LTC3564_15 Datasheet, PDF (9/20 Pages) Linear Technology – 2.25MHz, 1.25A Synchronous Step-Down Regulator

LTC3564
APPLICATIO S I FOR ATIO
L
V_IN
V_IN
SW
C_IN
C_F
LTC3564
RUN V_FB
R2
R1
GND
V_OUT
C_OUT
3564 F01
Figure 1. LTC3564 General Schematic
The basic LTC3564 application circuit is shown in Figure 1.
External component selection is driven by the load require-
ment and begins with the selection of L followed by C_IN and
C_OUT.
Inductor Selection
For most applications, the value of the inductor will fall in
the range of 0.47μH to 2.2μH. Its value is chosen based on
the desired ripple current. Large value inductors lower
ripple current and small value inductors result in higher
ripple currents. Higher V_IN or V_OUT also increases the ripple
current as shown in equation 1. A reasonable starting point
for setting ripple current is ΔI_L = 500mA (40% of 1.25A).
ΔI_L
=
1
(f)(L)
V_OUT
⎛
⎝⎜
1−
V_OUT
V_IN
⎞
⎠⎟
(1)
Table 1. Representative Surface Mount Inductors
MANUFATURER
PART NUMBER
Toko
A915AY-1R1M-DC53LC
1070AS-1R0N-DB3020C
Sumida
CDRH4D18C/LD-1R1
CDRH3D14-1R2
CR5D11-1R0
CDRH2D18/HP-2R2
FDK
MIPW3226D0R9M
Coilcraft
LPO6610-122ML
LPS4018-222ML
Vishay
IHLP1616ABERR47M01
IHLP1616ABER1R0M01
VALUE (μH)
1.1
1
1.1
1.2
1
2.2
0.9
1.2
2.2
0.47
1
The DC current rating of the inductor should be at least
equal to the maximum load current plus half the ripple
current to prevent core saturation. Thus, a 1.5A rated
inductor should be enough for most applications (1.25A
+ 250mA). For better efficiency, choose a low DC-resis-
tance inductor.
The inductor value also has an effect on Burst Mode
operation. The transition to low current operation begins
when the inductor current peaks fall to approximately
300mA. Lower inductor values (higher ΔI_L) will cause this
to occur at lower load currents, which can cause a dip in
efficiency in the upper range of low current operation. In
Burst Mode operation, lower inductance values will cause
the burst frequency to increase.
Inductor Core Selection
Different core materials and shapes will change the size/
current and price/current relationship of an inductor.
Toroid or shielded pot cores in ferrite or permalloy mate-
rials are small and don’t radiate much energy, but gener-
ally 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 and any radiated field/EMI requirements
than on what the LTC3564 requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3564 applications.
MAX DC CURRENT (A)
3.25
1.9
2.1
2.2
2.2
1.6
1.4
2.1
2.5
5
4
DCR (mΩ)
16
47
24
36
40
48
70
80
70
20
45
HEIGHT (mm)
3
2
2
1.5
1.2
2
1
1
1.8
1.2
1.2
3564f
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