LTC3407A Datasheet, PDF (8/16 Pages) Linear Technology – Dual Synchronous 600mA, 1.5MHz Step-Down DC/DC Regulator

LTC3407A
APPLICATIONS INFORMATION
V_IN = 2.5V TO 5.5V
R6
V_OUT2
C4
C_IN
BURST*
V_IN
MODE/SYNC
POR
PULSESKIP*
LTC3407A
RUN/SS2 RUN/SS1
L2
SW2
SW1
C2
R4
C_OUT2
V_FB2
V_FB1
GND
R3
R7
POWER-ON
RESET
L1
C1
R2
R1
R5
V_OUT1
C3
C_OUT1
*MODE/SYNC = 0V: PULSE SKIP
MODE/SYNC = V_IN: Burst Mode
3407A F01
Figure 1. LTC3407A General Schematic
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 :

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
specified 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 efficiency in
the upper range of low current operation. In Burst Mode
operation, lower inductance values will cause the burst
frequency to increase.
8
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 field/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-
MAX DC
ACTURER 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.
3407afa