LT3653

8

3653f

APPLICATIONS INFORMATION

Inductor Selection

A 4.7μH inductor is recommended for most LT3653 ap-

plications. This value provides a good tradeoff between

size and ripple current. The inductor’s RMS current rating

must be greater than the maximum load current and its

saturation current should be about 30% higher. The output

current limit circuit tightly controls the maximum average

inductor current therefore the inductor RMS current rating

does not have to be overrated to handle short-circuit or

overload conditions. For high efficiency, keep the series

resistance (DCR) less than 0.1Ω. Output voltage ripple

can be reduced by using a higher value inductor. The cost

is a larger physical size and poorer transient response.

A lower value inductor has higher ripple currents but is

physically smaller or, for the same size, has lower DCR

usually resulting in higher efficiency.

Catch Diode

The catch diode conducts current only during switch off

time. Average forward current in normal operation is

calculated from:

II

VV

V

D AVG

OUT

IN

OUT

IN

−

equal to the regulator input voltage. Use a Schottky diode

with a reverse voltage rating greater than the maximum

input voltage. The overvoltage protection feature in the

ranges up to 60V.

Input Capacitor

Bypass the input of the LT3653 circuit with a 4.7μF or higher

value ceramic capacitor of X7R or X5R type. Y5V types have

poor performance over temperature and applied voltage and

should not be used. If the input power source has high im-

pedance, or there is significant inductance due to long wires

or cables, additional bulk capacitance may be necessary.

This can be provided with a low performance electrolytic

capacitor. Step-down regulators draw current from the

input supply in pulses with very fast rise and fall times.

The input capacitor is required to reduce the resulting volt-

age ripple at the LT3653 and to force this very high frequency

switching current into a tight local loop, minimizing EMI.

Place the capacitor in close proximity to the LT3653 and

the catch diode (see the PCB Layout section).

Output Capacitor

A 10μF or greater ceramic capacitor is required for low

output ripple and good transient response. Ceramic ca-

pacitors have very low equivalent series resistance (ESR)

and provide the best ripple performance. Use X5R or X7R

types and keep in mind that a ceramic capacitor biased

High performance electrolytic capacitors can be used for

the output capacitor. Low ESR is important, so choose

one that is intended for use in switching regulators. Keep

the ESR less than 0.1Ω.

The LT3653 output current limit controls the maximum

current delivered from the LT3653 regulator. This allows

tighter control of the system power dissipation and also

protects the inductor and diode from overheating during

an overload or short-circuit condition. A resistor connected

desired output current limits

OUTPUT CURRENT LIMIT (A)

0.4

121

0.6

66.5

0.8

45.3

1

33.2

1.2

27.4

Boost Capacitor Selection

The boost capacitor is calculated with the following for-

mula:

C

IV

VV

MHz

BOOST

OUT MAX

OUT

IN

=

•

•• •

()

..

()

01

30

15

Typically, a 0.1μF capacitor is used.