LTC3546_15 Datasheet, PDF (15/30 Pages) Linear Technology – Dual Synchronous, 3A/1A or 2A/2A Configurable Step-Down DC/DC Regulator

LTC3546
Applications Information
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.
Input Capacitor (C_IN) Selection
In continuous mode, the input current of the converter can
be approximated by the sum of two square waves with
duty cycles of approximately V_OUT1/V_IN and V_OUT2/V_IN. To
prevent large voltage transients, a low equivalent series
resistance (ESR) input capacitor sized for the maximum
RMS current must be used. Some capacitors have a de-
rating spec for maximum RMS current. If the capacitor
being used has this requirement it is necessary to calculate
Table 1.
MANUFACTURER
Würth Elektronik
Würth Elektronik
Vishay
Vishay
Coilcraft
Coilcraft
Coiltronics
Coiltronics
Sumida
PART NUMBER
WE-PD2 MS
7447745012
WE-PD2 MS
74477450056
IHLP-1616AB-11
IHLP-1616AB-11
LPS6225-122
DO1813H-561
SD20-1R2
SD20-R47
CDRH3D23NP-1R5NC
VALUE (µH)
1.2
0.56
1.2
0.47
1.2
0.56
1.2
0.47
1
MAX DC CURRENT (A)
4.6
6.5
3.75
6
5.4
7.7
2.55
4
2.8
DCR
0.017
0.0078
0.068
0.019
0.04
0.01
0.0275
0.02
0.025
DIMENSIONS L × W × H (mm)
5.2 × 5.8 × 2
5.2 × 5.8 × 2
4.06 × 4.45 × 1.20
4.06 × 4.45 × 1.20
6.2 × 6.2 × 2.5
6.10 × 8.89 × 5.00
5.2 × 5.2 × 2
5.2 × 5.2 × 2
3.8 × 3.8 × 2.3
For more information www.linear.com/3546
3546fc
15