Datasheet

16
LTC4011
4011fa
APPLICATIO S I FOR ATIO
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Figure 4. Single-Cell Monitor Network
12
9
BAT
10k
10k
33nF
1 CELL
4011 F04
V
CDIV
8
V
CELL
actual application. An additional high temperature charge
qualification error of between 0°C and 5°C may occur
when using thermistors with β lower than 3750. Ther-
mistors with nominal β less than 3300 should be avoided.
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
TEMP
to GND 100k or less,
including R4, if present.
The LTC4011 is designed to work best with a 5% 10K NTC
thermistor with a β near 3750, such as the Siemens/
EPCOS B57620C103J062. In this case, the values for the
external network are given by:
R1 = 9.76k
R2 = 28k
R3 = 0
However, the LTC4011 will operate with other NTC ther-
mistors having different nominal values or exponential
temperature coefficients. For these thermistors, the de-
sign equations for the resistors in the external network are:
Figure 5. External NTC Thermistor Network
6
7
V
RT
R1
R2R
T
R4
51k
R3
C1
10nF
4011 F05
V
TEMP
Disabling Thermistor Functions
Temperature sensing is optional in LTC4011 applications.
For low cost systems where temperature sensing may not
be required, the V
TEMP
pin may simply be wired to V
RT
to
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
TEMP
to V
RT
if the pause function is
still desired.
INTV
DD
Regulator Output
If BGATE is left open, the INTV
DD
pin of the LTC4011 can
be used as an additional source of regulated voltage in the
host system any time READY is active. Switching loads on
INTV
DD
may reduce the accuracy of internal analog cir-
cuits used to monitor and terminate fast charging. In
addition, DC current drawn from the INTV
DD
pin can
where:
R
0
= thermistor resistance () at T
0
T
0
= thermistor reference temperature (°K)
β = exponential temperature coefficient of resistance
For thermistors with β less than 3750, the equation for R3
yields a negative number. This number should be used to
compute R2, even though R3 is replaced with a short in the