LM2576 datasheet(18/26 Pages) ONSEMI

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LM2576

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beyond approximately 6.0 in2 (4000 mm2) will not improve

heat

dissipation

significantly.

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’).

Thermal Analysis and Design

The following procedure must be performed to determine

whether or not a heatsink will be required. First determine:

1. PD(max)

maximum regulator power dissipation in the

application.

2. TA(max)

maximum ambient temperature in the

application.

3. TJ(max)

maximum allowed junction temperature

(125

°C for the LM2576). For a conservative

design, the maximum junction temperature

should not exceed 110

°C to assure safe

operation. For every additional +10

°C

temperature rise that the junction must

withstand, the estimated operating lifetime

of the component is halved.

4. RqJC

package thermal resistance junction−case.

5. RqJA

package thermal resistance junction−ambient.

(Refer to Maximum Ratings on page 2 of this data sheet or

RqJC and RqJA values).

The following formula is to calculate the approximate

total power dissipated by the LM2576:

PD = (Vin x IQ) + d x ILoad x Vsat

where d is the duty cycle and for buck converter

ton

(quiescent current) and Vsat can be found in the

LM2576 data sheet,

Vin

is minimum input voltage applied,

is the regulator output voltage,

ILoad is the load current.

The dynamic switching losses during turn−on and

turn−off can be neglected if proper type catch diode is used.

Packages Not on a Heatsink (Free−Standing)

For a free−standing application when no heatsink is used,

the junction temperature can be determined by the following

expression:

TJ = (RqJA) (PD) + TA

where (RqJA)(PD) represents the junction temperature rise

caused by the dissipated power and TA is the maximum

ambient temperature.

Packages on a Heatsink

If the actual operating junction temperature is greater than

the selected safe operating junction temperature determined

in step 3, than a heatsink is required. The junction

temperature will be calculated as follows:

TJ = PD (RqJA + RqCS + RqSA) + TA

where

RqJC is the thermal resistance junction−case,

RqCS is the thermal resistance case−heatsink,

RqSA is the thermal resistance heatsink−ambient.

If the actual operating temperature is greater than the

selected safe operating junction temperature, then a larger

heatsink is required.

Some Aspects That can Influence Thermal Design

It should be noted that the package thermal resistance and

the junction temperature rise numbers are all approximate,

and there are many factors that will affect these numbers,

such as PC board size, shape, thickness, physical position,

location, board temperature, as well as whether the

surrounding air is moving or still.

Other factors are trace width, total printed circuit copper

area, copper thickness, single− or double−sided, multilayer

board, the amount of solder on the board or even color of the

traces.

The size, quantity and spacing of other components on the

board can also influence its effectiveness to dissipate the heat.

Figure 26. Inverting Buck−Boost Develops −12 V

1N5822

Output

Feedback

12 to 40 V

Unregulated

DC Input

Cin

100

3ON/OFF

+Vin

−12 V @ 0.7 A

Regulated

Output

Cout

2200

LM2576−12

ADDITIONAL APPLICATIONS

Inverting Regulator

An inverting buck−boost regulator using the LM2576−12

is shown in Figure 26. This circuit converts a positive input

voltage to a negative output voltage with a common ground

by bootstrapping the regulators ground to the negative

output voltage. By grounding the feedback pin, the regulator

senses the inverted output voltage and regulates it.

In this example the LM2576−12 is used to generate a

−12 V output. The maximum input voltage in this case

cannot exceed +28 V because the maximum voltage

appearing across the regulator is the absolute sum of the

input and output voltages and this must be limited to a

maximum of 40 V.

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