Datasheet

LTC3455/LTC3455-1
24
3455fc
APPLICATIONS INFORMATION
Figure 15. Recommended Board Layout and Component Placement for Power Sections of LTC3455/LTC3455-1
(Refer to Schematic on Back Page)
5V WALL
ADAPTER
GNDGND
C1
C6
C7
C5
C4
D1
C2
L1
1
L2
GND
V
BAT
V
MAX
V
OUT1
V
OUT2
USB
VIAS TO LOCAL GROUND PLANE.
Standalone USB Power Supply
with Temporary Backup Power
Although designed primarily for Li-Ion powered portable
applications, the LTC3455/LTC3455-1 are also good choices
for systems that are always powered by a USB supply
or wall adapter. The battery charger can then be used
to charge up a large capacitor or backup battery, which
briefl y provides power to the system after the external
power has been removed. This gives the microcontroller
enough time to follow proper shutdown procedures when
the main power source is abruptly removed. Figure 14
shows a standalone power supply for USB high power
applications (500mA maximum USB current) using the
LTC3455/LTC3455-1. The total system power should be
kept below 1.8W to ensure clean operation even under
worst-case USB conditions. With the resistor values
shown, the low-battery indicator (AI and AO pins) trig-
gers when the V
MAX
pin voltage drops to 4V, notifying
the microcontroller of an impending dropout condition.
The 1MΩ resistor connected between the AI and AO pins
provides 150mV of hysteresis (the dropout indicator
stays low until the V
MAX
pin rises back above 4.15V). A
4700μF backup capacitor connected to the V
BAT
pin briefl y
provides power to the system after the USB supply has
been removed, and also helps support transient loads
that slightly exceed the USB current limit. Connecting this
large capacitance to the V
BAT
pin has several advantages.
It provides a large energy reservoir that is isolated from
both the USB pin (the USB specifi cation limits capacitance
on the USB supply pin to 10μF or less) and the V
MAX
pin
(using a very large capacitance on this pin will delay the
system turn-on), and it prevents large inrush currents by