Datasheet

LTC3407A
8
3407afa
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
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
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
specifi ed maximum, the inductor value should be chosen
according to the following 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
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.
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
MANUF-
ACTURER PART NUMBER VALUE
MAX DC
CURRENT DCR HEIGHT
Taiyo
Yuden
CB2016T2R2M
CB2012T2R2M
CB2016T3R3M
2.2µH
2.2µH
3.3µH
510mA
530mA
410mA
0.13
0.33
0.27
1.6mm
1.25mm
1.6mm
Panasonic ELT5KT4R7M 4.7µH 950mA 0.2 1.2mm
Sumida CDRH2D18/LD 4.7µH 630mA 0.086 2mm
Murata LQH32CN4R7M23 4.7µH 450mA 0.2 2mm
Taiyo
Yuden
NR30102R2M
NR30104R7M
2.2µH
4.7µH
1100mA
750mA
0.1
0.19
1mm
1mm
FDK FDKMIPF2520D
FDKMIPF2520D
FDKMIPF2520D
4.7µH
3.3µH
2.2µH
1100mA
1200mA
1300mA
0.11
0.1
0.08
1mm
1mm
1mm
TDK VLF3010AT4R7-
MR70
VLF3010AT3R3-
MR87
VLF3010AT2R2-
M1R0
4.7µH
3.3µH
2.2µH
700mA
870mA
1000mA
0.28
0.17
0.12
1mm
1mm
1mm
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:
I
RMS
≈I
MAX
V
OUT
(V
IN
–V
OUT
)
V
IN
where the maximum average output current I
MAX
equals
the peak current minus half the peak-to-peak ripple cur-
rent, I
MAX
= I
LIM
– ΔI
L
/2.
APPLICATIONS INFORMATION
Figure 1. LTC3407A 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
LTC3407A
C
IN
R7
POWER-ON
RESET
C1C2
L1
L2
R4 R2
R1
R3
C
OUT2
C4 C3
C
OUT1
3407A F01
PULSESKIP*
BURST*
*MODE/SYNC = 0V: PULSE SKIP
MODE/SYNC = V
IN
: Burst Mode
R6 R5