Datasheet

LTC3127
12
3127f
A typical LTC3127 application circuit is shown on the front
page of this data sheet. The external component selection
is determined by the desired output voltage, input current
and ripple voltage requirements for each particular applica-
tion. However, basic guidelines and considerations for the
design process are provided in this section.
Buck-Boost Output Voltage Programming
The buck-boost output voltage is set by a resistive divider
according to the following formula:
V V
R
R
V
OUT
= • +






1 195 1
2
1
.
The external divider is connected to the output as shown
in Figure 3. The buck-boost converter utilizes input current
mode control, and the output divider resistance does not
play a role in the stability.
Figure 3. Setting the Buck-Boost Output Voltage
LTC3127
GND
1.8V b V
OUT
b 5.25V
FB
R1
3127 F03
R2
The LTC3127 can utilize small surface mount inductors
due to its fast 1.35MHz switching frequency. Inductor
values between 2.2μH and 4.7μH are suitable for most
applications. Larger values of inductance will allow slightly
greater output current capability by reducing the inductor
ripple current. Increasing the inductance above 10μH will
increase size while providing little improvement in output
current capability.
The inductor current ripple is typically set for 20% to
40% of the maximum inductor current. High frequency
ferrite core inductor materials reduce frequency depen-
dent power losses compared to cheaper powdered iron
types, improving efficiency. The inductor should have
low ESR (series resistance of the windings) to reduce the
I2R power losses, and must be able to support the peak
inductor current without saturating. Molded chokes and
some chip inductors usually do not have enough core area
to support the peak inductor currents of 2.5A seen on
the LTC3127. To minimize radiated noise, use a shielded
inductor. See Table 1 and the reference schematics for
suggested components and suppliers.
Table 1. Recommended Inductors
VENDOR PART/STYLE
Coilcraft
847-639-6400
www.coilcraft.com
LPO2506
LPS4012, LPS4018
MSS6122
MSS4020
MOS6020
DS1605, DO1608
XPL4020
Coiltronics
www.cooperet.com
SD52, SD53
SD3114, SD3118
Murata
714-852-2001
www.murata.com
LQH55D
Sumida
847-956-0666
www.sumida.com
CDH40D11
Taiyo-Yuden
www.t-yuden.com
NP04SB
NR3015
NR4018
TDK
847-803-6100
www.component.tdk.com
VLP, LTF
VLF, VLCF
Würth Elektronik
201-785-8800
www.we-online.com
WE-TPC Type S, M, MH
applicaTions inForMaTion
Buck-Boost Inductor Selection
To achieve high efficiency, a low ESR inductor should
be utilized for the buck-boost converter. The inductor
must have a saturation rating greater than the worst case
average inductor current plus half the ripple current.
The peak-to-peak inductor current ripple will be larger
in buck and boost mode than in the buck-boost region.
The peak-to-peak inductor current ripple for each mode
can be calculated from the following formulas, where L
is the inductance in μH:
∆
( )
( . )
(
, ,
I
V V V
V L MHz
L P P BUCK
OUT IN OUT
IN
−
=
−
• • 1 35
AA
I
V V V
V L M
L P P BOOST
IN OUT IN
OUT
)
∆
( )
( .
, ,−
=
−
• • 1 35 HHz
A
)
( )