Datasheet

LM2595
www.ti.com SNVS122B –MAY 1999–REVISED APRIL 2013
Table 3. LM2595 Series Buck Regulator Design Procedure (Adjustable Output) (continued)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
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
than the maximum load current. Also, if the power supply design
Schottky diodes provide the best performance, and in this example a
must withstand a continuous output short, the diode should have a
3A, 40V, 1N5822 Schottky diode would be a good choice.
current rating equal to the maximum current limit of the LM2595. The
The 3A diode rating is more than adequate and will not be
most stressful condition for this diode is an overload or shorted
overstressed even for a shorted output.
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 LM2595 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.
6. Input Capacitor (C
IN
) 6. Input Capacitor (C
IN
)
A low ESR aluminum or tantalum bypass capacitor is needed The important parameters for the Input capacitor are the input
between the input pin and ground to prevent large voltage transients voltage rating and the RMS current rating. With a nominal input
from appearing at the input. In addition, the RMS current rating of voltage of 28V, an aluminum electrolytic aluminum electrolytic
the input capacitor should be selected to be at least ½ the DC load capacitor with a voltage rating greater than 42V (1.5 × V
IN
) would be
current. The capacitor manufacturers data sheet must be checked to needed. Since the the next higher capacitor voltage rating is 50V, a
assure that this current rating is not exceeded. The curve shown in 50V capacitor should be used. The capacitor voltage rating of (1.5 ×
Figure 26 shows typical RMS current ratings for several different V
IN
) is a conservative guideline, and can be modified somewhat if
aluminum electrolytic capacitor values. desired.
This capacitor should be located close to the IC using short leads The RMS current rating requirement for the input capacitor of a buck
and the voltage rating should be approximately 1.5 times the regulator is approximately ½ the DC load current. In this example,
maximum input voltage. with a 1A load, a capacitor with a RMS current rating of at least 500
mA is needed.
If solid tantalum input capacitors are used, it is recomended that they
be surge current tested by the manufacturer. The curves shown in Figure 26 can be used to select an appropriate
input capacitor. From the curves, locate the 50V line and note which
se caution when using a high dielectric constant ceramic capacitor
capacitor values have RMS current ratings greater than 500 mA.
for input bypassing, because it may cause severe ringing at the V
IN
Either a 100 μF or 120 μF, 50V capacitor could be used.
pin.
For a through hole design, a 120 μF/50V electrolytic capacitor
For additional information, see section on input capacitors in
(Panasonic HFQ series or Nichicon PL series or equivalent) would
Application Information section.
be adequate. Other types or other manufacturers capacitors can be
used provided the RMS ripple current ratings are adequate.
For surface mount designs, solid tantalum capacitors can be used,
but caution must be exercised with regard to the capacitor surge
current rting (see Application Information or input capacitors in this
data sheet). The TPS series available from AVX, and the 593D
series from Sprague are both surge current tested.
To further simplify the buck regulator design procedure, Texas
Instruments is making available computer design software to be
used with the Simple Switcher line ot switching regulators.
Switchers Made Simple (version 4.2 or later) is available on a 3½″
diskette for IBM compatible computers.
Table 4. Output Capacitor and Feedforward Capacitor Selection Table
Output Through Hole Electrolytic Output Capacitor Surface Mount Tantalum Output Capacitor
Voltage
Panasonic Nichicon PL Feedforward AVX TPS Sprague Feedforward
(V)
HFQ Series Series Capacitor Series 595D Series Capacitor
(μF/V) (μF/V) (μF/V) (μF/V)
1.2 330/50 330/50 0 330/6.3 330/6.3 0
4 220/25 220/25 4.7 nF 220/10 220/10 4.7 nF
6 220/25 220/25 3.3 nF 220/10 220/10 3.3 nF
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