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LM2585 Datasheet(PDF) 22 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Part No. LM2585
Description  SIMPLE SWITCHER 3A Flyback Regulator
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM2585 Datasheet(HTML) 22 Page - National Semiconductor (TI)

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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
θ
JA and θJC (given
in the Electrical Characteristics).
Total power dissipated (P
D) by the LM2585 can be estimated
as follows:
V
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:
T
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:
T
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
ona312" 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
www.national.com
22


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