Datasheet

LTC4007
12
4007fc
OPERATION
by the timing resistor connected to R
T
, keeps switch S1
closed to sample the thermistor:
t
SAMPLE
= 127.5 • 20 • R
RT
• 17.5pF = 13.8ms,
for R
RT
= 309k
The external RC network is driven to approximately 4.5V
and settles to a fi nal value across the thermistor of:
V
VR
RR
RTH FINAL
TH
TH
()
.•
=
+
45
9
This voltage is stored by C7. Then the switch is opened
for a short period of time to read the voltage across the
thermistor.
t
HOLD
= 10 • R
RT
• 17.5pF = 54μs,
for R
RT
= 309k
When the t
HOLD
interval ends the result of the thermistor
testing is stored in the D fl ip-fl op (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
BAD
to zero and charging will
continue. If the voltage at NTC is outside of the resistor
divider limits, then the DFF will set T
BAD
to one, the charger
will be shut down, FAULT pin is set low and the timer will
be suspended until T
BAD
returns to zero (see Figure 4).
9
R9
32.4k
C7
0.47μF
R
TH
10k
NTC
–
+
–
+
–
+
NTC
LTC4007
S1
60k
~4.5V
CLK
45k
15k
TBAD
4007 F03
D
C
Q
CLK
(NOT TO
SCALE)
V
NTC
t
SAMPLE
VOLTAGE ACROSS THERMISTOR
t
HOLD
4007 F04
COMPARATOR HIGH LIMIT
COMPARATOR LOW LIMIT
Figure 3
Figure 4