LM2595 Series Buck Regulator Design Procedure (Adjustable Output)
(Continued)
PROCEDURE (Adjustable Output Voltage Version)
2. Inductor Selection (L1)
A.
Calculate the inductor Volt
•
microsecond constant E
•
T
(V
•
µs), from the following formula:
EXAMPLE (Adjustable Output Voltage Version)
2. Inductor Selection (L1)
A.
Calculate the inductor Volt
•
microsecond constant
(E
•
T),
where V
SAT
= internal switch saturation voltage = 1V
and V
D
= diode forward voltage drop = 0.5V
B.
Use the E
•
T value from the previous formula and match
it with the E
•
T number on the vertical axis of the Inductor
Value Selection Guide shown in
Figure 7.
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 manufactur-
er’s part numbers listed in
Figure 8.
3. Output Capacitor Selection (C
OUT
)
A.
In the majority of applications, low ESR electrolytic or solid
tantalum capacitors between 47 µF and 330 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 330 µF.
For additional informa-
tion, see section on output capacitors in application in-
formation section.
B.
To simplify the capacitor selection procedure, refer to the
quick design table shown in
Figure 3.
This table contains dif-
ferent 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 volt-
age ratings are needed to satisfy the low ESR requirements
needed for low output ripple voltage.
B.
E
•
T = 34.8 (V
•
µs)
C.
I
LOAD
(max) = 1A
D.
From the inductor value selection guide shown in
Figure 7,
the inductance region intersected by the 35 (V
•
µs) horizon-
tal line and the 1A vertical line is 100 µH, and the inductor
code is L29.
E.
From the table in
Figure 8,
locate line L29, and select an
inductor part number from the list of manufacturers part num-
bers.
3. Output Capacitor SeIection (C
OUT
)
A.
See section on C
OUT
in Application Information section.
B.
From the quick design table shown in
Figure 3,
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 output capacitor
section, select a capacitor from the list of through hole elec-
trolytic 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 capaci-
tors from several different manufacturers are available.
82 µF, 35V Panasonic HFQ Series
82 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 capaci-
tor would work. A 35V rating was chosen, although a 50V rat-
ing 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. Re-
fer to the capacitor manufacturers data sheet for this informa-
tion.
4. Feedforward Capacitor (C
FF
)
The table shown in
Figure 3
contains feed forward capacitor
values for various output voltages. In this example, a 1 nF
capacitor is needed.
4. Feedforward Capacitor (C
FF
) (See
Figure 1
)
For output voltages greater than approximately 10V, an ad-
ditional capacitor is required. The compensation capacitor is
typically between 50 pF and 10 nF, and is wired in parallel
with the output voltage setting resistor, R
2
. It provides addi-
tional stability for high output voltages, low input-output volt-
ages, and/or very low ESR output capacitors, such as solid
tantalum capacitors.
This capacitor type can be ceramic, plastic, silver mica, etc.
(Because of the unstable characteristics of ceramic capaci-
tors made with Z5U material, they are not recommended.)
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