6

LT3471

3471f

tions section for examples). In this case:

VV

R

R

OUT

REF

=

⎛

⎝

⎜

⎞

⎠

⎟

1

2

Select values of R1 and R2 according to the following

equation:

RR

V

V

OUT

REF

12

=

⎛

⎝

⎜

⎞

⎠

⎟

A good value for R2 is 15k, which sets the current in the

resistor divider chain to 1.00V/15k = 67µA.

Switching Frequency and Inductor Selection

The LT3471 switches at 1.2 MHz, allowing for small valued

inductors to be used. 4.7µH or 10µH will usually suffice.

Choose an inductor that can handle at least 1.4A without

saturating, and ensure that the inductor has a low DCR

Note that in some applications, the current handling

requirements of the inductor can be lower, such as in the

SEPIC topology where each inductor only carries one half

of the total switch current. For better efficiency, use similar

valued inductors with a larger volume. Many different

sizes and shapes are available from various manufactur-

ers. Choose a core material that has low losses at 1.2 MHz,

such as ferrite core.

Table 1. Inductor Manufacturers

Sumida

(847) 956-0666

www.sumida.com

TDK

(847) 803-6100

www.tdk.com

Murata

(714) 852-2001

www.murata.com

Soft-Start and Shutdown Features

To shut down the part, ground both SHDN/SS pins. To

shut down one switcher but not the other one, ground that

switcher’s SHDN/SS pin. The soft-start feature provides a

way to limit the inrush current drawn from the supply upon

start-up. To use the soft-start feature for either switcher,

slowly ramp up that switcher’s SHDN/SS pin. The rate of

voltage rise at the output of the switcher’s comparator (A1

or A3 for switcher 1 or switcher 2 respectively) tracks the

rate of voltage rise at the SHDN/SS pin once the SHDN/SS

pin has reached about 1.1V. The soft-start function will go

away once the voltage at the SHDN/SS pin exceeds 1.8V.

See the Peak Switch Current vs SHDN/SS Voltage graph in

the Typical Performance Characteristics section. The rate

of voltage rise at the SHDN/SS pin can easily be controlled

with a simple RC network connected between the control

signal and the SHDN/SS pin. Typical values for the RC

network are 4.7kΩ and 0.33µF, giving start-up times on

the order of milliseconds. This RC time constant can be

adjusted to give different start-up times. If different values

of resistance are to be used, keep in mind the SHDN/SS

Current vs SHDN/SS voltage graph along with the Peak

Switch Current vs SHDN/SS Voltage graph, both found in

the Typical Performance Characteristics section. The im-

pedance looking into the SHDN/SS pin depends on whether

like 100kΩ in series with a diode. If the voltage of the

50kΩ in series with a diode. This 100kΩ or 50kΩ imped-

ance can have a slight effect on the start-up time if you

choose the R in the RC soft-start network too large.

Another consideration is selecting the soft-start time so

that the soft-start feature is dominated by the RC network

section of the Applications Information for details.)

CAPACITOR SELECTION

Low ESR (equivalent series resistance) capacitors should

be used at the output to minimize the output ripple voltage.

Multi-layer ceramic capacitors are an excellent choice, as

they have extremely low ESR and are available in very

small packages. X5R dielectrics are preferred, followed by

X7R, as these materials retain the capacitance over wide

voltage and temperature ranges. A 4.7µF to 15µF output

capacitor is sufficient for most applications, but systems

with very low output currents may need only a 1µF or 2.2µF

output capacitor. Solid tantalum or OS-CON capacitors

can be used, but they will occupy more board area than a

ceramic and will have a higher ESR. Always use a capacitor

with a sufficient voltage rating.

Ceramic capacitors also make a good choice for the input

decoupling capacitor, which should be placed as close as

possible to the LT3471. A 4.7µF to 10µF input capacitor is