LTC3557_15 Datasheet, PDF (12/28 Pages) Linear Technology – USB Power Manager with Li-Ion Charger and Three Step-Down Regulators

LTC3557/LTC3557-1
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 specifically 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 specifications.
The ideal diode from BAT to V_OUT guarantees that ample
power is always available to V_OUT even if there is insufficient
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 configured 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
efficiency 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.
FROM AC ADAPTER (OR HIGH VOLTAGE BUCK OUTPUT)
4.3V
(RISING)
–
3.2V
(FALLING)
+
WALL
3
+
–
26
V_C
OPTIONAL CONTROL
FOR HIGH VOLTAGE BUCK REGS
LT3480, LT3481 OR LT3505
FROM
USB
V_BUS
24
+–
75mV (RISING)
25mV (FALLING)
ENABLE
V_OUT
USB CURRENT LIMIT
CONSTANT CURRENT
CONSTANT VOLTAGE
BATTERY CHARGER
IDEAL
DIODE
+
–
+– _15mV
ACPR
25
V_OUT
23
GATE
21
SYSTEM
LOAD
OPTIONAL
EXTERNAL
IDEAL DIODE
PMOS
12
BAT
Figure 1. Simplified PowerPath Block Diagram
BAT
22
35571 F01
+
Li-Ion
35571fc