Datasheet

LTC3548A
9
3548afa
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
inductances, 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
=
0.3 • I
LIM
, where I
LIM
is the peak switch current limit. The
largest ripple current 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)
⎛
⎝
⎜
⎞
⎠
⎟
The inductor value will also have an effect on Burst Mode
operation. 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 this transition to occur
at lower load currents. This causes 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
more than powdered iron core inductors with similar elec-
trical characteristics. The choice of which style inductor
to use often depends more on the price vs size require-
ments and any radiated field/EMI requirements than on
what the LTC3548A requires to operate. Table 1 shows
some typical surface mount inductors that work well in
LTC3548A applications.
APPLICATIONS INFORMATION
Figure 1. LTC3548A General Schematic
V
OUT2
RUN/SS2
V
IN
V
IN
= 2.5V TO 5.5V
V
OUT1
RUN/SS1
POR
SW1
V
FB1
GND
V
FB2
SW2
MODE/SYNC
LTC3548A
C
IN
R7
POWER-ON
RESET
C1C2
L1
L2
R4 R2
R1
R3
C
OUT2
C4
C3
C
OUT1
3548A F01
PULSE SKIP*
BURST*
*MODE/SYNC = 0V: PULSE SKIP
MODE/SYNC = V
IN
: Burst Mode OPERATION
R6 R5
OPERATION