LTC4011 Linear Technology, LTC4011 Datasheet - Page 15

• Manufacturer Part Number: LTC4011
• Description: High Efficiency Standalone Nickel Battery Charger
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
Thermistor Network Design
The network for proper temperature sensing using a
thermistor with a negative temperature coefficient (NTC)
is shown in Figure 5. R3 is only required for thermistors
with a room temperature value above 10k. For thermistors
with a room temperature value of 10k or less, replace R3
with a short.
The LTC4011 is designed to work best with a 1% 10k NTC
thermistor with a β of 3750. In this case, the values for the
external network are given by:
However, the LTC4011 will operate with other NTC ther-
mistors having different nominal values or exponential
temperature coefficients. For these thermistors, the gen-
eral-purpose design equations for the passive resistors in
the external linearization network are as follows:
where:
4
R1 = 10.7k
R2 = 30.9k
R3 = 0Ω
R
T
β = Thermistor exponential temperature coefficient of
O
O
= Thermistor reference temperature (°K)
= Thermistor (R
resistance
Figure 4. Single-Cell Monitor Network
12
9
8
U
BAT
V
V
CDIV
CELL
T
) value at T
R
R
R
U
10k
1
2
3
=
=
=
1 164
11 46 2
0 141
10k
4011 F04
R
.
.
33nF
O
.
O
(
W
1 CELL
β
(Ω)
R
β
R
–
O
β
+
2
(
β
T
2
O
β R1
–
T
)
O
2
T
U
O
R
)
O
(
R
–
1
(
+
R R
1 2
R T
2
)
+
O
The filter formed by R4 and C1 in Figure 5 is optional but
recommended for rejecting PWM switching noise. Alter-
natively, R4 may be replaced by a short, and a value
chosen for C1 which will provide adequate filtering from
the Thevenin impedance of the remaining thermistor net-
work. The filter pole frequency, which should be less than
500Hz, will vary more with battery temperature without
R4. External components should be chosen to make the
Thevenin impedance from V
including R4, if present.
Disabling Thermistor Functions
Temperature sensing is optional in LTC4011 applications.
For low cost systems where temperature sensing may not
be required, the V
disable temperature qualification of all charging opera-
tions. However, this practice is not recommended for
NiMH cells charged well above or below their 1C rate,
because fast charge termination based solely on voltage
inflection may not be adequate to protect the battery from
a severe overcharge. A resistor between 10k and 20k may
be used to connect V
still desired.
R R
1
O
+
R R T
Figure 5. External NTC Thermistor Network
2
O O
)
TEMP
R
R3
T
,
if R
TEMP
pin may simply be wired to V
O
R1
R2
<
to V
51k
R4
TEMP
10
RT
k
V
4011 F05
TEMP
to GND 100kΩ or less,
if the pause function is
C1
10nF
V
RT
6
7
LTC4011
15
RT
4011p
to