10
LTC4050
4050f
If a PNP transistor is chosen as the pass transistor, a
1000pF capacitor is required from the DRV pin to V
CC
. This
capacitor is needed to help stabilize the voltage loop. A
10µF capacitor at the BAT pin is also recommended when
a battery is not present.
V
CC
Bypass Capacitor
Many types of capacitors can be used for input bypassing.
However, caution must be exercised when using multi-
layer ceramic capacitors. Because of the self resonant and
high Q characteristics of some types of ceramic capaci-
tors, high voltage transients can be generated under some
start-up conditions, such as connecting the charger input
to a hot power source. These transients can be minimized
by using X5R dielectric capacitors and/or by adding a 1.5Ω
resistor in series with the ceramic input capacitor. For
more information, refer to Application Note 88.
Thermistor Interface
A thermistor connected to the NTC (negative temperature
coefficient) pin can be used to sense the battery tempera-
ture to determine if the battery is within an acceptable
temperature range for charging (between 0°C and 50°C).
A Dale (curve 2) 10k thermistor is recommended although
many other types of thermistors can also be used. For
example, a BetaCHIP (curve 7) 10k thermistor or other 10k
thermistors with a room temperature beta of approxi-
mately 3400 will work well. The thermistor is connected
from NTC (pin 2) to ground and is biased up by an internal
28.6k trimmed thin film resistor that connects to V
CC
through a P-channel MOSFET. This MOSFET also biases
an internal resistor string to ground, from which voltage
thresholds of approximately V
CC
/2 and V
CC
/8 are derived.
The NTC pin is compared to these thresholds by two
comparators that have wired-OR outputs. The thresholds
are selected such that an overtemperature condition will
occur when the thermistor resistance is less than approxi-
mately 4.1k and undertemperature condition will occur
when the thermistor resistance is greater than approxi-
mately 28.5k. These correspond to thermistor tempera-
tures of 50°C and 0°C for the specific type of thermistor
listed above (many others will be close enough for most
purposes). The MOSFET is turned off during undervoltage
conditions, preventing the dividers that are biased from it
from drawing current from the battery when input power
is removed. The drop across the MOSFET is common to
both resistor dividers and does not cause any loss of
accuracy in the circuit. The comparators have approxi-
mately 10mV of hysteresis to prevent oscillations around
the trip points.
When an undertemperature or overtemperature condition
is sensed, the current amplifier pulldown is disabled and
DRV is pulled high, the timer is placed in a hold condition
with the count frozen until the battery temperature is
within an acceptable range. The end-of-charge compara-
tor is also disabled to prevent a premature end of charge
signal due to the lack of battery charging current.
APPLICATIONS INFORMATION
WUU
U
–
+
–
+
V
CC
UV
28.6k
4050 AI
10k DALE
CURVE 2
NTC
THERMISTOR
TBAD
(TO CA,
EOC, TIMER)
2
NTC
NTC Interface Circuitry
