LTC3548-1 Datasheet, PDF (8/16 Pages) Linear Technology – Dual Synchronous, Fixed Output 2.25MHz Step-Down DC/DC Regulator

LTC3548-1
APPLICATIONS INFORMATION
A general LTC3548-1 application circuit is shown in
Figure 2. External component selection is driven by the
load requirement, and begins with the selection of the
inductor L. Once the inductor is chosen, C_IN and C_OUT
can be selected.
higher ripple current which causes this 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.
V_IN
2.7V TO 5.5V
V_OUT2
1.575V
400mA
C_OUT2
10μF
CER
C_IN
10μF
CER
4.7μH
C_FF2
330pF
V_IN RUN1 RUN2
LTC3548-1
SW2
SW1
V_OUT2
V_OUT1
V_FB2
V_FB1
GND
2.2μH
C_FF1
330pF
V_OUT1
1.8V
800mA
C_OUT1
10μF
CER
3548-1 F01
Figure 2. LTC3548-1 General Schematic
Inductor Selection
Although the inductor does not influence the operating fre-
quency, the inductor value has a direct effect on ripple cur-
rent. The inductor ripple current ΔI_L decreases with higher
inductance and increases with higher V_IN or V_OUT:

I_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 = 0.3 • I_OUT(MAX), where I_OUT(MAX) is 0.8A for channel 1
and 400mA for channel 2. 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 follow-
ing equation:
L
=
V_OUT
f_O •
I_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
8
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 characterisitics. 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 LTC3548-1 requires to operate. Table 1 shows
some typical surface mount inductors that work well in
LTC3548-1 applications.
Table 1. Representative Surface Mount Inductors
PART
NUMBER
VALUE DCR
MAX DC
SIZE
(μH) (Ω MAX) CURRENT (A) W × L × H (mm³)
Sumida
2.2
0.075
CDRH3D16
3.3
0.110
4.7
0.162
1.20
3.8 × 3.8 × 1.8
1.10
0.90
Sumida
CMD4D06
2.2
0.089
4.7
0.166
0.95
4.1 × 3.2 × 0.8
0.75
Sumida
CMD4D11
2.2
0.116
3.3
0.174
0.950
0.770
4.4 × 5.8 × 1.2
Murata
LQH32CN
1.0
0.060
2.2
0.097
1.00
2.5 × 3.2 × 2.0
0.79
Toko
D312F
2.2
0.060
3.3
0.260
1.08
2.5 × 3.2 × 2.0
0.92
Panasonic
3.3
0.17
ELT5KT
4.7
0.20
1.00
4.5 × 5.4 × 1.2
0.95
Input Capacitor (C_IN) Selection
In continuous mode, the input current of the converter is a
square wave with a duty cycle of approximately V_OUT/V_IN.
To prevent large voltage transients, a low equivalent series
resistance (ESR) input capacitor sized for the maximum
RMS current must be used. The maximum RMS capacitor
current is given by:
( ) _{IRMS ≈IMAX}
V_OUT V_IN – V_OUT
V_IN
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