10
LT3503
3503f
transient response. If your load is lower than 1A, then you
can decrease the value of the inductor and operate with
higher ripple current. This allows you to use a physically
smaller inductor, or one with a lower DCR resulting in
higher efficiency. There are several graphs in the Typical
Performance Characteristics section of this data sheet that
show the maximum load current as a function of input
voltage and inductor value for several popular output
voltages. Low inductance may result in discontinuous
mode operation, which is okay, but further reduces maxi-
mum load current. For details of the maximum output
current and discontinuous mode operation, see Linear
Technology Application Note 44.
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
maximum input voltage. The ON Semiconductor MBRM140
is a good choice; it is rated for 1A continuous forward
current and a maximum reverse voltage of 40V.
Input Capacitor
Bypass the input of the LT3503 circuit with a 1µF 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. A 1µF ceramic is adequate
to bypass the LT3503 and will easily handle the ripple
current. However, if the input power source has high
APPLICATIO S I FOR ATIO
WUU
U
impedance, or there is significant inductance due to long
wires or cables, additional bulk capacitance may be nec-
essary. 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 voltage ripple
at the LT3503 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 1µF capacitor is capable of this task, but only if it is
placed close to the LT3503 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 LT3503. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (underdamped) tank circuit. If the LT3503 circuit is
plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT3503’s
voltage rating. This situation is easily avoided; see the Hot
Plugging Safely section.
Output Capacitor
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by
the LT3503 to produce the DC output. In this role it
determines the output ripple so low impedance at the
switching frequency is important. The second function is
to store energy in order to satisfy transient loads and
stabilize the LT3503’s control loop.
Table 1. Inductor Vendors
Vendor URL Part Series Inductance Range (µH) Size (mm)
Sumida www.sumida.com CDRH4D28 1.2 to 4.7 4.5 × 4.5
CDRH5D28 2.5 to 10 5.5 × 5.5
CDRH8D28 2.5 to 33 8.3 × 8.3
Toko www.toko.com A916CY 2 to 12 6.3 × 6.2
D585LC 1.1 to 39 8.1 × 8.0
Würth Elektronik www.we-online.com WE-TPC(M) 1 to 10 4.8 × 4.8
WE-PD2(M) 2.2 to 22 5.2 × 5.8
WE-PD(S) 1 to 27 7.3 × 7.3
