LTC3428 Datasheet, PDF (7/12 Pages) Linear Technology – 4A, 2MHz Dual Phase Step-Up DC/DC Converter in 3mm 3mm DFN

LTC3428
APPLICATIO S I FOR ATIO
5.0
4.5
1 PHASE
CONVERTER
4.0
3.5
3.0
2 PHASE
2.5
CONVERTER
2.0
1.5
1.0
0.5
0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
TIME (µs)
3428 F02
Figure 2. Output Ripple Current Comparison
Between Single Phase and Two Phase Boost
Converters with a 2A Load and 50% Duty Cycle
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 condi-
tions, the inductance is given as follows:
L
≥
V_IN(MIN)
• (V_OUT
–
V_IN(MIN)
)
,
L
>
2µH
R • V_OUT
where:
R = Allowable inductor current ripple (Amps P-P)
V_IN(MIN) = Minimum input voltage (V)
V_OUT = Output voltage (V)
For high efficiency, 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 I²R 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,
Sumida CDRH4D22C/LD or CDRH5D28 series, Toko
FDV0630 or D62CB series.
Table 1. Inductor Vendor Information
Supplier Phone
Fax
Coilcraft [phone] [phone]
Murata USA:
USA:
[phone] [phone]
Sumida
USA:
USA:
[phone] [phone]
Japan:
Japan:
81-3-3607-5111 81-3-3607-5144
TDK
[phone] [phone]
Toko [phone] [phone]
Wurth [phone] [phone]
Website
www.coilcraft.com
www.murata.com
www.sumida.com
www.component.tdk.com
www.toko.com
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_RIPPLE(C)
=
1
2
•
I_PEAK • (V_OUT – V_IN(MIN))
C_OUT • V_OUT • f
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:
V_RIPPLE(ESR) = I_PEAK •R_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.
3428f
7