LTC4011_15 Datasheet, PDF (17/26 Pages) Linear Technology – High Efficiency Standalone Nickel Battery Charger

LTC4011
Applications Information
is recommended for rejecting PWM switching noise. The
value of C1 should be chosen to yield a 1st order lowpass
frequency of less than 500Hz. In the case of a single cell,
the external application circuit shown in Figure 4 is rec-
ommended to provide the necessary noise filtering and
missing battery detection.
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 present for thermistors
with an exponential temperature coefficient (β) above
3750. For thermistors with β below 3750, replace R3
with a short.
BAT
12
10k
9 ^VCDIV
8 ^VCELL
1 CELL
10k
33nF
4011 F04
Figure 4. Single-Cell Monitor Network
V_{RT 6}
R1 ^R4
51k V_TEMP
R3
7
R_T
R2
C1
10nF
4011 F05
Figure 5. External NTC Thermistor Network
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 = 0Ω
However, the LTC4011 will operate with other NTC therm-
istors having different nominal values or exponential
temperature coefficients. For these thermistors, the design
equations for the resistors in the external network are:
where:
R₀ = thermistor resistance (Ω) at T₀
T₀ = 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
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. Thermistors
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.
Alternatively, 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 100kΩ or less,
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_TEMP pin may simply be wired to
V_RT to disable temperature qualification of all charging
4011fb
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