LTC3417 Datasheet, PDF (10/20 Pages) Linear Technology – Dual Synchronous 1.4A/800mA 4MHz Step-Down DC/DC Regulator

LTC3417
APPLICATIO S I FOR ATIO
A reasonable starting point for setting ripple current is
∆I_L = 0.35I_LOAD(MAX), where I_LOAD(MAX) is the maximum
current output. The largest ripple, ∆I_L, occurs at the
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:
L
=
V_OUT
f_O • ∆I_L
⎛
⎝⎜^1–
V_OUT ^⎞
V_{IN(MAX) ⎠⎟}
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.
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) 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 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_OUT1 > V_OUT2:
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
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
VALUE (µH)
1.5
1.5
1.5
1.5
1.7
1.4
2.0
2.2
2.2
2.2
2.2
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)
where:
D1= ^VOUT1 and D2 = ^VOUT2
V_IN
V_IN
MAX DC CURRENT (A)
DCR DIMENSIONS L × W × H (mm)
2.8
0.014 6 × 6 × 2.5
2.9
0.018 6 × 6 × 2
2.6
0.06 6.6 × 4.5 × 2.9
3.9
0.031 5 × 5 × 2.4
1.8
0.035 3.2 × 3.2 × 2
5.5
0.033 4.3 × 4.8 × 3.5
3.9
0.027 5 × 5 × 3
2.3
0.07 6.6 × 4.5 × 2.9
1.75
0.047 4 × 4 × 1.8
1.6
0.035 3.2 × 3.2 × 2
3.9
0.047 4.3 × 4.8 × 3.5
3417fb
10