LTC3455_15 Datasheet, PDF (24/28 Pages) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

LTC3455/LTC3455-1
APPLICATIONS INFORMATION
1
L1
V_OUT1
C1
L2
V_OUT2
C2
C6
GND
C5
C4
C7
GND
USB V_BAT
D1
GND
V_MAX 5V WALL
ADAPTER
VIAS TO LOCAL GROUND PLANE.
Figure 15. Recommended Board Layout and Component Placement for Power Sections of LTC3455/LTC3455-1
(Refer to Schematic on Back Page)
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
briefly 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 briefly
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 specification 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
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