARATOR HIGH LIMIT
t
t
for R
HOLD
HOLD
t
SAMPLE
RT
= 10 • R
= 150k
HOLD
interval ends the result of the thermistor
RT
4008 F04
• 17.5pF = 26µs,
BAD
returns to zero (see Figure 4).
BAD
to zero and charging will
BAD
to one, the charger
LTC4008
11
4008fa