LM2596 Series Buck Regulator Design Procedure (Adjustable Output)
(Continued)
PROCEDURE (Adjustable Output Voltage Version)
3. Output Capacitor Selection (COUT
EXAMPLE (Adjustable Output Voltage Version)
3. Output Capacitor SeIection (COUT
)
)
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 informa-
tion, see section on output capacitors in application in-
formation section.
A. See section on COUT 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 sec-
tion, 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 manufactur-
ers and the manufacturers series that are listed in the table be
used.
B. To simplify the capacitor selection procedure, refer to the
quick design table shown in Figure 3. This table contains
different output voltages, and lists various output capacitors
that will provide the best design solutions.
In this example, through hole aluminum electrolytic capacitors
from several different manufacturers are available.
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.
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 (CFF) (See Figure 1)
4. Feedforward Capacitor (CFF)
For output voltages greater than approximately 10V, an addi-
tional 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, R2. 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.
The table shown in Figure 3 contains feed forward capacitor
values for various output voltages. In this example, a 560 pF
capacitor is needed.
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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