LM2574 datasheet(16/24 Pages) NSC

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LM2574 Series Buck Regulator Design Procedure (Continued)

Application Hints

INPUT CAPACITOR (C

IN)

To maintain stability, the regulator input pin must be by-

passed with at least a 22 µF electrolytic capacitor. The ca-

pacitor’s leads must be kept short, and located near the

regulator.

If the operating temperature range includes temperatures

below −25˚C, the input capacitor value may need to be

larger. With most electrolytic capacitors, the capacitance

value decreases and the ESR increases with lower tempera-

tures and age. Paralleling a ceramic or solid tantalum ca-

pacitor will increase the regulator stability at cold tempera-

tures. For maximum capacitor operating lifetime, the

capacitor’s RMS ripple current rating should be greater than

INDUCTOR SELECTION

All switching regulators have two basic modes of operation:

continuous and discontinuous. The difference between the

two types relates to the inductor current, whether it is flowing

continuously, or if it drops to zero for a period of time in the

normal switching cycle. Each mode has distinctively different

operating characteristics, which can affect the regulator per-

formance and requirements.

The LM2574 (or any of the Simple Switcher family) can be

used for both continuous and discontinuous modes of opera-

tion.

In many cases the preferred mode of operation is in the con-

tinuous mode. It offers better load regulation, lower peak

switch, inductor and diode currents, and can have lower out-

put ripple voltage. But it does require relatively large inductor

values to keep the inductor current flowing continuously, es-

pecially at low output load currents.

To simplify the inductor selection process, an inductor selec-

tion guide (nomograph) was designed (see

Figure 4 through

Figure 8). This guide assumes continuous mode operation,

and selects an inductor that will allow a peak-to-peak induc-

tor ripple current (

∆I

IND) to be a certain percentage of the

maximum design load current. In the LM2574 SIMPLE

SWITCHER, the peak-to-peak inductor ripple current per-

centage (of load current) is allowed to change as different

design load currents are selected. By allowing the percent-

age of inductor ripple current to increase for lower current

applications, the inductor size and value can be kept rela-

tively low.

1 Amp Diodes

Schottky

Fast Recovery

20V

1N5817

SR102

MBR120P

30V

1N5818

SR103

11DQ03

The

MBR130P

following

10JQ030

diodes

40V

1N5819

are all

SR104

rated to

11DQ04

100V

11JQ04

MBR140P

50V

MBR150

11DF1

SR105

10JF1

11DQ05

MUR110

11JQ05

HER102

60V

MBR160

SR106

11DQ06

11JQ06

90V

11DQ09

FIGURE 9. Diode Selection Guide

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