Datasheet

LM2598
SNVS125C –MARCH 1998–REVISED APRIL 2013 www.ti.com
Table 3. LM2598 Series Buck Regulator Design Procedure (Adjustable Output) (continued)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
B. To simplify the capacitor selection procedure, refer to the quick B. From the quick design table shown in Table 4, locate the output
design table shown in Table 4. This table contains different output voltage column. From that column, locate the output voltage closest
voltages, and lists various output capacitors that will provide the best to the output voltage in your application. In this example, select the
design solutions. 24V line. Under the OUTPUT CAPACITOR 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.
82 μF 35V Panasonic HFQ Series
82 μF 35V Nichicon PL Series
C. The capacitor voltage rating should be at least 1.5 times greater C. For a 20V output, a capacitor rating of at least 30V or more is
than the output voltage, and often much higher voltage ratings are needed. In this example, either a 35V or 50V capacitor would work.
needed to satisfy the low ESR requirements needed for low output A 35V rating was chosen although a 50V rating could also be used if
ripple voltage. 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 Figure 24) 4. Feedforward Capacitor (C
FF
)
For output voltages greater than approximately 10V, an additional The table shown in Table 4 contains feed forward capacitor values
capacitor is required. The compensation capacitor is typically for various output voltages. In this example, a 1 nF capacitor is
between 50 pF and 10 nF, and is wired in parallel with the output needed.
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.
(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.)
5. Catch Diode Selection (D1) 5. Catch Diode Selection (D1)
A. The catch diode current rating must be at least 1.3 times greater A. Refer to the table shown in Table 8. Schottky diodes provide the
than the maximum load current. Also, if the power supply design best performance, and in this example a 3A, 40V, 1N5822 Schottky
must withstand a continuous output short, the diode should have a diode would be a good choice. The 3A diode rating is more than
current rating equal to the maximum current limit of the LM2598. The adequate and will not be overstressed even for a shorted output.
most stressful condition for this diode is an overload or shorted
output condition.
B. The reverse voltage rating of the diode should be at least 1.25
times the maximum input voltage.
C. This diode must be fast (short reverse recovery time) and must be
located close to the LM2598 using short leads and short printed
circuit traces. Because of their fast switching speed and low forward
voltage drop, Schottky diodes provide the best performance and
efficiency, and should be the first choice, especially in low output
voltage applications. Ultra-fast recovery, or High-Efficiency rectifiers
are also a good choice, but some types with an abrupt turn-off
characteristic may cause instability or EMl problems. Ultra-fast
recovery diodes typically have reverse recovery times of 50 ns or
less. Rectifiers such as the 1N4001 series are much too slow and
should not be used.
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