LTC4052-4.2_15 Datasheet, PDF (7/12 Pages) Linear Technology – Lithium-Ion Battery Pulse Charger with Overcurrent Protection

LTC4052-4.2
APPLICATIO S I FOR ATIO
Input Voltage (Wall Adapter)
The input voltage to the LTC4052 must have some
method of current limit capability. The current limit
level of the input power source must be lower than
the overcurrent limit (I_MAX) set by the sense resistor
I_MAX = 105mV/(R_SENSE + 10mΩ). The 10mΩ represents
bond wire resistance internal to the IC. If a wall adapter
without current limit is used, or the current limit level is
above I_MAX, the charger will turn on briefly and then
immediately turn off when the overcurrent condition is
detected. This cycle will be resumed every 640ms (C_TIMER
= 0.1µF) until the total charge time has run out. If overcurrent
protection is not needed, short the SENSE pin to V_IN. The
overcurrent limit selected should be approximately twice
the charge current (input supply current limit).
Trickle Charge and Defective Battery Detection
At the beginning of the charge cycle, if the cell voltage is
low (less than 2.45V) the charger goes into a 24mA trickle
charge mode. If the low cell voltage persists for one
quarter of the total charge time, the battery is considered
defective and the charge cycle is terminated. The CHRG
pin output is then forced to a high impedance state.
Battery Charge Current
The battery charge current is determined by the current
limit of the input supply (wall adapter). However, this
current must not exceed the maximum charge overcurrent,
I_MAX. If an overcurrent condition is detected, the internal
switch immediately turns off, the GATE pin is pulled to
ground and the charge pump turns off. The charging will
resume after a 640ms time off (C_TIMER = 0.1µF).
Programming the Timer
The programmable timer is used to terminate the charge
cycle and set the minimum ON/OFF time and the overcurrent
time-off period. The length of the timer is programmed by
an external capacitor from the TIMER pin to ground. The
total charge time is:
Time (Hours) = (3 Hours)(C_TIMER/0.1µF) or
C_TIMER = 0.1µF • Time (Hours)/3 (Hours)
The timer starts when an input voltage which is at least
40mV greater than V_BAT is applied. After a time-out has
occurred, the charge cycle stops and the CHRG pin be-
comes high impedance.
CHRG Status Output Pin
This open-drain output can report three different charger
conditions:
Charger Status
Not Charging
Charging
Charging, C/10 Reached
CHRG Pin Behavior
High Impedance
Strong Pulldown
Weak 40µA Pulldown
Using a simple two-resistor network a microprocessor
can distinguish all three states. See Figure 1.
V_IN
3
V_IN
LTC4052
6
CHRG
620k
2k
V_DD
MICROPROCESSOR
OUT
IN
4052 F01
Figure 1. Interfacing with Microprocessor
When the LTC4052 is charging a battery, an internal N-
MOSFET pulls the CHRG pin to ground. When C/10 is
reached, the strong NMOS pulldown is replaced by a weak
40µA current source pulldown. When the LTC4052 is not
charging a battery, the CHRG pin is high impedance.
Using the network in Figure 1, the microprocessor can
determine charger state using the following procedure:
1. Force the digital output pin, OUT, high and read the logic
value at the digital input, IN. If IN reads low, a charger cycle
is in progress.
2. If the IN pin is high, force the OUT pin to be high
impedance, then read the logic level at the IN pin again. If
IN is low, the charge cycle is still in progress, the timer is
still running, but the charge current has dropped below
10% of the programmed value indicating that the charge
cycle is nearly complete.
3. If the logic level at the IN pin is high, the charge cycle has
ended.
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