LTC4088-1/LTC4088-2
12
40881fc
operaTion
Ideal Diode from BAT to V
OUT
The LTC4088-1/LTC4088-2 has an internal ideal diode as
well as a controller for an external ideal diode. Both the
internal and the external ideal diodes are always on and
will respond quickly whenever V
OUT
drops below BAT.
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 power) is removed, then
all of the application power will be provided by the bat-
tery via the ideal diodes. The ideal diodes will be fast
enough to keep V
OUT
from drooping with only the stor-
age 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. An external
P-channel MOSFET transistor should be added from
BAT
to
V
OUT
. The GATE pin of the LTC4088-1/LTC4088-2 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 GATE pin can control an external P-channel
MOSFET transistor having an on-resistance of 30mΩ or
lower. When V
BUS
is unavailable, the forward voltage of
the ideal diode amplifier will be reduced from 15mV to
nearly zero.
Suspend LDO
The LTC4088-1/LTC4088-2 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 4.6V, this LDO only
becomes active when the switching converter is disabled.
To remain compliant 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 current 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.
V
BUS
Undervoltage Lockout (UVLO)
An internal undervoltage lockout circuit monitors V
BUS
and
keeps the switching regulator off until V
BUS
rises above the
rising UVLO threshold (4.3V). If V
BUS
falls below the falling
UVLO threshold (4V), system power at V
OUT
will be drawn
from the battery via the ideal diodes. The voltage at V
BUS
must also be higher than the voltage at BAT by approxi-
mately 170mV for the switching regulator to operate.
Battery Charger
The LTC4088-1/LTC4088-2 includes a constant-current/
constant-voltage battery charger with automatic recharge,
automatic termination by safety timer, low voltage trickle
charging, bad cell detection and thermistor sensor input
for out of temperature charge pausing.
Figure 3. Ideal Diode V-I Characteristics
FORWARD VOLTAGE (mV) (BAT – V
OUT
)
0
CURRENT (mA)
600
1800
2000
2200
120
240
300
408812 F03
200
1400
1000
400
1600
0
1200
800
60
180
360
480420
VISHAY Si2333
EXTERNAL
IDEAL DIODE
LTC4088-1/
LTC4088-2
IDEAL DIODE
ON
SEMICONDUCTOR
MBRM120LT3
V
BUS
= 5V
