LTC3546 Datasheet, PDF (16/28 Pages) Linear Technology – Dual Synchronous, 3A/1A or 2A/2A Confi gurable Step-Down DC/DC Regulator

LTC3546
APPLICATIONS INFORMATION
The minimum frequency is limited by leakage and noise
coupling due to the large resistance of R_T.
Inductor Selection
Although the inductor does not influence the operat-
ing frequency, the inductor value has a direct effect on
ripple current. The inductor ripple current ΔI_L decreases
with higher inductance and increases with higher V_IN or
V_OUT.
ΔI_L
=
V_OUT
f_O •L
⎛
⎝⎜
1−
V_OUT
V_IN
⎞
⎠⎟
Accepting larger values of ΔI_L allows the use of low induc-
tances, but results in higher output voltage ripple, greater
core losses, and lower output current capability.
A reasonable starting point for setting ripple current is ΔI_L
= 0.35I_LOAD(MAX), where I_LOAD(MAX) is the maximum output
current. The largest ripple ΔI_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 equation:
L
≥
V_OUT
f_O • ΔI_L
⎛
⎜
⎝
1−
V_OUT
V_IN(MAX)
⎞
⎟
⎠
Burst Mode Operation Considerations
There are two factors that determine the load current at which
the LTC3546 enters Burst Mode operation: the inductor value
and the BMC pin voltage. The transition from low current
operation begins when the peak inductor current falls below
a level set by the burst clamp. Lower inductor values result in
higher ripple current which causes Burst Mode operation to
occur at lower load currents. Lower inductor values will also
cause a dip in efficiency in the upper range of low current
operation. Lower inductor values will also cause the burst
frequency to increase in Burst Mode operation.
The burst clamp level can be set by the voltage on the BMC
pin. If BMC is tied to V_IN, an internally set level is used. A
BMC pin voltage between 0V and 0.6V will set the burst
clamp level (see charts OUT1 Minimum Peak Current vs
V_BMC1 and OUT2 Minimum Peak Current vs V_BMC2 in the
Typical Performance Characteristics section). Generally, a
higher clamp level results in improved light load efficiency
and higher output voltage ripple, while a lower clamp level
results in small output voltage ripple at the expense of
efficiency. The BMC pin should be connected to ground
when Burst Mode operation is not selected.
Inductor Core Selection
Different core materials and shapes will change the size/cur-
rent 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 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 of any radiated field/EMI
requirements than on what the LTC3546 requires to oper-
ate. Table 1 shows some typical surface mount inductors
that work well in LTC3546 applications.
Table 1.
MANUFACTURER
PART NUMBER
VALUE (μH) MAX DC CURRENT (A)
DCR
DIMENSIONS L × W × H (mm)
Wurth
WE-PD2 MS
1.2
7447745012
4.6
0.017
5.2 × 5.8 × 2
Wurth
WE-PD2 MS
0.56
74477450056
6.5
0.0078
5.2 × 5.8 × 2
Vishay
IHLP-1616AB-11
1.2
3.75
0.068
4.06 × 4.45 × 1.20
Vishay
IHLP-1616AB-11
0.47
6
0.019
4.06 × 4.45 × 1.20
Coilcraft
LPS6225-122
1.2
5.4
0.04
6.2 × 6.2 × 2.5
Coilcraft
DO1813H-561
0.56
7.7
0.01
6.10 × 8.89 × 5.00
Coiltronics
SD20-1R2
1.2
2.55
0.0275
5.2 × 5.2 × 2
Coiltronics
SD20-R47
0.47
4
0.02
5.2 × 5.2 × 2
Sumida
CDRH3D23NP-1R5NC
1
2.8
0.025
3.8 × 3.8 × 2.3
3546f
16