LTC4061 Datasheet, PDF (12/20 Pages) Linear Technology – Standalone Linear Li-lon Battery Charger with Thermistor Input

LTC4061
APPLICATIO S I FOR ATIO
When the programmed time has elapsed, the charge
cycle terminates and the charger enters standby mode.
Subsequent recharge cycles terminate when 50% of the
programmed time has elapsed. The I_DET pin determines
the behavior of the ^⎯C^⎯H^⎯R^⎯G output. Connecting a resistor
(R_DET) from the I_DET pin to ground sets the charge current
detection threshold, I_DETECT:
I_DETECT
=
R_PROG
10R_DET
• I_CHG
=
100V
R_DET
or
R_DET
=
100V
I_DETECT
When the charge current (I_BAT) is greater than
I_DETECT, the ^⎯C^⎯H^⎯R^⎯G output is in its pull-down state. When
the charger enters constant voltage mode operation and
the charge current falls below I_DETECT, the ^⎯C^⎯H^⎯R^⎯G output
becomes high impedance, indicating that the battery is
almost fully charged. The ^⎯C^⎯H^⎯R^⎯G output will also become
high impedance once the charge time elapses. If the I_DET
pin is not connected, the ^⎯C^⎯H^⎯R^⎯G output remains in its pull-
down state until the charge time elapses and terminates
the charge cycle.
Figure 2 shows a charger circuit using charge time termi-
nation that is programmed to charge at 500mA. Once the
charge current drops below 100mA in constant voltage
mode (as set by R_DET), the ^⎯C^⎯H^⎯R^⎯G output turns off the
LED. This indicates to the user that the battery is almost
fully charged and ready to use. The LTC4061 continues
to charge the battery until the internal timer reaches 3
hours (as set by C_TIMER). During recharge cycles, the
LTC4061 charges the battery until the internal timer reaches
V_IN
R_PROG
2k
R_DET
1k
V_CC
BAT
C/5 LTC4061
CHRG
PROG TIMER
I_DET
GND
500mA
+
C_TIMER
0.1µF
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Figure 2. Time Termination Mode.
The Charge Cycle Ends After 3 Hours.
1.5 hours. Figure 3 describes the operation of the LTC4061
charger when charge time termination is used.
Charge Current Termination
Connecting the TIMER pin to ground selects charge cur-
rent termination. With this method, the timer is disabled
and a resistor (R_DET) must be connected from the I_DET
pin to ground. I_DETECT is programmed using the same
equation stated in the previous section. The charge cycle
terminates when the charge current falls below I_DETECT.
This condition is detected using an internal filtered
comparator to monitor the I_DET pin. When the I_DET pin
falls below 100mV for longer than t_TERM (typically 1ms),
charging is terminated.
When charging, transient loads on the BAT pin can cause
the I_DET pin to fall below 100mV for short periods of time
before the DC current has dropped below the I_DETECT
threshold. The 1.5ms filter time (t_TERM) on the internal
comparator ensures that transient loads of this nature do
not result in premature charge cycle termination. Once the
average charge current drops below I_DETECT, the charger
terminates the charge cycle.
The ^⎯C^⎯H^⎯R^⎯G output is in a pull-down state while charging
and in a high impedance state once charging has stopped.
Figure 4 describes the operation of the LTC4061 charger
when charge current termination is used.
User-Selectable Charge Termination
Connecting the TIMER pin to V_CC selects user-selectable
charge termination, in which all of the internal termination
features are disabled. The charge cycle continues indefi-
nitely until the charger is shut down through the ^⎯E^⎯N pin.
The I_DET pin programs the behavior of the ^⎯C^⎯H^⎯R^⎯G output in
the same manner as when using charge time termination.
If the I_DET pin is not connected, the ^⎯C^⎯H^⎯R^⎯G output remains
in its pull-down state until the charger is shut down.
With user-selectable charge termination, the SmartStart
feature is disabled; when the charger is powered on or
enabled, the LTC4061 automatically begins charging,
regardless of the battery voltage. Figure 5 describes
charger operation when user-selectable charge termina-
tion is used.
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