LTC3560_15 Datasheet, PDF (8/16 Pages) Linear Technology – 2.25MHz, 800mA Synchronous Step-Down Regulator in ThinSOT

LTC3560
OPERATION (Refer to Functional Diagram)
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
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 ΔI_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 efficiency, 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 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 materials are
small and don’t radiate much energy, but generally cost
Table 1. Representative Surface Mount Inductors
MANUFACTURER
Toko
Sumida
TDK
Coilcraft
Cooper
EPCO
PART NUMBER
A960AW-1R2M-518LC
A960AW-2R3M-518LC
A997AS-3R3M-DB318L
CDRH2D11/HP-1R5
CDRH3D11/HP-1R5
CDRH2D18/HP-2R2
CDRH2D14-3R3
VLF3010AT-1R5M1R2
VLF3010AT-2R2M1R0
D01608C-222
LP01704-222M
SD3112-2R2
B82470A1222M
VALUE
1.2μH
2.3μH
3.3μH
1.5μH
1.5μH
2.2μH
3.3μH
1.5μH
2.2μH
2.2μH
2.2μH
2.2μH
2.2μH
MAX DC
CURRENT
1.8A
1.5A
1.2A
1.35A
2A
1.6A
1.2A
1.2A
1.0A
2.3A
2.4A
1.1A
1.6A
DCR
46mΩ
63mΩ
70mΩ
64mΩ
80mΩ
48mΩ
100mΩ
68mΩ
100mΩ
70mΩ
120mΩ
140mΩ
90mΩ
HEIGHT
1.8mm
1.8mm
1.8mm
1.2mm
1.2mm
2.0mm
1.55mm
1.0mm
1.0mm
3.0mm
1.0mm
1.2mm
1.2mm
8
3560fb