Datasheet

LTC4098-3.6
17
409836f
transistor may be added from BAT to V
OUT
. The IDGATE pin
of the LTC4098-3.6 drives the gate of the external P-chan-
nel 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. Figure 5 shows the
decreased forward voltage compared to a conventional
Schottky diode.
Suspend LDO
The LTC4098-3.6 provides a small amount of power to V
OUT
in suspend mode by including an LDO from V
BUS
to V
OUT
.
This LDO will prevent the battery from running down when
the portable product has access to a suspended USB port.
Regulating at 3.8V, this LDO only becomes active when the
switching converter is disabled. In accordance with the
USB specification, the input to the LDO is current limited so
that it will not exceed the low power or high power suspend
specification. If the load on V
OUT
exceeds the suspend cur-
rent limit, the additional current will come from the battery
via the ideal diodes. The suspend LDO sends a scaled copy
of the V
BUS
current to the CLPROG pin, which will servo to
approximately 100mV in this mode. Thus, the high power
and low power suspend settings are related to the levels
programmed by the same resistor for 1x and 5x modes.
Battery Charger
The LTC4098-3.6 includes a constant-current/constant-
voltage battery charger with automatic recharge, automatic
termination by safety timer and thermistor sensor input
for out-of-temperature charge pausing.
The charger begins charging in full power constant-cur-
rent mode. The current delivered to the battery will try to
reach 1030V/R
PROG
. 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 bat-
tery charge current. The USB current limit programming
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.
Charge Termination
The battery charger has a built-in safety timer. Once the
voltage on the battery reaches the preprogrammed float
voltage of 3.600V, the charger will regulate the battery volt-
age there and the charge current will decrease naturally.
Once the charger detects that the battery has reached
3.600V, the 1-hour safety timer is started. After the safety
timer expires, charging of the battery will discontinue and
no more current will be delivered.
Automatic Recharge
Once the battery charger terminates, it will remain off
drawing only microamperes of current from the battery.
If the portable product remains in this state long enough,
the battery will eventually self discharge. To ensure that
the battery is always topped off, a charge cycle will au-
tomatically begin when the battery voltage falls below
V
RECHRG
(typically 3.5V). In the event that the safety timer
is running when the battery voltage falls below V
RECHRG
, it
will reset back to zero. To prevent brief excursions below
V
RECHRG
from resetting the safety timer, the battery voltage
must be below V
RECHRG
for more than 1.5ms. The charge
cycle and safety timer will also restart if the V
BUS
UVLO
cycles low and then high (e.g., V
BUS
is removed and then
replaced) or if the charger is momentarily disabled using
the D2 pin.
Charge Current
The charge current is programmed using a single resistor
from PROG to ground. 1/1030th of the battery charge cur-
rent is delivered to PROG, which will attempt to servo to
1.000V. Thus, the battery charge current will try to reach
1030 times the current in the PROG pin. The program
resistor and the charge current are calculated using the
following equations:
R
V
I
I
V
R
PROG
CHG
CHG
PROG
==
1030 1030
,
In either the constant-current or constant-voltage charg-
ing modes, the voltage at the PROG pin will be propor-
tional to the actual charge current delivered to the
battery. The charge current can be determined at any time
OPERATION