LTC4008_15 Datasheet, PDF (12/24 Pages) Linear Technology – 4A, High Effi ciency, Multi-Chemistry Battery Charger

LTC4008
OPERATION
The thermistor detector performs a sample-and-hold
function. An internal clock, whose frequency is determined
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 = 6.7ms,
for R_RT = 150k
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 + R10
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 = 26μs,
for R_RT = 150k
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).
CLK
(NOT TO
SCALE)
t_SAMPLE
t_HOLD
VOLTAGE ACROSS THERMISTOR
V_NTC
COMPARATOR HIGH LIMIT
COMPARATOR LOW LIMIT
4008 F04
Figure 4
4008fb
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