LM2587SX-3.3 National Semiconductor, LM2587SX-3.3 Datasheet - Page 24

• Manufacturer Part Number: LM2587SX-3.3
• Description: SIMPLE SWITCHER 5A Flyback Regulator
• Manufacturer: National Semiconductor

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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:
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:
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).
F
SAT
is the forward biased voltage of the diode and is
T
J
is the switch saturation voltage and can be
= P
D
x (
T
T
T
J
JC
J
J
= T
= T
= P
+
D
Interface
(Continued)
J
J
x
+ T
+ T
JA
A
A
.
.
+
Heat Sink
)
24
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 Na-
tional
(1-800-272-9959).
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