LTC3417A-2_15 Datasheet, PDF (10/20 Pages) Linear Technology – Dual Synchronous 1.5A/1A 4MHz Step-Down DC/DC Regulator

LTC3417A-2
APPLICATIONS INFORMATION
The inductor value will also have an effect on Burst Mode
operation. The transition from low current operation begins
when the peak inductor current falls below a level set by the
burst clamp. Lower inductor values result in higher ripple
current which causes this to occur at lower load currents.
This causes a dip in efficiency in the upper range of low
current operation. In Burst Mode operation, lower inductor
values will cause the burst frequency to increase.
Inductor Core Selection
Different core materials and shapes will change the size/
current relationship of an inductor. Toroid or shielded
pot cores in ferrite or permalloy materials are small and
don’t radiate much energy, but generally cost more than
powdered iron core inductors with similar electrical
characteristics. The choice of which style inductor to use
often depends more on the price vs size requirements
of any radiated field/EMI requirements than on what the
LTC3417A-2 requires to operate. Table 1 shows some
typical surface mount inductors that work well in
LTC3417A-2 applications.
Input Capacitor (C_IN) Selection
In continuous mode, the input current of the converter can
be approximated by the sum of two square waves with
duty cycles of approximately V_OUT1/V_IN and V_OUT2/V_IN. To
prevent large voltage transients, a low equivalent series
resistance (ESR) input capacitor sized for the maximum
RMS current must be used. Some capacitors have a
de-rating spec for maximum RMS current. If the capaci-
tor being used has this requirement, it is necessary to
calculate the maximum RMS current. The RMS current
calculation is different if the part is used in “in phase” or
“out of phase”.
For “in phase”, there are two different equations:
V_OUT1 > V_OUT2:
I_RMS = 2 •I₁ •I₂ • D2(1– D1) + I₂₂(D2 – D2²) + I₁₂(D1– D1²)
V_OUT2 > V_OUT1:
I_RMS = 2 •I₁ •I₂ • D1(1– D2) + I₂₂(D2 – D2²) + I₁₂(D1– D1²)
Table 1
MANUFACTURER
L1 on OUT1
Toko
Coilcraft
Sumida
Midcom
L2 on OUT2
Toko
Coilcraft
Sumida
Midcom
PART NUMBER
A920CY-1R5M-D62CB
A918CY-1R5M-D62LCB
D01608C-152ML
CDRH4D22/HP 1R5
DUP-1813-1R4R
A915AY-2ROM-D53LC
D01608C-222ML
CDRH3D16/HP 2R2
DUP-1813-2R2R
VALUE (μH)
1.5
1.5
1.5
1.5
1.4
2.0
2.2
2.2
2.2
2.2
where.
D1=
V_OUT1
V_IN
and
D2 =
V_OUT2
V_IN
MAX DC CURRENT (A)
2.8
2.9
2.6
3.9
5.5
3.9
2.3
1.75
1.6
3.9
DCR DIMENSIONS L × W × H (mm)
0.014
0.018
0.06
0.031
0.033
6 × 6 × 2.5
6×6×2
6.6 × 4.5 × 2.9
5 × 5 × 2.4
4.3 × 4.8 × 3.5
0.027
0.07
0.047
0.035
0.047
5×5×3
6.6 × 4.5 × 2.9
4 × 4 × 1.8
3.2 × 3.2 × 2
4.3 × 4.8 × 3.5
10
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