LTC4008_15 Datasheet, PDF (18/24 Pages) Linear Technology – 4A, High Effi ciency, Multi-Chemistry Battery Charger

LTC4008
APPLICATIONS INFORMATION
LTC4008
R9
NTC 8
C7 R9A
R_TH
4008 F09
Figure 9. General Thermistor Network
If you are using a thermistor that doesn’t have a 1:7
HIGH/LOW ratio, or you wish to set the HIGH/LOW limits
to different temperatures, then the more generic network
in Figure 9 should work.
Once the thermistor, R_TH, has been selected and the
thermistor value is known at the temperature limits, then
resistors R9 and R9A are given by:
For NTC thermistors:
R9 = 6 R_TH(LOW) • R_TH(HIGH)/(R_TH(LOW) – R_TH(HIGH))
R9A = 6 R_TH(LOW) • R_TH(HIGH)/(R_TH(LOW) – 7 • R_TH(HIGH))
Where R_TH(LOW) > 7 • R_TH(HIGH)
For PTC thermistors:
R9 = 6 R_TH(LOW) • R_TH(HIGH)/(R_TH(HIGH) – R_TH(LOW))
R9A = 6 R_TH(LOW) • R_TH(HIGH)/(R_TH(HIGH) – 7 • R_TH(LOW))
Where R_TH(HIGH) > 7 • R_TH(LOW)
Example #1: 10kΩ NTC with custom limits
TLOW = 0°C, THIGH = 50°C
R_TH = 10k at 25°C,
R_TH(LOW) = 32.582k at 0°C
R_TH(HIGH) = 3.635k at 50°C
R9 = 24.55k → 24.3k (nearest 1% value)
R9A = 99.6k → 100k (nearest 1% value)
Example #2: 100kΩ NTC
TLOW = 5°C, THIGH = 50°C
R_TH = 100k at 25°C,
R_TH(LOW) = 272.05k at 5°C
R_TH(HIGH) = 33.195k at 50°C
R9 = 226.9k → 226k (nearest 1% value)
R9A = 1.365M → 1.37M (nearest 1% value)
Example #3: 22kΩ PTC
TLOW = 0°C, THIGH = 50°C
R_TH = 22k at 25°C,
R_TH(LOW) = 6.53k at 0°C
R_TH(HIGH) = 61.4k at 50°C
R9 = 43.9k → 44.2k (nearest 1% value)
R9A = 154k
Sizing the Thermistor Hold Capacitor
During the hold interval, C7 must hold the voltage across
the thermistor relatively constant to avoid false readings.
A reasonable amount of ripple on NTC during the hold
interval is about 10mV to 15mV. Therefore, the value of
C7 is given by:
C7 = t_HOLD/(R9/7 • –ln(1 – 8 • 15mV/4.5V))
= 10 • R_RT • 17.5pF/(R9/7 • –ln(1 – 8 • 15mV/4.5V)
Example:
R9 = 24.3k
R_RT = 150k
C7 = 0.28μF → 0.27μF (nearest value)
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