LTC3548A Datasheet, PDF (9/20 Pages) Linear Technology – Dual Synchronous 400mA/800mA, 2.25MHz Step-Down DC/DC Regulator

OPERATION
LTC3548A
V_IN = 2.5V TO 5.5V
C_IN
R6
BURST*
V_IN
MODE/SYNC
POR
PULSE SKIP*
C4
V_OUT2
L2
C2
RUN/SS2 RUN/SS1
LTC3548A
SW2
SW1
R7
POWER-ON
R5
RESET
L1
C1
C3
V_OUT1
C_OUT2 ^R4
V_FB2
V_FB1
R3
GND
R2
R1
*MODE/SYNC = 0V: PULSE SKIP
MODE/SYNC = V_IN: Burst Mode OPERATION
Figure 1. LTC3548A General Schematic
C_OUT1
3548A F01
APPLICATIONS INFORMATION
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.
3548af
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