LTC3555 Datasheet, PDF (16/32 Pages) Linear Technology – High Effi ciency USB Power Manager + Triple Step-Down DC/DC

LTC3555/LTC3555-X
OPERATION
If the load current increases beyond the power allowed
from the switching regulator, additional power will be
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
2200
2000
1800
VISHAY Si2333
OPTIONAL EXTERNAL
IDEAL DIODE
1600
1400
1200
1000
LTC3555
IDEAL DIODE
800
600
ON
400
SEMICONDUCTOR
MBRM120LT3
200
0
0 60 120 180 240 300 360 420 480
FORWARD VOLTAGE (mV) (BAT – V_OUT)
3555 F02
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
TO USB
OR WALL
ADAPTER
16
V_BUS
25
SW
26
I_SWITCH/
h_CLPROG
PWM AND
GATE DRIVE
CONSTANT CURRENT
CONSTANT VOLTAGE
BATTERY CHARGER
IDEAL
DIODE
+
–
15mV ^+–
CLPROG –
2
+
1.188V
AVERAGE INPUT
CURRENT LIMIT
CONTROLLER
0.3V
+–
3.6V
AVERAGE OUTPUT
VOLTAGE LIMIT
CONTROLLER
Figure 3. PowerPath Block Diagram
V_OUT
24
GATE
22
BAT
23
3555 F03
3.5V TO
(BAT + 0.3V)
TO SYSTEM
LOAD
OPTIONAL
EXTERNAL
IDEAL DIODE
PMOS
+ _{SINGLE CELL}
Li-Ion
3555fd