LT3650-8.2/LT3650-8.4
15
36508284fd
Status Pins
The LT3650 reports charger status through two open-
collector outputs, the CHRG and FAULT pins. These pins
can accept voltages as high as V
IN
, and can sink up to
10mA when enabled.
The CHRG pin indicates that the charger is delivering cur-
rent at greater than a C/10 rate, or one-tenth of the pro-
grammed maximum charge current. The FAULT pin signals
bad-battery and NTC faults. These pins are binary coded,
and signal following the table below, where On indicates
the pin pulled low, and Off indicates pin high impedance:
voltage is achieved, the charge current falls until the C/10
threshold is reached, at which time the charger terminates
and the LT3650 enters standby mode. The CHRG status
pin follows the charger cycle and is high impedance when
the charger is not actively charging.
When V
BAT
drops below 97.5% of the full-charged float
voltage, whether by battery loading or replacement of the
battery, the charger automatically re-engages and starts
charging.
There is no provision for bad-battery detection if C/10
termination is used.
Timer Termination
The LT3650 supports a timer-based termination scheme, in
which a battery charge cycle is terminated after a
specific
amount
of time elapses. Timer termination is engaged
when a capacitor (C
TIMER
) is connected from the TIMER
pin to ground. The timer cycle EOC (t
EOC
) occurs based
on C
TIMER
following the relation:
C
TIMER
= t
EOC
• 2.27 • 10
–7
(Hours)
Timer EOC is typically set to three hours, which requires
a 0.68µF capacitor.
The CHRG status pin continues to signal charging at a C/10
rate, regardless of which termination scheme is used. When
timer termination is used, the CHRG status pin is pulled
low during a charging cycle until the charger output cur-
rent falls below the C/10 threshold. The charger continues
to top off the battery until timer EOC, when the LT3650
terminates the charging cycle and enters standby mode.
Termination at the end of the timer cycle only occurs if
the charging cycle was successful. A successful charge
cycle occurs when the battery is charged to within 2.5%
of the full-charge float voltage. If a charge cycle is not
successful at EOC, the timer cycle resets and charging
continues for another full timer cycle.
When V
BAT
drops below 97.5% of the full-charge float
voltage, whether by battery loading or replacement of
the battery, the charger automatically re-engages and
starts charging.
applicaTions inForMaTion
Table 1. Status Pins State Table
STATUS PINS STATE
CHARGER STATUSCHRG FAULT
Off Off Not Charging—Standby or Shutdown Mode
Off On Bad-Battery Fault
(Precondition Timeout/EOC Failure)
On Off Normal Charging at C/10 or Greater
On On NTC Fault (Pause)
If the battery is removed from an LT3650 charger that is
configured for C/10 termination, a sawtooth waveform
of approximately 100mV appears at the charger output,
due to cycling between termination and recharge events.
This cycling results in pulsing at the CHRG output. An
LED connected to this pin will exhibit a blinking pattern,
indicating to the user that a battery is not present. The
frequency of this blinking pattern is dependent on the
output capacitance.
C/10 Termination
The LT3650 supports a low current based termination
scheme, where a battery charge cycle terminates when
the current output from the charger falls to below one-
tenth the maximum current, as programmed with R
SENSE
.
The C/10 threshold current corresponds to 10mV across
R
SENSE
. This termination mode is engaged by shorting
the TIMER pin to ground.
When C/10 termination is used, an LT3650 charger sources
battery charge current as long as the average current level
remains above the C/10 threshold. As the full-charge float
