LTC3427_15 Datasheet, PDF (7/12 Pages) Linear Technology – 500mA, 1.25MHz Synchronous Step-Up DC/DC Converter in 2mm 2mm DFN Package

APPLICATIO S I FOR ATIO
LTC3427
SW 1
LTC3427
6 V_OUT
GND 2
5 FB
MINIMIZE
TRACE ON FB
+
V_IN
V_IN 3
4 SHDN
MULTIPLE VIAS
TO GROUND PLANE
3427 F01
Figure 1. Recommended Component Placement for a Single Layer Board. Traces Carrying High Current are Direct (GND, SW, V_IN,
V_OUT). Trace Area at FB is Kept Low. Lead Length to Battery Should be Kept Short. V_IN and V_OUT Ceramic Capacitors Should be as
Close to the LTC3427 as Possible. A Multilayer Board with a Separate Ground Plane is Ideal, but not Absolutely Necessary
COMPONENT SELECTION
Inductor Selection
The LTC3427 can utilize small surface mount and chip
inductors due to its fast 1.25MHz switching frequency. A
minimum inductance value of 3.3µH is necessary for 3.6V
and lower voltage applications and a 4.7µH for output
voltages greater than 3.6V. Larger values of inductance
will allow greater output current capability by reducing the
inductor ripple current. Increasing the inductance above
10µH will increase size while providing little improvement
in output current capability.
The approximate output current capability of the LTC3427
vs Inductance value is given below in Equation 1 and
illustrated graphically in Figure 2.
I_OUT(MAX)
=
n
•
⎛
⎝⎜
I_P
–
V_IN
f •L
•D
•2
⎞
⎠⎟
•
(1– D)
(1)
where:
n = estimated efficiency
I_P = peak current limit value (0.5A min)
V_IN = input (battery) voltage
D = steady-state duty ratio = (V_OUT – V_IN)/V_OUT
f = switching frequency (1.25MHz typical)
L = inductance value
The inductor current ripple is typically set for 20% to 40%
of the maximum inductor current (I_P). High frequency
ferrite core inductor materials reduce frequency depen-
dent power losses compared to cheaper powdered iron
types, improving efficiency. The inductor should have low
ESR (series resistance of the windings) to reduce the I²R
power losses, and must be able to handle the peak
inductor current without saturating. Molded chokes and
some chip inductors usually do not have enough core to
support the peak inductor currents of greater than 500mA
seen on the LTC3427. To minimize radiated noise, use a
toroid, pot core or shielded bobbin inductor. See Table 1
for suggested suppliers.
Output and Input Capacitor Selection
Low ESR (equivalent series resistance) capacitors should
be used to minimize the output voltage ripple. Multilayer
ceramic capacitors are an excellent choice as they have
extremely low ESR and are available in small footprints. A
2.2µF to 10µF output capacitor is sufficient for most
applications. Larger values up to 22µF may be used to
obtain extremely low output voltage ripple and improve
transient response. An additional phase lead capacitor
may be required with output capacitors larger than 10µF to
maintain acceptable phase margin. X5R and X7R dielec-
tric materials are preferred for their ability to maintain
capacitance over wide voltage and temperature ranges.
3427fa
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