LM2576 datasheet(17/24 Pages) NSC

17 / 24 page

Application Hints (Continued)

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 LM2576 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 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 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 LM2576 (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 LM2576

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

5-lead TO-220 and TO-263 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.

HEAT SINK/THERMAL CONSIDERATIONS

In many cases, only a small heat sink is required to keep the

LM2576 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:

Maximum ambient temperature (in the application).

Maximum regulator power dissipation (in application).

Maximum allowed junction temperature (125˚C for the

LM2576). For a safe, conservative design, a tempera-

ture approximately 15˚C cooler than the maximum tem-

peratures should be selected.

LM2576 package thermal resistances

JA and

JC.

Total power dissipated by the LM2576 can be estimated as

follows:

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)(

JA)

To arrive at the actual operating junction temperature, add

the junction temperature rise to the maximum ambient tem-

perature.

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)(

JC +

interface +

Heat sink)

The operating junction temperature will be:

J =TA +

∆T

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).

Included on the Switcher Made Simple design software is a

more precise (non-linear) thermal model that can be used to

determine junction temperature with different input-output

parameters or different component values. It can also calcu-

late the heat sink thermal resistance required to maintain the

regulators junction temperature below the maximum operat-

ing temperature.

Additional Applications

INVERTING REGULATOR

Figure 10 shows a LM2576-12 in a buck-boost configuration

to generate a negative 12V output from a positive input

voltage. This circuit bootstraps the regulator’s ground pin to

the negative output voltage, then by grounding the feedback

pin, the regulator senses the inverted output voltage and

regulates it to −12V.

For an input voltage of 12V or more, the maximum available

output current in this configuration is approximately 700 mA.

At lighter loads, the minimum input voltage required drops to

approximately 4.7V.

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