LM2594 datasheet(13/28 Pages) NSC

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LM2594/LM2594HV Series Buck Regulator Design Procedure (Adjustable

Output) (Continued)

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

4. Feedforward Capacitor (C

FF) (See Figure 1 )

For output voltages greater than approximately 10V, an addi-

tional capacitor is required. The compensation capacitor is

typically between 50 pF and 10 nF, and is wired in parallel

with the output voltage setting resistor, R

2. 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.

This capacitor type can be ceramic, plastic, silver mica, etc.

(Because of the unstable characteristics of ceramic capaci-

tors made with Z5U material, they are not recommended.)

4. Feedforward Capacitor (C

FF)

The table shown in

Figure 3 contains feed forward capacitor

values for various output voltages. In this example,a1nF

capacitor is needed.

5. Catch Diode Selection (D1)

A. The catch diode current rating must be at least 1.3 times

greater than the maximum load current. Also, if the power

supply design must withstand a continuous output short, the

diode should have a 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 are also a

good choice, but some types with an abrupt turn-off charac-

teristic may cause instability or EMl problems. Ultra-fast re-

covery 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.

5. Catch Diode Selection (D1)

A. Refer to the table shown in

Figure 11. Schottky diodes

provide the best performance, and in this example a 1A, 40V,

1N5819 Schottky diode would be a good choice. The 1A di-

ode rating is more than adequate and will not be over-

stressed even for a shorted output.

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