LTC4007_15 Datasheet, PDF (12/24 Pages) Linear Technology – 4A, High Effi ciency, Standalone Li-Ion Battery Charger

LTC4007
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 final value across the thermistor of:
V_RTH(FINAL)
=
4.5V
R_TH
• R_TH
+ R9
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 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_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).
LTC4007
R9
32.4k _NTC
9
R_TH
10k
NTC
C7
0.47μF
S1
CLK
~4.5V
60k
12
45k
DQ
C
15k
TBAD
4007 F03
Figure 3
CLK
(NOT TO
SCALE)
t_SAMPLE
t_HOLD
V_NTC
VOLTAGE ACROSS THERMISTOR
COMPARATOR HIGH LIMIT
COMPARATOR LOW LIMIT
Figure 4
4007 F04
4007fc