LM2576 datasheet(17/28 Pages) MOTOROLA

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LM2576

MOTOROLA ANALOG IC DEVICE DATA

Do Not Operate an Inductor Beyond its

Maximum Rated Current

Exceeding an inductor’s maximum current rating may

cause the inductor to overheat because of the copper wire

losses, or the core may saturate. Core saturation occurs

when the flux density is too high and consequently the cross

sectional area of the core can no longer support additional

lines of magnetic flux.

This causes the permeability of the core to drop, the

inductance value decreases rapidly and the inductor begins

to look mainly resistive. It has only the DC resistance of the

winding. This can cause the switch current to rise very rapidly

and force the LM2576 internal switch into cycle–by–cycle

current limit, thus reducing the DC output load current. This

can also result in overheating of the inductor and/or the

LM2576. Different inductor types have different saturation

characteristics, and this should be kept in mind when

selecting an inductor.

0.4 A

0 A

0.4 A

0 A

Inductor

Current

Waveform

Power

Switch

Current

Waveform

Figure 24. Discontinuous Mode Switching Current

Waveforms

HORIZONTAL TIME BASE: 5.0

µs/DIV

GENERAL RECOMMENDATIONS

Output Voltage Ripple and Transients

Source of the Output Ripple

Since the LM2576 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 25). 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.

Unfiltered

Output

Voltage

Filtered

Output

Voltage

HORIZONTAL TIME BASE: 5.0

µs/DIV

Figure 25. Output Ripple Voltage Waveforms

Voltage spikes

caused by

switching action

of the output

switch and the

parasitic

inductance of the

output capacitor

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 34) 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 25 shows the

difference between filtered and unfiltered output waveforms

of the regulator shown in Figure 34.

The lower waveform is from the normal unfiltered output of

the converter, while the upper waveform shows the output

ripple voltage filtered by an additional LC filter.

Heatsinking and Thermal Considerations

The Through–Hole Package TO–220

The LM2576 is available in two packages, a 5–pin

TO–220(T, TV) and a 5–pin surface mount D2PAK(D2T).

Although the TO–220(T) package needs a heatsink under

most conditions, there are some applications that require no

heatsink to keep the LM2576 junction temperature within the

allowed operating range. Higher ambient temperatures

require some heat sinking, either to the printed circuit (PC)

board or an external heatsink.

The Surface Mount Package D2PAK and its Heatsinking

The other type of package, the surface mount D2PAK, is

designed to be soldered to the copper on the PC board. The

copper and the board are the heatsink for this package and

the other heat producing components, such as the catch

diode and inductor. The PC board copper area that the

package is soldered to should be at least 0.4 in2 (or

260 mm2) and ideally should have 2 or more square inches

(1300 mm2) of 0.0028 inch copper. Additional increases of

copper area beyond approximately 6.0 in2 (4000 mm2) will

not improve heat dissipation significantly. If further thermal

improvements are needed, double sided or multilayer PC

boards with large copper areas should be considered. In

order to achieve the best thermal performance, it is highly

recommended to use wide copper traces as well as large

areas of copper in the printed circuit board layout. The only

exception to this is the OUTPUT (switch) pin, which should

not have large areas of copper (see page 8 ’PCB Layout

Guideline’).

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