Datasheet

LTC3428
7
3428fb
APPLICATIONS INFORMATION
COMPONENT SELECTION
Inductor Selection
The high frequency operation of the LTC3428 allows for the
use of small surface mount inductors. The inductor ripple
current is typically set to between 20% and 40% of the
maximum inductor current. For a given set of conditions,
the inductance is given as follows:
L
VVV
RV
LH
IN MIN OUT IN MIN
OUT
≥ >
() ()
•( – )
•
,2μ
where:
R = Allowable inductor current ripple (Amps P-P)
V
IN(MIN)
= Minimum input voltage (V)
V
OUT
= Output voltage (V)
For high effi ciency, the inductor should have a high
frequency core material, such as ferrite, to reduce core
losses. The inductor should have a low ESR (equivalent
series resistance) to reduce I
2
R losses and must be able
to handle the peak inductor current without saturating.
Use of a toroid, pot core, or shielded bobbin inductor will
minimize radiated noise. See Table 1 for a list of inductor
manufacturers. Some example inductor part types are:
Coilcraft 1608 and 3316 series, Murata LQH55D series,
TIME (µs)
0 1.6
3428 F02
0.40.2 0.6 1.0 1.4 1.8
0.8
1.2
2.0
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
OUTPUT (DIODE) CURRENT (A)
1 PHASE
CONVERTER
2 PHASE
CONVERTER
Figure 2. Output Ripple Current Comparison
Between Single Phase and Two Phase Boost
Converters with a 2A Load and 50% Duty Cycle
Sumida CDRH4D22C/LD or CDRH5D28 series, Toko
FDV0630 or D62CB series.
Table 1. Inductor Vendor Information
SUPPLIER PHONE FAX WEBSITE
Coilcraft [phone] [phone] www.coilcraft.com
Murata USA:
[phone]
USA:
[phone]
www.murata.com
Sumida USA:
[phone]
Japan:
81-3-3607-5111
USA:
[phone]
Japan:
81-3-3607-5144
www.sumida.com
TDK [phone] [phone] www.component.tdk.com
Toko [phone] [phone] www.toko.com
Wurth [phone] [phone] www.we-online.com
Output Capacitor Selection
The minimum value of the capacitor is set to reduce the
output ripple voltage due to charging and discharging the
capacitor each cycle. The steady state ripple due to this
charging is given by:
V
IVV
CVf
RIPPLE C
PEAK OUT IN MIN
OUT OUT
()
()
•
•( – )
••
=
1
2
where: I
PEAK
= Peak inductor current (A)
The equivalent series resistance (ESR) of the output
capacitor will contribute another term to output voltage
ripple. Ripple voltage due to capacitor ESR is:
VIR
RIPPLE ESR PEAK ESR C() ()
•=
where:
R
ESR(C)
= Capacitor ESR
The ESL (Equivalent Series Inductance) is another
capacitor characteristic that needs to be minimized. ESL
will be minimized by using small surface mount ceramic
capacitors, placed as close to the V
OUT
pin as possible.
Input Capacitor Selection
Since the V
IN
pin directly powers most of the internal
circuitry, it is recommended to place at least a 4.7µF, low
ESR bypass capacitor between V
IN
and AGND, as close
to the IC as possible. See Table 2 for a list of capacitor
manufacturers.