LM2575T-5.0 datasheet(19/27 Pages) NSC

Teilenummer	LM2575T-5.0

Bauteilbeschribung	SIMPLE SWITCHER 1A Step-Down Voltage Regulator

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Application Hints (Continued)

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 tan-

talum can be used in parallel with aluminum electrolytics,

with the tantalum making up 10% or 20% of the total capaci-

tance.

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 LM2575 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 prob-

lems. 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 8 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

capacitor. (See the inductor selection in the application

hints.)

The voltage spikes are present because of the fast switching

action of the output switch, and the parasitic inductance 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 evaluate

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 15) 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 LM2575 (fixed voltage versions) feedback pin must be

wired to the output voltage point of the switching power

supply. When using the adjustable version, physically locate

both output voltage programming resistors near the LM2575

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

To maintain output voltage stability, the power ground con-

nections must be low-impedance (see Figure 2). For the

TO-3 style package, the case is ground. For the 5-lead

TO-220 style package, both the tab and pin 3 are ground and

either connection may be used, as they are both part of the

same copper lead frame.

With the N or M packages, all the pins labeled ground, power

ground, or signal ground should be soldered directly to wide

printed circuit board copper traces. This assures both low

inductance connections and good thermal properties.

HEAT SINK/THERMAL CONSIDERATIONS

In many cases, no heat sink is required to keep the LM2575

junction temperature within the allowed operating range. For

each application, to determine whether or not a heat sink will

be required, the following must be identified:

1.

Maximum ambient temperature (in the application).

2.

Maximum regulator power dissipation (in application).

3.

Maximum allowed junction temperature (150˚C for the

LM1575 or 125˚C for the LM2575). For a safe, conser-

vative design, a temperature approximately 15˚C cooler

than the maximum temperature should be selected.

4.

LM2575 package thermal resistances

θ

JA and

θ

JC.

Total power dissipated by the LM2575 can be estimated as

follows:

P

D =(VIN)(IQ)+(VO/VIN)(ILOAD)(VSAT)

where I

Q (quiescent current) and VSAT can be found in the

Characteristic Curves shown previously, V

IN is the applied

minimum input voltage, V

O is the regulated output voltage,

and I

LOAD is the load current. The dynamic losses during

turn-on and turn-off are negligible if a Schottky type catch

diode is used.

When no heat sink is used, the junction temperature rise can

be determined by the following:

∆T

J =(PD)(

θ

JA)

To arrive at the actual operating junction temperature, add

the junction temperature rise to the maximum ambient tem-

perature.

T

J =

∆T

J +TA

If the actual operating junction temperature is greater than

the selected safe operating junction temperature determined

in step 3, then a heat sink is required.

When using a heat sink, the junction temperature rise can be

determined by the following:

∆T

J =(PD)(

θ

JC +

θ

interface +

θ

Heat sink)

The operating junction temperature will be:

T

J =TA +

∆T

J

As above, if the actual operating junction temperature is

greater than the selected safe operating junction tempera-

ture, then a larger heat sink is required (one that has a lower

thermal resistance).

When using the LM2575 in the plastic DIP (N) or surface

mount (M) packages, several items about the thermal prop-

erties of the packages should be understood. The majority of

the heat is conducted out of the package through the leads,

with a minor portion through the plastic parts of the package.

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