LM2574 datasheet(18/24 Pages) NSC

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Application Hints (Continued)

pacitor and the amplitude of the inductor ripple current

(

∆I

IND). See the section on inductor ripple current in Applica-

tion Hints.

The lower capacitor values (100 µF- 330 µF) will allow typi-

cally 50 mV to 150 mV of output ripple voltage, while larger-

value capacitors will reduce the ripple to approximately

20 mV to 50 mV.

Output Ripple Voltage = (

∆I

IND) (ESR of COUT)

To further reduce the output ripple voltage, several standard

electrolytic capacitors may be paralleled, or a higher-grade

capacitor may be used. Such capacitors are often called

“high-frequency,” “low-inductance,” or “low-ESR.” These will

reduce the output ripple to 10 mV or 20 mV. However, when

operating in the continuous mode, reducing the ESR below

0.03

Ω can cause instability in the regulator.

Tantalum capacitors can have a very low ESR, and should

be carefully evaluated if it is the only output capacitor. Be-

cause of their good low temperature characteristics, a tanta-

lum can be used in parallel with aluminum electrolytics, with

the tantalum making up 10% or 20% of the total capacitance.

The capacitor’s ripple current rating at 52 kHz should be at

least 50% higher than the peak-to-peak inductor ripple cur-

rent.

CATCH DIODE

Buck regulators require a diode to provide a return path for

the inductor current when the switch is off. This diode should

be located close to the LM2574 using short leads and short

printed circuit traces.

Because of their fast switching speed and low forward volt-

age drop, Schottky diodes provide the best efficiency, espe-

cially in low output voltage switching regulators (less than

5V). Fast-Recovery, High-Efficiency, or Ultra-Fast Recovery

diodes are also suitable, but some types with an abrupt turn-

off characteristic may cause instability and EMI problems. A

fast-recovery diode with soft recovery characteristics is a

better choice. Standard 60 Hz diodes (e.g., 1N4001 or

1N5400, etc.) are also not suitable. See

Figure 9 for Schot-

tky and “soft” fast-recovery diode selection guide.

OUTPUT VOLTAGE RIPPLE AND TRANSIENTS

The output voltage of a switching power supply will contain a

sawtooth ripple voltage at the switcher frequency, typically

about 1% of the output voltage, and may also contain short

voltage spikes at the peaks of the sawtooth waveform.

The output ripple voltage is due mainly to the inductor saw-

tooth ripple current multiplied by the ESR of the output ca-

pacitor. (See the inductor selection in the application hints.)

The voltage spikes are present because of the the fast

switching action of the output switch, and the parasitic induc-

tance of the output filter capacitor. To minimize these voltage

spikes, special low inductance capacitors can be used, and

their lead lengths must be kept short. Wiring inductance,

stray capacitance, as well as the scope probe used to evalu-

ate these transients, all contribute to the amplitude of these

spikes.

An additional small LC filter (20 µH & 100 µF) can be added

to the output (as shown in

Figure 16 ) to further reduce the

amount of output ripple and transients. A 10 x reduction in

output ripple voltage and transients is possible with this filter.

FEEDBACK CONNECTION

The LM2574 (fixed voltage versions) feedback pin must be

wired to the output voltage point of the switching power sup-

ply. When using the adjustable version, physically locate

both output voltage programming resistors near the LM2574

to avoid picking up unwanted noise. Avoid using resistors

greater than 100 k

Ω because of the increased chance of

noise pickup.

ON /OFF INPUT

For normal operation, the ON /OFF pin should be grounded

or driven with a low-level TTL voltage (typically below 1.6V).

To put the regulator into standby mode, drive this pin with a

high-level TTL or CMOS signal. The ON /OFF pin can be

safely pulled up to +V

IN without a resistor in series with it.

The ON /OFF pin should not be left open.

GROUNDING

The 8-pin molded DIP and the 14-pin surface mount pack-

age have separate power and signal ground pins. Both

ground pins should be soldered directly to wide printed cir-

cuit board copper traces to assure low inductance connec-

tions and good thermal properties.

THERMAL CONSIDERATIONS

The 8-pin DIP (N) package and the 14-pin Surface Mount

(M) package are molded plastic packages with solid copper

lead frames. The copper lead frame conducts the majority of

the heat from the die, through the leads, to the printed circuit

board copper, which acts as the heat sink. For 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 (lower thermal resistance) to the surrounding air, and

even double-sided or multilayer boards provide better heat

paths to the surrounding air. Unless the power levels are

small, using a socket for the 8-pin package is not recom-

mended because of the additional thermal resistance it intro-

duces, and the resultant higher junction temperature.

Because of the 0.5A current rating of the LM2574, the total

package power dissipation for this switcher is quite low,

ranging from approximately 0.1W up to 0.75W under varying

conditions. In a carefully engineered printed circuit board,

both the N and the M package can easily dissipate up to

0.75W, even at ambient temperatures of 60˚C, and still keep

the maximum junction temperature below 125˚C.

A curve displaying thermal resistance vs. pc board area for

the two packages is shown in the Typical Performance Char-

acteristics curves section of this data sheet.

These thermal resistance numbers are approximate, and

there can be many factors that will affect the final thermal re-

sistance. Some of these factors include board size, shape,

thickness, position, location, and board temperature. Other

factors are, the area of printed circuit copper, copper thick-

ness, trace width, multi-layer, single- or double-sided, and

the amount of solder on the board. The effectiveness of the

pc board to dissipate heat also depends on the size, number

and spacing of other components on the board. Further-

more, some of these components, such as the catch diode

and inductor will generate some additional heat. Also, the

thermal resistance decreases as the power level increases

because of the increased air current activity at the higher

power levels, and the lower surface to air resistance coeffi-

cient at higher temperatures.

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