LTC4053-4.2_15 Datasheet, PDF (11/16 Pages) Linear Technology – USB Compatible Lithium-Ion Battery Charger with Thermal Regulation

LTC4053-4.2
APPLICATIO S I FOR ATIO
V⁺
V_DD
8
V_CC
LTC4053
3
CHRG
400k
2k
µPROCESSOR
OUT
IN
4053 F03
Figure 3. Microprocessor Interface
V_CC
R_HOT
1%
7/8 V_CC
–
+
NTC
R_NTC
10k
1/2 V_CC
+
–
TOO COLD
TOO HOT
full-scale current (C/10), the N-channel MOSFET is turned
off and a 30µA current source is connected to the CHRG
pin. The IN pin will then be pulled high by the 2k pull-up.
By forcing the OUT pin to a high impedance state, the
current source will pull the pin low through the 400k
resistor. When the internal timer has expired, the CHRG
pin will assume a high impedance state and the 400k
resistor will then pull the pin high to indicate that charging
has terminated.
NTC Thermistor
The battery temperature is measured by placing a negative
temperature coefficient (NTC) thermistor close to the
battery pack. The NTC circuitry is shown in Figure 4. To use
this feature, connect a 10k NTC thermistor between the
NTC pin and ground and a resistor (R_HOT) from the NTC pin
to V_CC. R_HOT should be a 1% resistor with a value equal to
the value of the chosen NTC thermistor at 50°C (this value
is 4.1k for a Vishay NTHS0603N02N1002J thermistor).
The LTC4053 goes into hold mode when the resistance of
the NTC thermistor drops below 4.1k which should be
approximately 50°C. The hold mode freezes the timer and
stops the charge cycle until the thermistor indicates a
return to a valid temperature. As the temperature drops,
the resistance of the NTC thermistor rises. The LTC4053
is designed to go into hold mode when the value of the NTC
thermistor increases to seven times the value of R_HOT. For
a Vishay NTHS0603N02N1002J thermistor, this value is
28.7k which corresponds to approximately 0°C. The hot
and cold comparators each have approximately 2°C of
hysteresis to prevent oscillation about the trip point. The
NTC function can be disabled by grounding the NTC pin.
3/160 V_CC
+
–
LTC4053
Figure 4
DISABLE NTC
4053 F04
Thermistors
The LTC4053 NTC trip points were designed to work with
thermistors whose resistance-temperature characteris-
tics follow Vishay Dale’s “R-T Curve 2”. The Vishay
NTHS0603N02N1002J is an example of such a ther-
mistor. However, Vishay Dale has many thermistor prod-
ucts that follow the “R-T Curve 2” characteristic in a variety
of sizes. Futhermore, any thermistor whose ratio of R_COLD
to R_HOT is about 7.0 will also work (Vishay Dale R-T Curve
2 shows a ratio of R_COLD to R_HOT of 2.816/0.4086 = 6.9).
NTC Layout Considerations
It is important that the NTC thermistor not be in close
thermal contact with the LTC4053. Because the LTC4053
package can reach temperatures in excess of the 50°C trip
point, the NTC function can cause a hysteretic oscillation
which turns the charge current on and off according to the
package temperature rather than the battery temperature.
This problem can be eliminated by thermally coupling the
NTC thermistor to the battery and not to the LTC4053.
Furthermore, it is essential that the V_CC connection to
R_HOT is made according to standard Kelvin sense tech-
niques. Since V_CC is a high current path into the LTC4053,
it is essential to minimize voltage drops between the V_CC
input pin and the top of R_HOT.
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