Datasheet

LTC3419
9
3419fa
A general LTC3419 application circuit is shown in Figure 1.
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.
Inductor Selection
Although the inductor does not infl uence the operating
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
V
fL
V
V
L
OUT
O
OUT
IN
=−
⎛
⎝
⎜
⎞
⎠
⎟
•
•()11
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
40% of the maximum output load current. So, for a 600mA
regulator, ΔI
L
= 240mA (40% of 600mA).
The inductor value will also have an effect on Burst Mode
operation. The transition to low current operation begins
when the peak inductor current falls below a level set by
the internal burst clamp. Lower inductor values result in
higher ripple current which causes the transition to occur
at lower load currents. This causes a dip in effi ciency in
the upper range of low current operation. Furthermore,
lower inductance values will cause the bursts to occur
with increased frequency.
Inductor Core Selection
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. Toroid
APPLICATIONS INFORMATION
Figure 1. LTC3419 General Schematic
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 inductors with similar
electrical characteristics. The choice of which style
inductor to use often depends more on the price versus
size requirements, and any radiated fi eld/EMI requirements,
than on what the LTC3419 requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3419 applications.
Table 1. Representative Surface Mount Inductors
MANU-
FACTURER PART NUMBER VALUE
MAX DC
CURRENT DCR HEIGHT
Taiyo Yuden CB2016T2R2M
CB2012T2R2M
CB2016T3R3M
2.2μH
2.2μH
3.3μH
510mA
530mA
410mA
0.13Ω
0.33Ω
0.27Ω
1.6mm
1.25mm
1.6mm
Panasonic ELT5KT4R7M 4.7μH 950mA
0.2Ω
1.2mm
Sumida CDRH2D18/LD 4.7μH 630mA
0.086Ω
2mm
Murata
LQH32CN4R7M23 4.7μH 450mA
0.2Ω
2mm
Taiyo Yuden NR30102R2M
NR30104R7M
2.2μH
4.7μH
1100mA
750mA
0.1Ω
0.19Ω
1mm
1mm
FDK FDKMIPF2520D
FDKMIPF2520D
FDKMIPF2520D
4.7μH
3.3μH
2.2μH
1100mA
1200mA
1300mA
0.11Ω
0.1Ω
0.08Ω
1mm
1mm
1mm
TDK VLF3010AT4R7-
MR70
VLF3010AT3R3-
MR87
VLF3010AT2R2-
M1R0
4.7μH
3.3μH
2.2μH
700mA
870mA
1000mA
0.28Ω
0.17Ω
0.12Ω
1mm
1mm
1mm
V
IN
RUN2 RUN1
LTC3419
V
FB2
SW2
SW1
MODE
V
FB1
C
F2
C
F1
GND
V
IN
2.5V TO 5.5V
V
OUT2
V
OUT1
3419 F01
R4 R2
R3
R1
L2 L1
C
OUT2
C
OUT1
C1
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:
II
VVV
V
RMS MAX
OUT IN OUT
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. This formula has a maximum at
V
IN
= 2V
OUT
, where I
RMS
= I
OUT
/2. This simple worst-case
is commonly used to design because even signifi cant