Datasheet

LM2595
SNVS122B –MAY 1999–REVISED APRIL 2013 www.ti.com
Table 1. LM2595 Series Buck Regulator Design Procedure (Fixed Output) (continued)
PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)
3. Catch Diode Selection (D1) 3. 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 In this example, a 3A, 20V,
than the maximum load current. Also, if the power supply design 1N5820 Schottky diode will provide the best performance, and will
must withstand a continuous output short, the diode should have a not be overstressed even for a shorted output.
current rating equal to the maximum current limit of the LM2595. The
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 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
also provide good results. Ultra-fast recovery diodes typically have
reverse recovery times of 50 ns or less. Rectifiers such as the
1N5400 series are much too slow and should not be used.
4. Input Capacitor (C
IN
) 4. 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 pin to prevent large voltage voltage rating and the RMS current rating. With a nominal input
transients from appearing at the input. This capacitor should be voltage of 12V, an aluminum electrolytic capacitor with a voltage
located close to the IC using short leads. In addition, the RMS rating greater than 18V (1.5 × V
IN
) would be needed. The next
current rating of the input capacitor should be selected to be at least higher capacitor voltage rating is 25V.
½ the DC load current. The capacitor manufacturers data sheet must
The RMS current rating requirement for the input capacitor in a buck
be checked to assure that this current rating is not exceeded. The
regulator is approximately ½ the DC load current. In this example,
curve shown in Figure 26 shows typical RMS current ratings for
with a 1A load, a capacitor with a RMS current rating of at least 500
several different aluminum electrolytic capacitor values.
mA is needed. The curves shown in Figure 26 can be used to select
For an aluminum electrolytic, the capacitor voltage rating should be an appropriate input capacitor. From the curves, locate the 25V line
approximately 1.5 times the maximum input voltage. Caution must and note which capacitor values have RMS current ratings greater
be exercised if solid tantalum capacitors are used (see Application than 500 mA. Either a 180 μF or 220 μF, 25V capacitor could be
Information on input capacitor). The tantalum capacitor voltage rating used.
should be 2 times the maximum input voltage and it is recommended
For a through hole design, a 220 μF/25V electrolytic capacitor
that they be surge current tested by the manufacturer.
(Panasonic HFQ series or Nichicon PL series or equivalent) would
Use caution when using ceramic capacitors for input bypassing, be adequate. other types or other manufacturers capacitors can be
because it may cause severe ringing at the V
IN
pin. used provided the RMS ripple current ratings are adequate.
For additional information, see section on input capacitors in For surface mount designs, solid tantalum capacitors can be used,
Application Information section. but caution must be exercised with regard to the capacitor surge
current rating (see Application Information on input capacitors in this
data sheet). The TPS series available from AVX, and the 593D
series from Sprague are both surge current tested.
Table 2. LM2595 Fixed Voltage Quick Design Component Selection Table
Conditions Inductor Output Capacitor
Through Hole Electrolytic Surface Mount Tantalum
Output Load Max Input Inductance Inductor Panasonic Nichicon AVX TPS Sprague
Voltage Current Voltage (μH) (#) HFQ Series PL Series Series 595D Series
(V) (A) (V) (μF/V) (μF/V) (μF/V) (μF/V)
5 22 L24 330/16 330/16 220/10 330/10
7 33 L23 270/25 270/25 220/10 270/10
1
10 47 L31 220/25 220/35 220/10 220/10
3.3 40 68 L30 180/35 220/35 220/10 180/10
6 47 L13 220/25 220/16 220/10 220/10
0.5 10 68 L21 150/35 150/25 100/16 150/16
40 100 L20 150/35 82/35 100/16 100/20
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