Datasheet

LTC4088
10
4088fb
operaTion
Introduction
The LTC4088 includes a PowerPath controller, battery
charger, internal ideal diode, optional external ideal diode
controller, SUSPEND LDO and an always-on 3.3V LDO.
Designed specifically for USB applications, the PowerPath
controller incorporates a precision average input current
limited step-down switching regulator to make maximum
use of the allowable USB power. Because power is con-
served, the LTC4088 allows the load current on V
OUT
to
exceed the current drawn by the USB port without exceed-
ing the USB load specifications.
The switching regulator and battery charger communicate
to ensure that the average input current never exceeds the
USB specifications.
The ideal diodes from BAT to V
OUT
guarantee that ample
power is always available to V
OUT
even if there is insuf-
ficient or absent power at V
BUS
.
To prevent battery drain when a device is connected to a
suspended USB port, an LDO from V
BUS
to V
OUT
provides
either low power or high power suspend current to the
application.
Finally, 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 to a system microprocessor.
Input Current Limited
Step Down Switching Regulator
The
power delivered from V
BUS
to V
OUT
is controlled by a
2.25MHz constant frequency step-down switching regu-
lator. To meet the USB maximum load specification, the
switching regulator contains a measurement and control
system that ensures that the average input current remains
below the level programmed at CLPROG. V
OUT
drives the
combination of the external load and the battery charger.
If the combined load does not cause the switching power
supply to reach the programmed input current limit, V
OUT
will track approximately 0.3V above the battery voltage.
By keeping the voltage across the battery charger at this
low level, power lost to the battery charger is minimized.
Figure 1 shows the power path components.
If the combined external load plus battery charge current
is large enough to cause the switching power supply to
reach the programmed input current limit, the battery
charger will reduce its charge current by precisely the
amount necessary to enable the external load to be satis-
fied. Even if the battery charge current is programmed to
exceed the allowable USB current, the USB specification
for average input current will not be violated; the battery
charger will reduce its current as
needed. Furthermore,
if
the load current at V
OUT
exceeds the programmed power
Figure 1
+
–
+
+
–
0.3V
1.188V 3.6V
CLPROG
I
SWITCH
/N
+
–
+
–
15mV
OV
IDEAL
DIODE
PWM AND
GATE DRIVE
AVERAGE INPUT
CURRENT LIMIT
CONTROLLER
AVERAGE OUTPUT
VOLTAGE LIMIT
CONTROLLER
CONSTANT CURRENT
CONSTANT VOLTAGE
BATTERY CHARGER
+
–
2
GATE
8
V
OUT
10
SW
SYSTEM LOAD
3.5V TO
(BAT + 0.3V)
OPTIONAL
EXTERNAL
IDEAL DIODE
PMOS
SINGLE CELL
Li-Ion
4088 F01
12
BAT
9
V
BUS
TO USB
OR WALL
ADAPTER
11
+