LTC4061
10
4061fd
operaTion
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 EN 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 EN pin or pulling it low.
If shutdown is not required, leaving the pin disconnected
continuously enables the circuit.
T
rickle-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 defective
1
. The LTC4061
indicates the FAULT state by driving the CHRG open-drain
output with a square wave. The duty cycle of this oscillation
is 50% and the frequency is set by C
TIMER
:
f
µF
C
Hz
CHRG
TIMER
=
0 1
6
.
•
A LED driven by the CHRG 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 EN input or removing and reapplying
power to V
CC
.
Charge Status Output (CHRG)
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 CHRG pin
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 I
DET
pin is left unconnected (R
DET
= ∞,
I
DETECT
= 0), a high impedance state on CHRG indicates
that the LTC4061 is not charging.
Smart
Pulsing Error Feature
LTC4061 has two different pulsing states at CHRG pull-
down pin:
1. 6Hz
(50% duty cycle) due to defective battery detection
(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 NTH-
S0603N01N1003J 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 approximately
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 temperature. 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.
