LTC3419 Datasheet, PDF (9/16 Pages) Linear Technology – Dual Monolithic 600mA Synchronous Step-Down Regulator

LTC3419
APPLICATIONS INFORMATION
A general LTC3419 application circuit is shown in Figure 1.
External component selection is driven by the load require-
ment, 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 influence 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 :
ΔI_L
=
V_OUT
f_O • L
•
⎛
⎝⎜ ^1−
V_OUT
V_IN
⎞
⎠⎟
(1)
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 efficiency 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/cur-
rent and price/current relationship of an inductor. Toroid
V_IN
2.5V TO 5.5V
V_OUT2
C1
L2
C_F2
RUN2 V_IN RUN1
MODE
LTC3419
SW2
SW1
L1
C_F1
V_OUT1
C_OUT2 ^R4
V_FB2
V_FB1
GND
R3
R2
R1
C_OUT1
3419 F01
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 induc-
tor to use often depends more on the price versus size
requirements, and any radiated field/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-
MAX DC
FACTURER PART NUMBER VALUE CURRENT DCR HEIGHT
Taiyo Yuden CB2016T2R2M
CB2012T2R2M
CB2016T3R3M
2.2μH
2.2μH
3.3μH
510mA
530mA
410mA
0.13Ω 1.6mm
0.33Ω 1.25mm
0.27Ω 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 1100mA 0.1Ω 1mm
4.7μH 750mA 0.19Ω 1mm
FDK
FDKMIPF2520D 4.7μH 1100mA 0.11Ω 1mm
FDKMIPF2520D 3.3μH 1200mA 0.1Ω 1mm
FDKMIPF2520D 2.2μH 1300mA 0.08Ω 1mm
TDK
VLF3010AT4R7- 4.7μH 700mA 0.28Ω 1mm
MR70
VLF3010AT3R3- 3.3μH 870mA 0.17Ω 1mm
MR87
VLF3010AT2R2- 2.2μH 1000mA 0.12Ω 1mm
M1R0
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. 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 significant
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