LTC3456_15 Datasheet, PDF (23/28 Pages) Linear Technology – 2-Cell, Multi-Output DC/DC Converter with USB Power Manager

LTC3456
APPLICATIO S I FOR ATIO
USB POWER
(4.35V TO 5.5V)
AC
ADAPTER
(5V ±10%)
2 AA ⁺
CELLS
4.7µF
L2
4.7µH
L3
10µH
4.7µF
1Ω
4.7µF
1Ω
USB
CONTROLLER
100k
V_BATT
AIN
80.6k
AO
SW2_BST
SW2_BK
USBHP
SUSPEND
V_INT
V_MAIN
HSO
USB
LTC3456
SW1
1k
EXT_PWR
V_EXT
FB1
11.3k
10µF
4.32k
WALLFB
PWRKEY
V_MAX
PBSTAT RESET MODE PWRON PGND AGND
L1
10µH
100k
80.6k
220pF
V_INT
3.3V
22µF
1µF
1µF
10µF
MAIN OUTPUT
3.3V
150mA
Hot Swap OUTPUT
3.3V
50mA
CORE OUTPUT
1.8V
200mA
V_MAX
(POWERS
1µF ^{REAL-TIME CLOCK)}
µP
3456 TA01 BOLD LINES INDICATE HIGH CURRENT PATHS
Figure 17. Layout Diagram
BOARD LAYOUT CONSIDERATION
As with all switching regulators, careful attention must be
paid to the PCB board layout and component placement.
To prevent electromagnetic interference (EMI) problems,
proper layout of high frequency switching paths is essen-
tial. Minimize the length and area of all traces connected to
the switching node pins (SW1, SW2_BK, SW2_BST).
Keep the feedback pins FB1 and AIN away from the
switching nodes. The power traces shown as bold lines in
Figure 17 should be kept short, direct and wide.
The QFN package has an exposed paddle and it must be
connected to the system ground. The ground connection
for the feedback resistors should be tied directly to the
ground plane and not shared with any other component,
ensuring a clean, noise-free connection.
DESIGN EXAMPLE
As a design example, we target a 2 AA cell powered GPS
navigator application. Figure 18 shows LTC3456 being
used to provide power to the core and I/O peripherals. The
flash memory card is powered from the Hot Swap output.
Core output needs to be 1.8V and the maximum load
current is 200mA. The inductor current ripple, ∆I_L, for
buck mode of operation is given by:
( ) _∆IL1
=
V_OUT • V_IN – V_OUT
V_IN • f(L1)
The maximum inductor current in L1 is set by the core
converter current limit; i.e., 400mA (minimum). Choosing
∆I_L = 100mA (~25% of peak inductor current) is a reason-
able starting value.
Substituting V_OUT = 1.8V, V_IN(MAX) = 3.2V, ∆I_L1 = 100mA,
f = 1MHz in above equation gives:
L1= ^{1.8V • (3.2V – 1.8V)} = 7.8µH
3.2V • 100mA • 1MHz
3456fa
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