Datasheet

LTC4156
30
4156f
Battery Charger Operation
The LTC4156 contains a fully featured constant-current/
constant-voltage LiFePO
4
battery charger with automatic
recharge, bad cell detection, trickle charge, programmable
safety timer, thermistor temperature qualified charging,
programmable end-of-charge indication, programmable
float voltage, programmable charge current, detailed I
2
C
status reporting, and programmable interrupt generation.
Precharge/Low Battery
When a battery charge cycle begins, the battery charger
first determines if the
battery is deeply discharged. If
the battery voltage is below V
LOWBAT
, typically 2.7V, the
LTC4156 will report the LOWBAT condition via I
2
C (see
Table 17). If the low battery voltage persists for more than
one-half hour, the battery charger automatically terminates
and indicates via the I
2
C port that the battery was unre-
sponsive. When the battery voltage is low, charge current
is reduced, both
to protect the battery and to prevent ex-
cessive power dissipation in the external PMOS transistor.
Figure 1 shows the relationship between battery voltage
and charge current reduction. When input power (USB
or WALL) is unavailable, the I
2
C LOWBAT indication will
always be true, independent of the actual state of charge
of the battery and can be disregarded.
Constant-Current
When the battery voltage is above approximately 2.8V
, the
charger will attempt to deliver the programmed charge cur-
rent in constant-current mode. Depending on available input
power and external load conditions, the battery charger
may or may not be able to charge at the full programmed
rate. The external load will always be prioritized over the
battery charge current. Likewise, the USB and WALL input
current limit programming will always be observed and
only
additional power will be available to charge the bat-
tery. When system loads are light, battery charge current
will be maximized.
The upper limit of charge current is programmed by the
combination of a resistor from PROG to ground and the
PROG servo voltage value set in the I
2
C port. The maximum
charge current will be given by the following expression:
I
CHARGE
=
V
PROG
R
PROG
• 1000
V
PROG
can be set by the I
2
C port and ranges from 150mV
to 1.2V in 75mV steps. The default value of V
PROG
is 1.2V.
V
PROG
is controlled by bits ICHARGE[3:0] located at sub
address 0x02. See Table 10.
In either the constant-current or constant-voltage charging
modes, the voltage at the PROG pin will be proportional
to the actual charge current delivered to the battery
. The
charge current can be determined at any time by monitoring
the PROG pin voltage and using the following relationship:
I
BAT
=
V
PROG
R
PROG
• 1000
Recall, however, that in some cases the actual battery
charge current, I
BAT
, will be lower than the programmed
current, I
CHARGE
, due to limited input power available and
prioritization of the system load drawn from V
OUT
. R
PROG
should be set to match the capacity of the battery without
regard to input power limitations.
Constant-Voltage
Once the battery terminal voltage reaches the preset float
voltage, the battery
charger will hold the voltage steady
and the charge current will decrease naturally toward
zero. Four voltage settings are available for final float
voltage selection via the I
2
C port using bits VFLOAT[1:0]
(Table 11).
OPERATION