Datasheet

Data Sheet ADP3050
Rev. C | Page 13 of 20
low ESR (a 22 μF ceramic can have an ESR one-fifth that of a
22 μF solid tantalum), but may require more board area for the
same value of output capacitance. A few manufacturers have
recently improved upon their low voltage ceramic capacitors,
providing a smaller package with a lower ESR (NEC Tokin,
Murata, Taiyo Yuden, and AVX). Several ceramics can be used
in parallel to give an extremely low ESR and a good value of
capacitance. If the design is cost sensitive and not severely space
limited, several aluminum electrolytic capacitors can be used in
parallel (their size and ESR are larger than ceramic and solid
tantalum). OS-CON capacitors can also be used, but they are
typically larger and more expensive than ceramic or solid
tantalum capacitors.
Choosing an Output Capacitor
Use the following steps to choose an appropriate capacitor.
1. Decide the maximum output ripple voltage for the design,
and this determines your maximum ESR (remember that
V
RIPPLE
≈ ESR × I
RIPPLE
). Typical output ripple voltages range
between 0.5% and 2% of the output voltage. To lower the
output voltage ripple, there are only two choices: either
increase the inductor value, or use an output capacitor with
a lower ESR.
2. Decide what type of capacitor to use (tantalum, ceramic, or
others). Many more values, sizes, and voltage ratings are
available, so contact each manufacturer for a complete
product list. If a certain type of capacitor must be used and
space permits, use several devices in parallel to reduce the
total ESR.
3. Check the capacitor voltage rating and ripple current rating
to ensure it works for the application in question. These
ratings are derated for higher temperatures, so always check
the manufacturer’s data sheet.
4. Make sure the final choice for the output capacitor has
been optimized for cost, size, availability, and performance
yet still meets the required capacitance. The recommended
capacitance is in the 47 μF to 220 μF range.
CATCH DIODE SELECTION
The recommended catch diode is a Type 1N5818 Schottky or
equivalent. The low forward voltage drop (450 mV typical at
1 A) and fast switching speed of a Schottky rectifier provide the
best performance and efficiency. The 1N5818 is rated at 30 V
reverse voltage and 1 A average forward current. For lower
input voltages, use a lower voltage Schottky to reduce the diode
forward voltage drop and increase overall system efficiency; for
example, a 12 V to 5 V system does not need a 30 V diode. For
automotive applications, a 60 V Schottky may be necessary. The
average forward current for the catch diode is calculated by
IN
OUT
IN
OUT
AVGDIODE
V
VV
II
−
×=
)(
(5)
For the earlier continuous mode example (12 V to 5 V at
800 mA), the average diode current is
A47.0
12
512
8.0
)(
=
−
×=
AVGDIODE
I
(6)
For this system, a 1N5817 is a good choice (rated at 20 V and 1 A).
Do not use catch diodes rated less than 1 A. Even though the
average current can be less than 1 A under normal operating
conditions, as the diode current is much higher under fault
conditions. The worst-case fault condition for the diode occurs
when the regulator becomes slightly overloaded (sometimes
called a soft short). This is usually only a problem when the
input voltage to output voltage ratio is greater than 2.5. Under
this condition, the load current needed is slightly more than the
regulator can provide. The output voltage droops slightly, and
the switch stays on every cycle until the internal current limit is
reached. Under this condition, the load current can reach
around 1.2 A. For example, when using a system with an input
voltage of 24 V and an output voltage of 5 V, if a gradual overload
causes the output voltage to droop to 4 V, the average diode
current is
A0.1
24
424
2.1
)(
=
−
×=
AVGDIODE
I
(7)
If the system must survive such gradual overloads for a prolonged
period of time, ensure the diode chosen can survive these
conditions. A larger 2 A or 3 A diode can be used if necessary.
Table 4. Manufacturers
Inductor Manufacturers Capacitor Manufacturers Schottky Diode Manufacturers
Sumida AVX
Motorola
Coilcraft Kemet Diodes, Inc.
Cooper Bussmann Coiltronics Murata International Rectifier
NEC Tokin Nemco
Nihon Inter Electronics
Würth Elektronik
Vishay Sprague
Toko
NEC Tokin
Taiyo Yuden