lm2587sx-5.0 National Semiconductor Corporation, lm2587sx-5.0 Datasheet - Page 24

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

www.national.com
Application Hints
OUTPUT VOLTAGE LIMITATIONS
The maximum output voltage of a boost regulator is the
maximum switch voltage minus a diode drop. In a flyback
regulator, the maximum output voltage is determined by the
turns ratio, N, and the duty cycle, D, by the equation:
The duty cycle of a flyback regulator is determined by the
following equation:
Theoretically, the maximum output voltage can be as large
as desired — just keep increasing the turns ratio of the trans-
former. However, there exists some physical limitations that
prevent the turns ratio, and thus the output voltage, from
increasing to infinity. The physical limitations are capaci-
tances and inductances in the LM2587 switch, the output
diode(s), and the transformer — such as reverse recovery
time of the output diode (mentioned above).
NOISY INPUT LINE CONDITION)
A small, low-pass RC filter should be used at the input pin of
the LM2587 if the input voltage has an unusual large amount
of transient noise, such as with an input switch that bounces.
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
indicated in Figure 44). When using the Adjustable version,
physically locate the programming resistors as near the
regulator IC as possible, to keep the sensitive feedback
wiring short.
V
OUT
≈ N x V
IN
(Continued)
x D/(1 − D)
FIGURE 44. Circuit Board Layout
24
The circuit in Figure 43 demonstrates the layout of the filter,
with the capacitor placed from the input pin to ground and
the resistor placed between the input supply and the input
pin. Note that the values of R
matic are good enough for most applications, but some
readjusting might be required for a particular application. If
efficiency is a major concern, replace the resistor with a
small inductor (say 10 µH and rated at 100 mA).
STABILITY
All current-mode controlled regulators can suffer from an
instability, known as subharmonic oscillation, if they operate
with a duty cycle above 50%. To eliminate subharmonic
oscillations, a minimum value of inductance is required to
ensure stability for all boost and flyback regulators. The
minimum inductance is given by:
where V
found in the Characteristic Curves.
HEAT SINK/THERMAL CONSIDERATIONS
In many cases, no heat sink is required to keep the LM2587
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
LM2587). For a safe, conservative design, a temperature
approximately 15˚C cooler than the maximum junction tem-
perature should be selected (110˚C).
SAT
is the switch saturation voltage and can be
IN
and C
01231629
IN
shown in the sche-