LM2574 datasheet(17/28 Pages) ONSEMI

Part #	LM2574

Description	0.5 A, Adjustable Output Voltage, Step-Down Switching Regulator

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Manufacturer	ONSEMI [ON Semiconductor]

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LM2574

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17

HORIZONTAL TIME BASE: 5.0 µs/DIV

Figure 24. Continuous Mode Switching

Current Waveforms

0.5 A

0 A

0.5 A

0 A

Power

Switch

Current

Waveform

Inductor

Current

Waveform

HORIZONTAL TIME BASE: 5.0 µs/DIV

Figure 25. Continuous Mode Switching

Current Waveforms

0.1 A

0 A

0.1 A

0 A

Power

Switch

Current

Waveform

Inductor

Current

Waveform

GENERAL RECOMMENDATIONS

Output Voltage Ripple and Transients

Source of the Output Ripple

Since the LM2574 is a switch mode power supply

regulator, its output voltage, if left unfiltered, will contain a

sawtooth ripple voltage at the switching frequency. The

output ripple voltage value ranges from 0.5% to 3% of the

output voltage. It is caused mainly by the inductor sawtooth

ripple current multiplied by the ESR of the output capacitor.

Short Voltage Spikes and How to Reduce Them

The regulator output voltage may also contain short

voltage spikes at the peaks of the sawtooth waveform (see

Figure 26). These voltage spikes are present because of the

fast switching action of the output switch, and the parasitic

inductance of the output filter capacitor. There are some

other important factors such as wiring inductance, stray

capacitance, as well as the scope probe used to evaluate these

transients, all these contribute to the amplitude of these

spikes. To minimize these voltage spikes, low inductance

capacitors should be used, and their lead lengths must be

kept short. The importance of quality printed circuit board

layout design should also be highlighted.

HORIZONTAL TIME BASE: 5.0 µs/DIV

Voltage spikes caused by switching action of the output

switch and the parasitic inductance of the output capacitor

Figure 26. Output Ripple Voltage Waveforms

Unfiltered

Output

Voltage

Filtered

Output

Voltage

Minimizing the Output Ripple

In order to minimize the output ripple voltage it is possible

to enlarge the inductance value of the inductor L1 and/or to

use a larger value output capacitor. There is also another way

to smooth the output by means of an additional LC filter

(20

µH, 100 µF), that can be added to the output (see

Figure 35) to further reduce the amount of output ripple and

transients. With such a filter it is possible to reduce the

output ripple voltage transients 10 times or more. Figure 26

shows the difference between filtered and unfiltered output

waveforms of the regulator shown in Figure 34.

The upper waveform is from the normal unfiltered output

of the converter, while the lower waveform shows the output

ripple voltage filtered by an additional LC filter.

Heatsinking and Thermal Considerations

The LM2574 is available in both 8–pin DIP and SO–16L

packages. When used in the typical application the copper lead

frame conducts the majority of the heat from the die, through

the leads, to the printed circuit copper. The copper and the

board are the heatsink for this package and the other heat

producing components, such as the catch diode and inductor.

For the best thermal performance, wide copper traces

should be used and all ground and unused pins should be

soldered to generous amounts of printed circuit board

copper, such as a ground plane. Large areas of copper

provide the best transfer of heat to the surrounding air. One

exception to this is the output (switch) pin, which should not

have large areas of copper in order to minimize coupling to

sensitive circuitry.

Additional improvement in heat dissipation can be

achieved even by using of double sided or multilayer boards

which can provide even better heat path to the ambient.

Using a socket for the 8–pin DIP package is not

recommended because socket represents an additional

thermal resistance, and as a result the junction temperature

will be higher.

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