LTC3407A LINER [Linear Technology], LTC3407A Datasheet - Page 8

• Manufacturer Part Number: LTC3407A
• Description: Dual Synchronous 600mA, 1.5MHz Step-Down DC/DC Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LTC3407A
Inductor Selection
Although the inductor does not infl uence the operat-
ing frequency, the inductor value has a direct effect on
ripple current. The inductor ripple current ΔI
with higher inductance and increases with higher V
V
Accepting larger values of ΔI
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
0.3 • I
largest ripple current ΔI
voltage. To guarantee that the ripple current stays below a
specifi ed maximum, the inductor value should be chosen
according to the following equation:
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 effi ciency in
the upper range of low current operation. In Burst Mode
operation, lower inductance values will cause the burst
frequency to increase.
8
OUT
V
L =
IN
R6
V
= 2.5V TO 5.5V
OUT2
C4
:
I
LIM
L
f
O
=
V
, where I
OUT
• I
C
V
C
f
OUT2
O
IN
OUT
Figure 1. LTC3407A General Schematic
• L
L
BURST*
PULSESKIP*
C2
R4
• 1–
• 1–
L2
LIM
R3
*MODE/SYNC = 0V: PULSE SKIP
MODE/SYNC = V
V
is the peak switch current limit. The
MODE/SYNC
RUN/SS2
SW2
V
V
FB2
IN(MAX)
V
V
OUT
IN
OUT
L
LTC3407A
occurs at the maximum input
GND
V
IN
IN
: Burst Mode
RUN/SS1
L
SW1
POR
V
FB1
allows the use of low
R7
R1
L1
POWER-ON
RESET
C1
R2
L
decreases
V
OUT1
C3
C
R5
OUT1
3407A F01
IN
L
or
=
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 fi eld/EMI requirements than on
what the LTC3407A requires to operate. Table 1 shows
some typical surface mount inductors that work well in
LTC3407A applications.
Table 1. Representative Surface Mount Inductors
Taiyo
Yuden
Panasonic
Sumida
Murata
Taiyo
Yuden
FDK
TDK
Input Capacitor (C
In continuous mode, the input current of the converter is a
square wave with a duty cycle of approximately V
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:
where the maximum average output current I
the peak current minus half the peak-to-peak ripple cur-
rent, I
ACTURER
MANUF-
I
RMS
MAX
≈I
CB2016T2R2M
CB2012T2R2M
CB2016T3R3M
ELT5KT4R7M
CDRH2D18/LD
LQH32CN4R7M23 4.7μH
NR30102R2M
NR30104R7M
FDKMIPF2520D
FDKMIPF2520D
FDKMIPF2520D
VLF3010AT4R7-
MR70
VLF3010AT3R3-
MR87
VLF3010AT2R2-
M1R0
MAX
= I
PART NUMBER
LIM
– ΔI
V
OUT
IN
) Selection
L
/2.
(V
V
IN
IN
VALUE
2.2μH
2.2μH
3.3μH
4.7μH
4.7μH
2.2μH
4.7μH
4.7μH
3.3μH
2.2μH
4.7μH
3.3μH
2.2μH
– V
OUT
CURRENT
1100mA
1100mA
1200mA
1300mA
1000mA
MAX DC
510mA
530mA
410mA
950mA
630mA
450mA
750mA
700mA
870mA
)
0.086Ω
0.13Ω
0.33Ω
0.27Ω
0.19Ω
0.11Ω
0.08Ω
0.28Ω
0.17Ω
0.12Ω
0.2Ω
0.2Ω
0.1Ω
0.1Ω
DCR
MAX
OUT
1.25mm
HEIGHT
1.6mm
1.6mm
1.2mm
equals
2mm
1mm
1mm
1mm
1mm
1mm
1mm
1mm
1mm
2mm
3407afa
/V
IN
.