SNVS124C – NOVEMBER 1999 – REVISED APRIL 2013
PROCEDURE (Adjustable Output Voltage Version)
2. Inductor Selection (L1)
EXAMPLE (Adjustable Output Voltage Version)
2. Inductor Selection (L1)
A.
Calculate the inductor Volt • microsecond constant E • T (V •
μs),
A.
Calculate the inductor Volt • microsecond constant
from the following formula:
(E • T),
where
•
•
V
SAT
= internal switch saturation voltage =
B.
E • T = 34.2 (V •
μs)
1.16V
C.
I
LOAD
(max) = 3A
V
D
= diode forward voltage drop = 0.5V
(4)
(5)
D.
From the inductor value selection guide shown in
the
inductance region intersected by the 34 (V •
μs)
horizontal line and
B.
Use the E • T value from the previous formula and match it with the 3A vertical line is 47
μH,
and the inductor code is L39.
the E • T number on the vertical axis of the Inductor Value Selection
E.
From the table in
locate line L39, and select an inductor
Guide shown in
part number from the list of manufacturers part numbers.
C.
on the horizontal axis, select the maximum load current.
D.
Identify the inductance region intersected by the E • T value and
the Maximum Load Current value. Each region is identified by an
inductance value and an inductor code (LXX).
E.
Select an appropriate inductor from the four manufacturer's part
numbers listed in
3. Output Capacitor Selection (C
OUT
)
A.
In the majority of applications, low ESR electrolytic or solid
tantalum capacitors between 82
μF
and 820
μF
provide the best
results. This capacitor should be located close to the IC using short
capacitor leads and short copper traces. Do not use capacitors
larger than 820
μF.
For additional information, see section on
output capacitors in
section.
B.
To simplify the capacitor selection procedure, refer to the quick
design table shown in
This table contains different output
voltages, and lists various output capacitors that will provide the best
design solutions.
C.
The capacitor voltage rating should be at least 1.5 times greater
than the output voltage, and often much higher voltage ratings are
needed to satisfy the low ESR requirements needed for low output
ripple voltage.
3. Output Capacitor SeIection (C
OUT
)
A.
See section on C
OUT
in
section.
B.
From the quick design table shown in
locate the output
voltage column. From that column, locate the output voltage closest
to the output voltage in your application. In this example, select the
24V line. Under the
section, select a
capacitor from the list of through hole electrolytic or surface mount
tantalum types from four different capacitor manufacturers. It is
recommended that both the manufacturers and the manufacturers
series that are listed in the table be used.
In this example, through hole aluminum electrolytic capacitors from
several different manufacturers are available.
220
μF/35V
Panasonic HFQ Series
150
μF/35V
Nichicon PL Series
C.
For a 20V output, a capacitor rating of at least 30V or more is
needed. In this example, either a 35V or 50V capacitor would work.
A 35V rating was chosen, although a 50V rating could also be used
if a lower output ripple voltage is needed.
Other manufacturers or other types of capacitors may also be used,
provided the capacitor specifications (especially the 100 kHz ESR)
closely match the types listed in the table. Refer to the capacitor
manufacturers data sheet for this information.
4. Feedforward Capacitor (C
FF
)
(See
For output voltages greater than approximately 10V, an additional
capacitor is required. The compensation capacitor is typically
between 100 pF and 33 nF, and is wired in parallel with the output
voltage setting resistor, R
2
. It provides additional stability for high
output voltages, low input-output voltages, and/or very low ESR
output capacitors, such as solid tantalum capacitors.
4. Feedforward Capacitor (C
FF
)
The table shown in
contains feed forward capacitor values
for various output voltages. In this example, a 560 pF capacitor is
needed.
(6)
This capacitor type can be ceramic, plastic, silver mica, etc.
(Because of the unstable characteristics of ceramic capacitors made
with Z5U material, they are not recommended.)
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