LM2585 datasheet(22/25 Pages) NSC

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Application Hints (Continued)

CIRCUIT LAYOUT GUIDELINES

As in any switching regulator, layout is very important. Rap-

idly switching currents associated with wiring inductance

generate voltage transients which can cause problems. For

minimal inductance and ground loops, keep the length of the

leads and traces as short as possible. Use single point

grounding or ground plane construction for best results.

Separate the signal grounds from the power grounds (as in-

dicated in

Figure 41). When using the Adjustable version,

physically locate the programming resistors as near the

regulator IC as possible, to keep the sensitive feedback wir-

ing short.

HEAT SINK/THERMAL CONSIDERATIONS

In many cases, no heat sink is required to keep the LM2585

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 the application).

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

LM2585). For a safe, conservative design, a temperature ap-

proximately 15˚C cooler than the maximum junction tem-

perature should be selected (110˚C).

4) LM2585 package thermal resistances

JA and θJC (given

in the Electrical Characteristics).

Total power dissipated (P

D) by the LM2585 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 x θJA.

Adding the junction temperature rise to the maximum ambi-

ent temperature gives the actual operating junction tempera-

ture:

J = ∆TJ +TA.

If the operating junction temperature exceeds the maximum

junction temperatue in item 3 above, then a heat sink is re-

quired. When using a heat sink, the junction temperature rise

can be determined by the following:

∆T

J = PD x(θJC + θInterface + θHeat Sink)

Again, the operating junction temperature will be:

J = ∆TJ +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 Na-

tional Semiconductor sales office in your area or the National

Semiconductor

Customer

Response

Center

(1-800-272-9959).

DS012515-58

FIGURE 41. Circuit Board Layout

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