Datasheet

16
LTC4006
4006fa
you, then simply set R9 = R
TH(LOW)
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 11 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)
> 7R
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
APPLICATIO S I FOR ATIO
WUUU
Figure 10. Voltage Divider Thermistor Network Figure 11. General Thermistor Network
LTC4006
NTC
R9
C7 R
TH
4006 F10
6
LTC4006
NTC
R9
C7 R9A R
TH
4006 F11
6
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
= 309k (~2 hour timer)
C7 = 0.57µF → 0.56µF (nearest value)
Disabling the Thermistor Function
If the thermistor is not needed, connecting a resistor
between DCIN and NTC will disable it. The resistor should
be sized to provide at least 10µA with the minimum voltage
applied to DCIN and 10V at NTC. Do not exceed 30µA into
NTC. Generally, a 301k resistor will work for DCIN less
than 15V. A 499k resistor is recommended for DCIN
between 15V and 24V.
Optional Simple Battery Discharge Path Circuit
It is NOT recommended that one permit battery current to
flow backwards through R
SENSE
, inductor and out the
TGATE MOSFET internal diode to reach V
OUT
. The TGATE
MOSFET is off when V
IN
< V
BAT
. Figure 12 shows an op-
tional high efficiency discharge path for the battery such that
V
OUT
power comes from lossless “diode or” of V
IN
and V
BAT
.
Normally when V
IN
> V
BAT
, P-channel MOSFET Q1B V
GS
=