LTC4000
22
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For more information
applicaTions inForMaTion
The value of R3 and R
D
can now be set according to the
following formula:
R3
=
R
NTC
at cold_ threshold – R
NTC
at hot _ threshold
2.461
R
D
=
0.219 • R
NTC
at cold_ threshold –
1.219 • R
NTC
at hot _ threshold
Note the important caveat that this method can only be
used to desensitize the thermal effect on the thermistor
and hence push the hot and cold temperature thresholds
apart from each other. When using the formulas above,
if the user finds that a negative value is needed for R
D
,
the two temperature thresholds selected are too close to
each other and a higher sensitivity thermistor is needed.
For example, this method can be used to set the hot
and cold thresholds independently to 60°C and –5°C.
Using a Vishay Curve 2 thermistor whose nominal value
at 25°C is 100k, the formula results in R3 = 130k and
R
D
= 41.2k for the closest 1% resistors values.
To increase thermal sensitivity such that the valid charging
temperature band is much smaller than 40°C, it is pos-
sible to put a PTC (positive thermal coefficient) resistor
in series with R3 between the BIAS pin and the NTC pin.
This PTC resistor also needs to be thermally coupled with
the battery. Note that this method increases the number of
thermal sensing connections to the battery pack from one
wire to three wires. The exact value of the nominal PTC
resistor required can be calculated using a similar method
as described above, keeping in mind that the threshold at
the NTC pin is always 75% and 35% of V
BIAS
.
Leaving the NTC pin floating or connecting it to a capacitor
disables all NTC functionality.
The
FLT
and
CHRG
Indicator Pins
The
FLT
and
CHRG
pins in the LTC4000 provide status
indicators. Table 2 summarizes the mapping of the pin
states to the part status.
Table 2.
FLT
and
CHRG
Status Indicator
FLT
CHRG
STATUS
0
0
NTC Over Ranged – Charging Paused
1
0
Charging Normally
0
1
Charging Terminated and Bad Battery Detected
1
1
V
IBMON
< (V
C/X
– 10mV)
where 1 indicates a high impedance state and 0 indicates
a low impedance pull-down state.
Note that V
IBMON
< (V
CX
– 10mV) corresponds to charge
termination only if the C/X termination is selected. If the
charger timer termination is selected, constant voltage
charging may continue for the remaining charger timer
period even after the indicator pins indicate that V
IBMON
< (V
CX
– 10mV). This is also true when no termination is
selected, constant voltage charging will continue even after
the indicator pins indicate that V
IBMON
< (V
CX
– 10mV).
The BIAS Pin
For ease of use the LTC4000 provides a low dropout volt-
age regulator output on the BIAS pin. Designed to provide
up to 0.5mA of current at 2.9V, this pin requires at least
470nF of low ESR bypass capacitance for stability.
Use the BIAS pin as the pull-up source for the NTC resis-
tor networks, since the internal reference for the NTC
circuitry is based on a ratio of the voltage on the BIAS
pin. Furthermore, various 100k pull-up resistors can be
conveniently connected to the BIAS pin.
