LM2587 datasheet(24/26 Pages) NSC

24 / 26 page

Application Hints (Continued)

Boost:

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. Switchers Made Simple software is available

ona(31⁄2") diskette for IBM compatable computers from a

National Semiconductor sales office in your area or the Na-

tional

Semiconductor

Customer

Response

Center

(1-800-272-9959).

European Magnetic Vendor

Contacts

Please contact the following addresses for details of local

distributors or representatives:

Coilcraft

21 Napier Place

Wardpark North

Cumbernauld, Scotland G68 0LL

Phone: +44 1236 730 595

Fax: +44 1236 730 627

Pulse Engineering

Dunmore Road

Tuam

Co. Galway, Ireland

Phone: +353 93 24 107

Fax: +353 93 24 459

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