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 effi ciency, 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 effi ciency.
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
()
=
−
where I
OUT
is the maximum output load current pro-
grammed by the I
LIM
resistor. Peak reverse voltage is
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
LT3653 keeps the switch off when V
IN
> 33V (typical),
allowing the use of a 40V rated Schottky, even when V
IN
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 signifi cant 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
with V
OUT
has less than its nominal capacitance.
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.
I
LIM
Resistor
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
from the I
LIM
pin to GND programs the output current
limit. Table 1 details the I
LIM
resistor values for specifi c
desired output current limits
Table 1. Output Current Limit vs R
ILIM
Value
OUTPUT CURRENT LIMIT (A) R
ILIM
VALUE (kΩ)
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.
