Datasheet

LTC3560
8
3560fb
The basic LTC3560 application circuit is shown in Figure 1.
External component selection is driven by the load re-
quirement 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 1µH to 3.3µH. 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 ∆I
L
= 320mA
(40% of 800mA).
I
L
=
1
f
()
L
()
V
OUT
1
V
OUT
V
IN






(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 960mA rated
inductor should be enough for most applications (800mA +
160mA). For better effi ciency, choose a low DC-resistance
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
200mA. Lower inductor values (higher ∆I
L
) will cause this
to occur at lower load currents, which can cause a dip in
effi ciency 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 materials are
small and don’t radiate much energy, but generally cost
OPERATION
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant
frequency architectures by preventing subharmonic
oscillations at high duty cycles. It is accomplished internally
by adding a compensating ramp to the inductor current
signal at duty cycles in excess of 40%. Normally, this
results in a reduction of maximum inductor peak current
for duty cycles >40%. However, the LTC3560 uses a
patented scheme that counteracts this compensating
ramp, which allows the maximum inductor peak current
to remain unaffected throughout all duty cycles.
APPLICATIONS INFORMATION
Table 1. Representative Surface Mount Inductors
MANUFACTURER PART NUMBER VALUE
MAX DC
CURRENT DCR HEIGHT
Toko A960AW-1R2M-518LC
A960AW-2R3M-518LC
A997AS-3R3M-DB318L
1.2µH
2.3µH
3.3µH
1.8A
1.5A
1.2A
46m
63m
70m
1.8mm
1.8mm
1.8mm
Sumida CDRH2D11/HP-1R5
CDRH3D11/HP-1R5
CDRH2D18/HP-2R2
CDRH2D14-3R3
1.5µH
1.5µH
2.2µH
3.3µH
1.35A
2A
1.6A
1.2A
64m
80m
48m
100m
1.2mm
1.2mm
2.0mm
1.55mm
TDK VLF3010AT-1R5M1R2
VLF3010AT-2R2M1R0
1.5µH
2.2µH
1.2A
1.0A
68m
100m
1.0mm
1.0mm
Coilcraft D01608C-222
LP01704-222M
2.2µH
2.2µH
2.3A
2.4A
70m
120m
3.0mm
1.0mm
Cooper SD3112-2R2 2.2µH 1.1A 140m 1.2mm
EPCO B82470A1222M 2.2µH 1.6A 90m 1.2mm
(Refer to Functional Diagram)