Datasheet

LTC4099
17
4099fd
If the load current increases beyond the power allowed
from the switching regulator, additional power will be pulled
from the battery via the ideal diodes. Furthermore, if power
to V
BUS
(USB or wall adapter) is removed, then all of the
application power will be provided by the battery via the
ideal diodes. The ideal diodes will be fast enough to keep
V
OUT
from drooping with only the storage capacitance
required for the switching regulator. The internal ideal
diode consists of a precision amplifier that activates a
large on-chip MOSFET transistor whenever the voltage at
V
OUT
is approximately 15mV (V
FWD
) below the voltage at
BAT. Within the amplifier’s linear range, the small-signal
resistance of the ideal diode will be quite low, keeping
the forward drop near 15mV. At higher current levels, the
MOSFET will be in full conduction.
Precharge
When a
battery charge cycle begins, the battery charger
first determines if the battery is deeply discharged. If the
battery voltage is below V
TRKL
, typically 2.85V, an automatic
trickle charge feature sets the battery charge current to
one-fifth of the default charge current. If the low 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 unresponsive.
Constant-Current
On
ce the battery voltage is above V
TRKL
, the charger begins
charging in full power constant-current mode. The current
delivered to the battery will try to reach V
PROG
/R
PROG
•
1030 where V
PROG
can be set by the I
2
C port and ranges
from 500mV to 1.2V in 100mV steps. The default value
of V
PROG
is 500mV. 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 current limit program
-
ming will
always be observed and only additional power
will be available to charge the battery. When system loads
are light, battery charge current will be maximized.
As mentioned above, 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 charge current will be given by the following
expression:
I
CHG
=
V
PROG
R
PROG
•1030
Eight values of V
PROG
may be selected by the I
CHARGE2
,
I
CHARGE1
and I
CHARGE0
bits in the I
2
C port. See Table 3.
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
•1030
FORWARD VOLTAGE (mV) (BAT – V
OUT
)
0
CURRENT (mA)
600
1800
2000
2200
120
240
300
4099 F03
200
1400
1000
400
1600
0
1200
800
60
180
360
480420
VISHAY Si2333
EXTERNAL
IDEAL DIODE
LTC4099
IDEAL DIODE
ON
SEMICONDUCTOR
MBRM120LT3
Figure 3. Ideal Diode V-I Characteristics
operaTion
To supplement the internal ideal diode, an external
P-channel MOSFET transistor may be added from BAT to
V
OUT
. The IDGATE pin of the LTC4099 drives the gate of
the external P-channel MOSFET transistor for automatic
ideal diode control. The source of the external P-channel
MOSFET should be connected to V
OUT
and the drain should
be connected to BAT. Capable of driving a 1nF load, the
IDGATE pin can control an external P-channel MOSFET
transistor having an on-resistance of 30mΩ or lower.
Battery Charger
Th
e LTC4099 includes a battery charger with low voltage
precharge, constant-current/constant-voltage charging, C/x
state-of-charge detection, automatic termination by safety
timer, automatic recharge, bad cell detection and thermistor
sensor input for out-of-temperature charge pausing.