lm2587sx-5.0 National Semiconductor Corporation, lm2587sx-5.0 Datasheet - Page 25

• Manufacturer Part Number: lm2587sx-5.0
• Description: Simple Switcher 5a Flyback Regulator
• Manufacturer: National Semiconductor Corporation

Application Hints
4) LM2587 package thermal resistances θ
in the Electrical Characteristics).
Total power dissipated (P
as follows:
Boost:
V
N is the transformer turns ratio, D is the duty cycle, and I
is the maximum load current (and
maximum load currents for multiple-output flyback regula-
tors). The duty cycle is given by:
Boost:
where V
typically 0.5V for Schottky diodes and 0.8V for fast recovery
diodes. V
found in the Characteristic Curves.
When no heat sink is used, the junction temperature rise is:
Adding the junction temperature rise to the maximum ambi-
ent temperature gives the actual operating junction tempera-
ture:
IN
is the minimum input voltage, V
F
SAT
is the forward biased voltage of the diode and is
is the switch saturation voltage and can be
∆T
T
J
J
D
= ∆T
) by the LM2587 can be estimated
= P
D
J
(Continued)
x θ
+ T
OUT
∑
JA
A
I
.
LOAD
.
is the output voltage,
JA
is the sum of the
and θ
JC
(given
LOAD
25
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:
Again, the operating junction temperature will be:
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
on a (3
National Semiconductor sales office in your area or the
National 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:
[phone] Fax: [phone]
Pulse Engineering
Dunmore Road
Tuam Co. Galway, Ireland Phone: [phone] Fax: +353
93 24 459
1
⁄
2
") diskette for IBM compatable computers from a
∆T
J
= P
D
x (θ
T
JC
J
= ∆T
+ θ
Interface
J
+ T
A
+ θ
Heat Sink
www.national.com
)