LTC4061 Datasheet, PDF (10/20 Pages) Linear Technology – Standalone Linear Li-lon Battery Charger with Thermistor Input

LTC4061
U
OPERATIO
Undervoltage Lockout (UVLO)
An internal undervoltage lockout circuit monitors the
input voltage and keeps the charger in shutdown mode
until V_CC rises above the undervoltage lockout threshold
(3.8V). The UVLO circuit has a built-in hysteresis of
200mV. Furthermore, to protect against reverse current
in the power MOSFET, the UVLO circuit keeps the charger
in shutdown mode if V_CC falls to less than 45mV above
the battery voltage. Hysteresis of 145mV prevents the
charger from cycling in and out of shutdown.
Manual Shutdown
At any point in the charge cycle, the charger can be put
into shutdown mode by pulling the ^⎯E^⎯N pin high. This
reduces the supply current to less than 50µA and the
battery drain current of the charger to less than 2µA. A
new charge cycle can be initiated by floating the ^⎯E^⎯N pin
or pulling it low.
If shutdown is not required, leaving the pin disconnected
continuously enables the circuit.
Trickle-Charge and Defective Battery Detection
When the BAT pin voltage is below the 2.9V trickle charge
threshold (V_TRIKL), the charger reduces the charge current
to 10% of the programmed value. If the battery remains in
trickle charge for more than 25% of the total programmed
charge time, the charger stops charging and enters a FAULT
state, indicating that the battery is defective¹. The LTC4061
indicates the FAULT state by driving the ⎯C⎯H⎯R⎯G open-drain
output with a square wave. The duty cycle of this oscillation
is 50% and the frequency is set by C_TIMER:
f _CHRG
=
0.1µF
C _TIMER
• 6Hz
A LED driven by the ⎯C⎯H⎯R⎯G output exhibits a pulsing
pattern, indicating to the user that the battery needs
replacing. To exit the FAULT state, the charger must be
restarted either by toggling the ^⎯E^⎯N input or removing and
reapplying power to V_CC.
Charge Status Output (⎯C⎯H⎯R⎯G)
The charge status indicator pin has three states: pull-down,
pulse at 1.5Hz or 6Hz and high impedance. In the pull-down
state, an NMOS transistor pulls down on the ⎯C⎯H⎯R⎯G pin
10
capable of sinking up to 10mA. A pull-down state indicates
that the LTC4061 is charging a battery and the charge cur-
rent is greater than I_DETECT (which is set by the external
component R_DET). A high impedance state indicates that
the charge current has dropped below I_DETECT. In the
case where the IDET pin is left unconnected (R_DET = ∞,
I_DETECT = 0), a high impedance state on ⎯C⎯H⎯R⎯G indicates
that the LTC4061 is not charging.
Smart Pulsing Error Feature
LTC4061 has two different pulsing states at ⎯C⎯H⎯R⎯G pull-
down pin:
1) 6Hz (50% duty cycle) due to defective battery detec-
tion (see Trickle-Charge and Defective Battery Detection
section);
2) 1.5Hz (25% duty cycle if in time termination, 50% duty
cycle if in charge current or user termination) due to NTC
out-of-temperature condition.
NTC Thermistor (NTC)
The temperature of the battery is measured by placing
a negative temperature coefficient (NTC) thermistor
close to the battery pack. The NTC circuitry is shown in
Figure 1. To use this feature, connect the NTC thermistor,
R_NTC, between the NTC pin and ground and a resistor,
R_NOM, from the NTC pin to V_CC. R_NOM should be a 1%
resistor with a value equal to the value of the chosen
NTC thermistor at 25°C (this value is 100kΩ for a Vishay
NTHS0603N01N1003J thermistor). The LTC4061 goes
into hold mode when the resistance, R_HOT, of the NTC
thermistor drops to 0.53 times the value of R_NOM or ap-
proximately 53kΩ, which corresponds to approximately
40°C. Hold mode freezes the timer and stops the charge
cycle until the thermistor indicates a return to a valid tem-
perature. As the temperature drops, the resistance of the
NTC thermistor rises. The LTC4061 is designed to go into
hold mode when the value of the NTC thermistor increases
to 3.26 times the value of R_NOM. This resistance is R_COLD.
For a Vishay NTHS0603N01N1003J thermistor, this value
is 326kΩ, 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.
Grounding the NTC pin disables the NTC function. For more
details refer to the Application Information section.
1 _{The Defective Battery Detection feature is only available when time termination is being used.}
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