Datasheet

LM2594, LM2594HV
SNVS118C –DECEMBER 1999–REVISED APRIL 2013 www.ti.com
Table 1. LM2594/LM2594HV 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 1A, 20V,
than the maximum load current. Also, if the power supply design 1N5817 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 LM2594. 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 LM2594 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
1N4001 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 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 12V, an aluminum electrolytic capacitor with a voltage
the input capacitor should be selected to be at least ½ the DC load rating greater than 18V (1.5 × V
IN
) would be needed. The next
current. The capacitor manufacturers data sheet must be checked to higher capacitor voltage rating is 25V.
assure that this current rating is not exceeded. The curve shown in
The RMS current rating requirement for the input capacitor in a buck
Figure 25 shows typical RMS current ratings for several different
regulator is approximately ½ the DC load current. In this example,
aluminum electrolytic capacitor values.
with a 400 mA load, a capacitor with a RMS current rating of at least
This capacitor should be located close to the IC using short leads 200 mA is needed. The curves shown in Figure 25 can be used to
and the voltage rating should be approximately 1.5 times the select an appropriate input capacitor. From the curves, locate the
maximum input voltage. 25V line and note which capacitor values have RMS current ratings
greater than 200 mA. Either a 47 μF or 68 μF, 25V capacitor could
If solid tantalum input capacitors are used, it is recommended that
be used.
they be surge current tested by the manufacturer.
For a through hole design, a 68 μF/25V electrolytic capacitor
Use caution when using ceramic capacitors for input bypassing,
(Panasonic HFQ series or Nichicon PL series or equivalent) would
because it may cause severe ringing at the V
IN
pin.
be adequate. Other types or other manufacturers capacitors can be
For additional information, see EXTERNAL COMPONENTS
used provided the RMS ripple current ratings are adequate.
section on input capacitors in Application Information section.
For surface mount designs, solid tantalum capacitors are
recommended. The TPS series available from AVX, and the 593D
series from Sprague are both surge current tested.
Table 2. LM2594/LM2594HV Fixed Voltage Quick Design Component Selection Table
Conditions Inductor Output Capacitor
Through Hole Surface Mount
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)
3.3 0.5 5 33 L14 220/16 220/16 100/16 100/6.3
7 47 L13 120/25 120/25 100/16 100/6.3
10 68 L21 120/25 120/25 100/16 100/6.3
40 100 L20 120/35 120/35 100/16 100/6.3
6 68 L4 120/25 120/25 100/16 100/6.3
0.2 10 150 L10 120/16 120/16 100/16 100/6.3
40 220 L9 120/16 120/16 100/16 100/6.3
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