LM2586 datasheet(28/30 Pages) NSC

28 / 30 page

Application Hints (Continued)

indicated in Figure 45). When using the Adjustable version,

physically locate the programming resistors as near the

regulator IC as possible, to keep the sensitive feedback

wiring short.

HEAT SINK/THERMAL CONSIDERATIONS

In many cases, a heat sink is not required to keep the

LM2586 junction temperature within the allowed operating

temperature 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 the application).

3) Maximum allowed junction temperature (125˚C for the

LM2586). For a safe, conservative design, a temperature

approximately 15˚C cooler than the maximum junction tem-

perature should be selected (110˚C).

4) LM2586 package thermal resistances

JA and

JC (given

in the Electrical Characteristics).

Total power dissipated (P

D) by the LM2586 can be estimated

as follows:

IN is the minimum input voltage, VOUT is the output voltage,

N is the transformer turns ratio, D is the duty cycle, and I

LOAD

is the maximum load current (and

∑I

LOAD is the sum of the

maximum load currents for multiple-output flyback regula-

tors). The duty cycle is given by:

where V

F is the forward biased voltage of the diode and is

typically 0.5V for Schottky diodes and 0.8V for fast recovery

diodes. V

SAT is the switch saturation voltage and can be

found in the Characteristic Curves.

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

∆T

J =PD •

JA.

Adding the junction temperature rise to the maximum ambi-

ent temperature gives the actual operating junction tempera-

ture:

J =

∆T

J +TA.

If the operating junction temperature exceeds the maximum

junction temperatue in item 3 above, 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)

Again, the operating junction temperature will be:

J =

∆T

J +TA

As before, if the maximum junction temperature is exceeded,

a larger heat sink is required (one that has a lower thermal

resistance).

Included in the Switchers 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

regulator junction temperature below the maximum operat-

ing temperature.

To further simplify the flyback regulator design procedure,

National Semiconductor is making available computer de-

sign software to be used with the Simple Switcher® line of

switching regulators. Switchers Made Simple is available

ona31⁄2" diskette for IBM compatible computers from a

National Semiconductor sales office in your area or the

National

Semiconductor

Customer

Response

Center

(1-800-272-9959).

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