LTC4121/LTC4121-4.2
15
4121fc
For more information www.linear.com/LTC4121
OPERATION
ture returns to the safe charging range. The safe charging
range is determined by two comparators that monitor the
voltage at the NTC pin. NTC qualified charging is disabled
if the NTC pin is pulled below about 85mV (V
DIS
).
Thermistor manufacturers usually include either a tem-
perature lookup table identified with a characteristic curve
number, or a formula relating temperature to the resistor
value. Each thermistor is also typically designated by a
thermistor gain value B25/85.
The NTC pin should be connected to a voltage divider
from INTV
CC
to GND as shown in Figure 6. In the simple
application (R
ADJ
= 0) a 1% resistor, R
BIAS
, with a value
equal to the resistance of the thermistor at 25°C is con-
nected from INTV
CC
to NTC, and a thermistor is connected
from NTC to GND. With this setup, the LTC4121 pauses
charging when the resistance of the thermistor increases
to 285% the R
BIAS
resistor as the temperature drops. For
a Vishay Curve 2 thermistor with B25/85 = 3490 and 25°C
resistance of 10kΩ, this corresponds to a temperature
of about 0°C. The LTC4121 also pauses charging if the
thermistor resistance decreases to 58.8% of the R
BIAS
resistor. For the same Vishay Curve 2 thermistor, this
corresponds to approximately 40°C. With a Vishay Curve
2 thermistor, the hot and cold comparators both have
about 2°C of hysteresis to prevent oscillations about the
trip points. The NTC comparator trip points are ratio metric
to the INTV
CC
voltage, so NTC trip points are defined as a
percentage of INTV
CC
. The HOT threshold is calculated as
285%/385% = 74% of INTV
CC
and the COLD threshold is
calculated as 58.8%/158% = 37% of INTV
CC
.
The hot and cold trip points may be adjusted using a differ-
ent type of thermistor, or a different R
BIAS
resistor, or by
adding a desensitizing resistor, R
ADJ
, or by a combination
of these measures as shown in Figure 6. For example, by
increasing R
BIAS
to 12.4kΩ, with the same thermistor as
before, the cold trip point moves down to –5°C, and the
hot trip point moves down to 34°C. If a Vishay Curve 1
thermistor with B25/85 = 3964 and resistance of 100kΩ
at 25°C is used, a 1% R
BIAS
resistor of 118kΩ and a 1%
R
ADJ
resistor of 12.1kΩ results in a cold trip point of 0°C,
and a hot trip point of 39°C.
End-of-Charge Indication and Safety Timeout
The LTC4121 uses a safety timer to terminate charging.
Whenever the LTC4121 is in constant current mode the
timer is paused, and when FB rises or falls through the
V
RCHG
threshold the timer is reset. When the battery
voltage reaches the float voltage, the safety timer begins
counting down a 2-hour timeout. If charge current falls
below one tenth of the programmed maximum charge cur
-
rent (h
C/10
), the CHRG status pin rises, but top-off charge
current continues to flow until the timer finishes. After the
timeout, the LTC4121 enters a low-power sleep mode.
Automatic Recharge
In sleep mode, the IC continues to monitor battery voltage.
If the battery falls 2.2% (V
RCHG
or V
RCHG_42
) from the full-
charge float voltage, the LTC4121 engages an automatic
recharge cycle as the safety timer is reset. Automatic
recharge has a built in delay of about 0.5ms to prevent
triggering a new charge cycle if a load transient causes
the battery voltage to drop temporarily.
State of Charge and Fault Status Pins
The LTC4121 contains two open-drain outputs which
provide charge status and signal fault indications. The
CHRG pin pulls low to indicate charging at a rate higher
than C/10. The FAULT pin pulls low to indicate a bad bat-
tery timeout, or to indicate an NTC thermal fault condition.
During NTC faults the CHRG pin remains low, but when
a bad-battery timeout occurs the CHRG pin de-asserts.
When the open drain outputs are pulled up with a resistor,
Table 3 summarizes the charger state that is indicated by
the pin voltages.
Table 3 LTC4121 Open-Drain Indicators with Resistor Pull-Ups
FAULT CHRG CHARGER STATE
High High Off or Topping-Off Charge at a Rate Less Than C/10.
High Low Charging at Rate Higher Than C/10
Low High Bad Battery Fault
Low Low NTC Thermal Fault, Charging Paused
Low Battery Voltage Operation
The LTC4121 automatically preconditions heavily dis-
charged batteries. If the battery voltage is below V
LOWBAT
minus its hysteresis (typically 2.05V - e.g. battery pack
