LTC4060_15 Datasheet, PDF (11/20 Pages) Linear Technology – Standalone Linear NiMH/NiCd Fast Battery Charger

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OPERATIO
charging begins at the maximum current set by the
external programming resistor connected between the
PROG pin and GND.
If an external thermistor indicates sensed temperature is
beyond a range of 5°C to 55°C charging is suspended, the
charge timer is paused and the CHRG status output
remains low. Normal charging resumes from the previous
state when the sensed temperature rises above 5°C or falls
below 45°C. Voltage-based termination (–∆V) is then
reset and immediately enabled. If voltage-based termina-
tion was imminent when the temperature limits were
exceeded, charge termination will occur.
Charge Termination
Once fast charge begins and after an initial battery voltage
stabilization hold-off period shown in Table 1, voltage-
based termination (–∆V) is enabled. This period is used to
prevent falsely terminating on a –∆V event that can occur
almost immediately after initiating charging on some
heavily discharged or stored batteries. However, if V_CELL
was measured to be above 1.3V (V_IDT) immediately prior
to the precharge cycle, then a mostly charged battery is
assumed and voltage-based termination (–∆V) is enabled
without delay.
An internal 1.5mV resolution A/D converter measures the
cell voltage after each battery voltage sampling interval
indicated in Table 1. The peak cell voltage is stored and
compared to the current cell voltage. When the cell voltage
has dropped by at least V_MDV (magnitude selected by the
CHEM pin) from the peak for four consecutive battery
voltage sampling intervals, charging is terminated.
Back-up termination is provided by the charge time limiter,
whose time limit is indicated in Table 1, and by a battery
overvoltage detector. Once terminated by back-up termi-
nation, charge qualification can be reinitiated only by remov-
ing and replacing the battery or toggling the SHDN pin low
to high or removing and reapplying power to the charger.
Automatic Recharge
Once charging is complete, the optional programmable
automatic recharge state can be entered. This state, if
LTC4060
enabled, will automatically restart the charger from the
charge qualification state without user intervention when-
ever the battery cell voltage drops below a set level. With
the advent of low memory effect NiMH and improved NiCd
cells an automatic recharge feature is practical and elimi-
nates the need for very slow trickle charging.
The CHRG status output is high impedance in the auto-
matic recharge state until charging begins. If the V_CELL
voltage drops below the voltage set on the ARCT pin for at
least the automatic recharge entry delay time as shown in
Table 1, the charge qualification state is entered and
charging will begin anew in fast charge. An easy way of
setting the voltage on the ARCT pin is by using two series
current programming resistors and connecting their com-
mon to the ARCT pin as shown in Figure 2. The PROG pin
will provide a constant 1.5V (V_PROG). The programmable
voltage range of the ARCT pin is approximately 0.8V to
1.6V. A preprogrammed recharge threshold of 1.3V (V_ARDT)
is selected when the ARCT pin is connected to V_CC
(V_ARDEF). Automatic recharge is disabled when the ARCT
pin is connected to ground (V_ARDIS).
Pause
After charging is initiated, the PAUSE pin may be used to
pause operation at any time. Whenever the voltage on the
PAUSE pin is a logic high, the charge timer and all other
timers pause, charging is stopped and the fast charge ter-
mination algorithm is inhibited. The CHRG status output
remains at GND. If voltage-based termination was immi-
nent before pause, charge termination will occur. Otherwise,
when pause ends, the charge timer and all other timers
resume timing, charging restarts and voltage-based termi-
nation (–∆V) is reset and immediately enabled. If the bat-
tery is removed while the PAUSE pin is a logic high, then
battery removal is detected and shutdown is entered. If the
battery is replaced while the PAUSE pin is a logic high, it
will not be detected until pause is turned off.
For pause periods or a series of periods where the battery
capacity could be significantly depleted, consider using
shutdown instead of pause to avoid having the safety timer
expire before the battery can be fully charged. Shutdown
resets the safety timer.
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