Datasheet

LTC3555/LTC3555-X
16
3555fd
pulled from the battery via the ideal diode. 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 diode. The transition from input power to
battery power at V
OUT
will be quick enough to allow only
the 10µF capacitor to keep V
OUT
from drooping. The ideal
diode consists of a precision amplifier that enables a large
on-chip P-channel MOSFET transistor whenever the voltage
at V
OUT
is approximately 15mV (V
FWD
) below the voltage
at BAT. The resistance of the internal ideal diode is approxi-
mately 180mΩ. If this is sufficient for the application, then
no external components are necessary. However, if more
conductance is needed, an external P-channel MOSFET
transistor can be added from BAT to V
OUT
.
When an external P-channel MOSFET transistor is pres-
ent, the GATE pin of the LTC3555 family drives its gate 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 40mΩ or
lower.
Suspend LDO
If the LTC3555 family is configured for USB suspend
mode, the switching regulator is disabled and the suspend
LDO provides power to the V
OUT
pin (presuming there is
power available to V
BUS
). This LDO will prevent the bat-
tery 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 (suspended). To remain compliant with the
USB specification, the input to the LDO is current limited
so that it will not exceed the 500µA low power suspend
specification. If the load on V
OUT
exceeds the suspend
current limit, the additional current will come from the
OPERATION
Figure 3. PowerPath Block Diagram
FORWARD VOLTAGE (mV) (BAT – V
OUT
)
0
CURRENT (mA)
600
1800
2000
2200
120
240
300
3555 F02
200
1400
1000
400
1600
0
1200
800
60
180
360
480420
VISHAY Si2333
OPTIONAL EXTERNAL
IDEAL DIODE
LTC3555
IDEAL DIODE
ON
SEMICONDUCTOR
MBRM120LT3
+
–
+
+
–
0.3V
1.188V 3.6V
CLPROG
I
SWITCH
/
h
CLPROG
+
–
+
–
15mV
IDEAL
DIODE
PWM AND
GATE DRIVE
AVERAGE INPUT
CURRENT LIMIT
CONTROLLER
AVERAGE OUTPUT
VOLTAGE LIMIT
CONTROLLER
CONSTANT CURRENT
CONSTANT VOLTAGE
BATTERY CHARGER
+
–
2
GATE
22
V
OUT
24
SW
3.5V TO
(BAT + 0.3V)
TO SYSTEM
LOAD
OPTIONAL
EXTERNAL
IDEAL DIODE
PMOS
SINGLE CELL
Li-Ion
3555 F03
26
BAT
23
V
BUS
TO USB
OR WALL
ADAPTER
25
+