LTC3543_15 Datasheet, PDF (11/20 Pages) Linear Technology – 600mA Synchronous Step Down Buck Regulator with PLL, Soft-Start and Spread Spectrum

LTC3543
APPLICATIONS INFORMATION
The basic LTC3543 application circuit is shown on the
front page of this data sheet. External component selec-
tion is driven by the load requirement 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 4.7μH. 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 = 130mA
20% I_LOAD(MAX).
ΔI_L
=
V_OUT
f •L
•
⎛
⎝⎜
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 665mA rated induc-
tor should be enough for most applications (600mA +
65mA). For better efficiency, choose a low DC-resistance
inductor.
The inductor value also has an effect on Burst Mode opera-
tion. The transition to low current operation begins when
the inductor current peaks fall to approximately 100mA.
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 induc-
tor. Toroid or shielded pot cores in ferrite or permalloy
materials are small and do not radiate much en-
ergy, but generally 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 re-
quirements, than on what the LTC3543 requires to operate.
Table 1 shows some typical surface mount inductors that
work well in LTC3543 applications.
Table 1. Representative Surface Mount Inductors
MANUFACTURER
PART NUMBER
VALUE (μH)
TDK
VLF3010AT-2R2M1R0
2.2
VLF3012AT-2R2M1R0
2.2
VLCF4020T-2R2N1R7
2.2
VLCF5020-2R7N1R7
2.7
VLCF5020-3R3N1R6
3.3
VLCF5020-4R7N1R4
4.7
Sumida
CDRH2D18/HP-2R2NC
2.2
Taiyo Yuden
NR4018T4R7M
4.7
NP03SB4R7M
4.7
CoEv
DN4835-2R2
2.2
DN4835-3R3
3.3
DN4835-6R8
6.8
Murata
LQH32CN2R2M33
2.2
LQH55DN2R2M03
2.2
LQH55DN3R3M03
3.3
LQH55DN4R7M03
4.7
MAX DC CURRENT (A)
1.0
1.0
1.7
1.7
1.6
1.4
1.6
1.7
1.2
2.6
2.43
1.41
0.79
3.2
2.9
2.7
DCR (mΩ)
100
88
54
58
69
79
48
90
47
47
58
117
97
29
36
41
HEIGHT (mm)
1.0
1.2
2.0
2.0
2.0
2.0
2.0
1.8
1.8
3.5
3.5
3.5
3.2
4.7
4.7
4.7
3543fa
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