LTC3548-1_15 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 • 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
8
Inductor Core Selection
Different core materials and shapes will change the
size/ current and price/current relationship of an induc-
tor. Toroid or shielded pot cores in ferrite or permalloy
materials are small and do not radiate much energy,
but generally cost more than powdered iron core induc-
tors with similar electrical characteristics. The choice of
which style inductor to use often depends more on the
price vs size requirements 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
35481fc