Datasheet

LTC4088
12
4088fb
Ideal Diode from BAT to V
OUT
The LTC4088 has an internal ideal diode as well as a
controller for an optional 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 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. The on-resistance in
this case is approximately 180mΩ. If this is
sufficient for
the
application, then no external components are neces-
sary. However, if more conductance is needed, an external
P-channel MOSFET transistor can be added from BAT to
V
OUT
. The GATE pin of the LTC4088 drives the gate of the
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
operaTion
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 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.
3.3V Always-On Supply
The LTC4088 includes an ultralow quiescent current low
dropout regulator that is always powered. This LDO can
be used to provide power to a system pushbutton control-
ler or standby microcontroller. Designed to deliver up to
25mA, the always-on LDO requires a 1µF MLCC bypass
capacitor for compensation. The LDO is powered from
V
OUT
, and therefore will enter dropout at loads less than
25mA as V
OUT
falls near 3.3V. If the LDO3V3 output is
not used, it should be disabled by connecting it to V
OUT
.
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.
Figure 3. Ideal Diode V-I Characteristics
FORWARD VOLTAGE (mV) (BAT – V
OUT
)
0
CURRENT (mA)
600
1800
2000
2200
120
240
300
4088 F03
200
1400
1000
400
1600
0
1200
800
60
180
360
480420
VISHAY Si2333
OPTIONAL EXTERNAL
IDEAL DIODE
LTC4088
IDEAL DIODE
ON
SEMICONDUCTOR
MBRM120LT3
V
BUS
= 5V