LM2594 Просмотр технического описания (PDF) - ON Semiconductor

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LM2594

0.5 A, Step-Down Switching Regulator

ON Semiconductor 

LM2594

PROCEDURE (ADJUSTABLE OUTPUT VERSION: LM2594) (CONTINUED)

Procedure

Example

4. Inductor Selection (L1)

A. Use the following formula to calculate the inductor Volt x

microsecond [V x ms] constant:

E

ǒ Ǔ T + VIN * VOUT * VSAT

VOUT ) VD

VIN * VSAT ) VD

1000 ǒV

150 kHz

msǓ

B. Match the calculated E x T value with the corresponding

number on the vertical axis of the Inductor Value Selection

Guide shown in Figure 19. This E x T constant is a

measure of the energy handling capability of an inductor and

is dependent upon the type of core, the core area, the

number of turns, and the duty cycle.

C. Next step is to identify the inductance region intersected by

the E x T value and the maximum load current value on the

horizontal axis shown in Figure 19.

D. Select an appropriate inductor from Table 3.

The inductor chosen must be rated for a switching

frequency of 150 kHz and for a current rating of 1.15 x ILoad.

The inductor current rating can also be determined by

calculating the inductor peak current:

ǒ Ǔ Vin * Vout ton

Ip(max) + ILoad(max) )

2L

where ton is the “on” time of the power switch and

ton

+

Vout

Vin

x

1.0

fosc

4. Inductor Selection (L1)

A. Calculate E x T [V x ms] constant:

E T + ǒ12 * 5 * 1.0Ǔ

5 ) 0.5

1000 ǒV msǓ

12 * 1 ) 0.5 150 kHz

E T + ǒ6Ǔ 5.5 6.7ǒV msǓ

11.5

B. E x T = 19.2 [V x ms]

C. ILoad(max) = 0.5 A

Inductance Region = L20

D. Proper inductor value = 100 mH

Choose the inductor from Table 3.

5. Output Capacitor Selection (Cout)

A. Since the LM2594 is a forward−mode switching regulator

with voltage mode control, its open loop has 2−pole−1−zero

frequency characteristic. The loop stability is determined by

the output capacitor (capacitance, ESR) and inductance

values.

For stable operation use recommended values of the output

capacitors in Table 1.

Low ESR electrolytic capacitors between 180 mF and

1000 mF provide best results.

B. The capacitors voltage rating should be at least 1.5 times

greater than the output voltage, and often much higher

voltage rating is needed to satisfy low ESR requirement

6. Feedforward Capacitor (CFF)

It provides additional stability mainly for higher input voltages. For

Cff selection use Table 1. The compensation capacitor between

0.6 nF and 15 nF is wired in parallel with the output voltage setting

resistor R2, The capacitor type can be ceramic, plastic, etc..

5. Output Capacitor Selection (Cout)

A. In this example is recommended Nichicon PM

capacitors: 220 mF/25 V

6. Feedforward Capacitor (CFF)

In this example is recommended feedforward capacitor

1.5 nF.

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