LT3505
APPLICATIONS INFORMATION
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
L = 1.2 (VOUT + VD)/fSW
Catch Diode
Depending on load current, a 1A to 2A Schottky diode is
recommended for the catch diode, D1. The diode must
have a reverse voltage rating equal to or greater than the
maximuminputvoltage.TheONSemiconductorMBRM140
is a good choice; it is rated for 1A continuous forward
current and a maximum reverse voltage of 40V.
where VD is the voltage drop of the catch diode (~0.4V),
L is in µH and fSW is in MHz. With this value there will
be no subharmonic oscillation for applications with 50%
or greater duty cycle. The inductor’s RMS current rating
must be greater than your maximum load current and
its saturation current should be about 30% higher. For
robustoperationinfaultconditions, thesaturationcurrent
should be above 2.2A. To keep efficiency high, the series
resistance (DCR) should be less than 0.1 . Table 2 lists
several vendors and types that are suitable.
Input Capacitor
The input of the LT3505 circuit must be bypassed with a
X7R or X5R type ceramic capacitor. Y5V types have poor
performance over temperature and applied voltage and
should not be used. For switching frequencies higher than
750kHz,bypasstheinputwitha1µForhighervalueceramic
capacitor.Forswitchingfrequenciesbelow750kHz,bypass
the input with a 2.2µF or higher value ceramic capacitor.
If the input power source has high impedance, or there is
significantinductanceduetolongwiresorcables,additional
bulk capacitance may be necessary. This can be provided
with a low performance electrolytic capacitor.
Of course, such a simple design guide will not always
result in the optimum inductor for your application. A
larger value provides a higher maximum load current and
reduces output voltage ripple at the expense of slower
transient response. If your load is lower than 1.2A, then
you can decrease the value of the inductor and operate
with higher ripple current. This allows you to use a physi-
cally smaller inductor, or one with a lower DCR resulting
in higher efficiency. There are several graphs in the Typical
PerformanceCharacteristicssectionofthisdatasheetthat
show the maximum load current as a function of input
voltage and inductor value for several popular output volt-
ages. Low inductance may result in discontinuous mode
operation, which is okay, but further reduces maximum
load current. For details on maximum output current and
discontinuous mode operation, see Linear Technology
Application Note 44.
Step-down regulators draw current from the input sup-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT3505 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
To accomplish this task, the input bypass capacitor must
be placed close to the LT3505 and the catch diode; see
the PCB Layout section. A second precaution regarding
the ceramic input capacitor concerns the maximum input
voltage rating of the LT3505. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (underdamped) tank circuit. If the LT3505 circuit
is plugged into a live supply, the input voltage can ring to
Table 2. Inductor Vendors
VENDOR
URL
PART SERIES
INDUCTANCE RANGE (µH)
Size (mm)
Sumida
www.sumida.com
CDRH4D28
CDRH5D28
CDRH5D28
1.2 to 4.7
2.5 to 10
2.5 to 33
4.5 × 4.5
5.5 × 5.5
8.3 × 8.3
Toko
www.toko.com
A916CY
D585LC
2 to 12
1.1 to 39
6.3 × 6.2
8.1 × 8.0
Würth Elektronik
www.we-online.com
WE-TPC(M)
WE-PD2(M)
WE-PD(S)
1 to 10
2.2 to 22
1 to 27
4.8 × 4.8
5.2 × 5.8
7.3 × 7.3
3505fc
13
Linear [ Linear ]
