LTC3417 Linear Technology, LTC3417 Datasheet - Page 10

• Manufacturer Part Number: LTC3417
• Description: Dual Synchronous 1.4A/800mA 4MHz Step-Down DC/DC Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC3417
A reasonable starting point for setting ripple current is
∆I
current output. The largest ripple, ∆I
maximum input voltage. To guarantee that the ripple
current stays below a specified maximum, the inductor
value should be chosen according to the following equa-
tion:
The inductor value will also have an effect on Burst Mode
operation. The transition from low current operation be-
gins 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 pow-
dered iron core inductors with similar electrical character-
istics. The choice of which style inductor to use often
depends more on the price vs size requirements of any
Table 1
MANUFACTURER
L1 on OUT1
Toko
Coilcraft
Sumida
Midcom
L2 on OUT2
Toko
Coilcraft
Sumida
Midcom
10
L
L
= 0.35I
=
f
O
V
OUT
•
LOAD(MAX)
∆
I
L
⎛
⎜
⎝
1
–
PART NUMBER
A920CY-1R5M-D62CB
A918CY-1R5M-D62LCB
DO1608C-152ML
CDRH4D22/HP 1R5
CDRH2D18/HP 1R7
DUP-1813-1R4R
A915AY-2R0M-D53LC
DO1608C-222ML
CDRH3D16/HP 2R2
CDRH2D18/HP 2R2
DUP-1813-2R2R
U
V
, where I
IN MAX
V
OUT
(
U
)
⎞
⎟
⎠
LOAD(MAX)
W
L
is the maximum
, occurs at the
VALUE (µH)
U
1.5
1.5
1.5
1.5
1.7
1.4
2.0
2.2
2.2
2.2
2.2
MAX DC CURRENT (A)
radiated field/EMI requirements than on what the LTC3417
requires to operate. Table 1 shows some typical surface
mount inductors that work well in LTC3417 applications.
Input Capacitor (C
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
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 capacitor
being used has this requirement, it is necessary to calcu-
late the maximum RMS current. The RMS current calcu-
lation is different if the part is used in “in phase” or “out of
phase”.
For “in phase”, there are two different equations:
V
V
where:
I
I
RMS
RMS
OUT1
OUT2
D
1
=
=
1.75
=
2.8
2.6
3.9
5.5
2.3
3.9
2.9
1.8
3.9
1.6
> V
> V
V
2
2
OUT
V
OUT2
OUT1
• • •
• • • ( –
IN
I I
I I
1 2
1 2
1
:
:
and D
D
D
IN
2 1
1 1
) Selection
( – )
0.014
0.018
0.031
0.035
0.033
0.027
0.047
0.035
0.047
DCR
2
0.06
0.07
=
D
D
2
1
V
)
OUT
V
+
+
DIMENSIONS L × W × H (mm)
6 × 6 × 2.5
6 × 6 × 2
6.6 × 4.5 × 2.9
5 × 5 × 2.4
3.2 × 3.2 × 2
4.3 × 4.8 × 3.5
5 × 5 × 3
6.6 × 4.5 × 2.9
4 × 4 × 1.8
3.2 × 3.2 × 2
4.3 × 4.8 × 3.5
IN
I
I
OUT1
2
2
2
2
2
(
(
www.DataSheet4U.com
D
D
/V
2
2
–
–
IN
D
D
and V
2
2
2
2
)
)
+
+
OUT2
I D
I D
1
1
2
2
( –
( –
/V
1
1
IN
D
D
3417fb
. To
1
1
2
2
)
)