Datasheet
LTC3557/LTC3557-1
12
35571fc
OPERATION
Introduction
The LTC3557/LTC3557-1 is a highly integrated power
management IC that includes a PowerPath controller,
battery charger, an ideal diode, an always-on LDO, three
synchronous step-down switching regulators as well as
a buck regulator V
C
controller. Designed specifi cally for
USB applications, the PowerPath controller incorporates
a precision input current limit which communicates with
the battery charger to ensure that input current never
violates the USB specifi cations.
The ideal diode from BAT to V
OUT
guarantees that ample
power is always available to V
OUT
even if there is insuffi cient
or absent power at V
BUS
.
The LTC3557/LTC3557-1 also has the ability to receive
power from a wall adapter or other non-current-limited
power source. Such a power supply can be connected
to the V
OUT
pin of the LTC3557/LTC3557-1 through an
external device such as a power Schottky or FET as shown
in Figure 1.
The LTC3557/LTC3557-1 has the unique ability to use
the output, which is powered by an external supply, to
charge the battery while providing power to the load. A
comparator on the WALL pin is confi gured to detect the
presence of the wall adapter and shut off the connection
to the USB. This prevents reverse conduction from V
OUT
to V
BUS
when a wall adapter is present.
The LTC3557/LTC3557-1 provides a V
C
output pin which
can be used to drive the V
C
pin of an external high voltage
buck switching regulator such as the LT3480, LT3481, or
LT3505 to provide power to the V
OUT
pin. The V
C
control
circuitry adjusts the regulation point of the switching
regulator to a small voltage above the BAT pin voltage.
This control method provides a high input voltage, high
effi ciency battery charger and PowerPath function.
An “always on” LDO provides a regulated 3.3V from V
OUT
.
This LDO will be on at all times and can be used to supply
up to 25mA.
Figure 1. Simplifi ed PowerPath Block Diagram
+
–
+
–
+
–
+
–
4.3V
(RISING)
3.2V
(FALLING)
75mV (RISING)
25mV (FALLING)
ENABLE
USB CURRENT LIMIT
CONSTANT CURRENT
CONSTANT VOLTAGE
BATTERY CHARGER
WALL
FROM AC ADAPTER (OR HIGH VOLTAGE BUCK OUTPUT)
3
V
BUS
FROM
USB
24
21
+
–
15mV
GATE
BAT
35571 F01
Li-Ion
OPTIONAL
EXTERNAL
IDEAL DIODE
PMOS
BAT
IDEAL
DIODE
22
23
V
OUT
V
OUT
V
C
OPTIONAL CONTROL
FOR HIGH VOLTAGE BUCK REGS
LT3480, LT3481 OR LT3505
25
ACPR
26
+
SYSTEM
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