LTC4055/LTC4055-1
11
4055fb
OPERATION
The LTC4055/LTC4055-1are complete PowerPath control-
lers for battery-powered USB applications. The LTC4055/
LTC4055-1 are designed to provide device power and Li-ion
battery charging from the USB V
BUS
while maintaining
the current limits as specifi ed in the USB specifi cation.
This is accomplished by reducing battery charge current
as output/load current is increased. In this scenario, the
available bus current is maximized in an effort to minimize
battery charge time.
An ideal diode function provides power from the battery
when output/load current exceeds the input current limit
set for the part or when input power is removed. The
advantage to powering the load through the ideal diode
(rather than connecting the load directly to the battery)
is that when the bus is connected and the battery is fully
charged, the battery remains fully charged until bus power
is removed. Once bus power is removed the output drops
until the ideal diode is forward biased. The forward biased
ideal diode will then provide the output power to the load
from the battery.
Another advantage to powering the load from the bus when
the bus is available is in cases where the load is a switching
regulator. The input power to a switching regulator can be
thought of as constant. A higher voltage across a constant
power load will require less current. Less load current in
USB applications means more available charge current.
More charge current translates to shorter charge times.
The LTC4055/LTC4055-1 also have the ability to
accommodate power from a wall adapter. Wall adapter
power can be connected to the output (load side) of the
LTC4055/LTC4055-1 through an external device such as a
power Schottky or FET, as shown in Figure 1. The LTC4055/
LTC4055-1 have the unique ability to use the output, which
is powered by the wall adapter, as an alternate path to
charge the battery while providing power to the load. A
wall adapter comparator on the LTC4055/LTC4055-1 can
be confi gured to detect the presence of the wall adapter
and shut off the connection to the USB to prevent reverse
conduction out to the bus.
–
+
1
+
INPUT CHARGER
CONTROL
ENABLE
IN2
5
WALL
1V
4
IN1
V
BUS
WALL
ADAPTER
3
OUT
2
BAT
4055 F01
UVLO
CURRENT LIMIT
CONTROL
ENABLE
OUTPUT CHARGER
CONTROL
ENABLE
IDEAL
Li-Ion
LOAD
Figure 1. Simplifi ed Block Diagram —PowerPath
