Datasheet

LTC3417A-2
10
3417a2fa
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 effi ciency 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 fi eld/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
:
V
OUT2
> V
OUT1
:
where.
Table 1
MANUFACTURER PART NUMBER VALUE (μH) MAX DC CURRENT (A) DCR DIMENSIONS L × W × H (mm)
L1 on OUT1
Toko A920CY-1R5M-D62CB
A918CY-1R5M-D62LCB
1.5
1.5
2.8
2.9
0.014
0.018
6 × 6 × 2.5
6 × 6 × 2
Coilcraft D01608C-152ML 1.5 2.6 0.06 6.6 × 4.5 × 2.9
Sumida CDRH4D22/HP 1R5 1.5 3.9 0.031 5 × 5 × 2.4
Midcom DUP-1813-1R4R 1.4 5.5 0.033 4.3 × 4.8 × 3.5
L2 on OUT2
Toko A915AY-2ROM-D53LC 2.0 3.9 0.027 5 × 5 × 3
Coilcraft D01608C-222ML 2.2 2.3 0.07 6.6 × 4.5 × 2.9
Sumida CDRH3D16/HP 2R2 2.2
2.2
1.75
1.6
0.047
0.035
4 × 4 × 1.8
3.2 × 3.2 × 2
Midcom DUP-1813-2R2R 2.2 3.9 0.047 4.3 × 4.8 × 3.5
APPLICATIONS INFORMATION
I
RMS
= 2•I
1
•I
2
•D2(1–D1)+I
2
2
(D2 – D2
2
)+I
1
2
(D1– D1
2
)
I
RMS
= 2•I
1
•I
2
•D1(1–D2)+I
2
2
(D2 – D2
2
)+I
1
2
(D1– D1
2
)
D1=
V
OUT1
V
IN
and D2 =
V
OUT2
V
IN