LM2586T-ADJ National Semiconductor, LM2586T-ADJ Datasheet - Page 24

• Manufacturer Part Number: LM2586T-ADJ
• Description: DC/DC Converter IC
• Manufacturer: National Semiconductor

Specifications of LM2586T-ADJ

• Input Voltage: 40V
• Output Voltage: 37V
• No. Of Pins: 5
• Termination Type: Through Hole
• Mounting Type: Through Hole
• Output Current Max: 3A
• Peak Reflow Compatible (260 C): No
• Supply Voltage Max: 40V
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Application Hints
OUTPUT VOLTAGE LIMITATIONS
(Continued)
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:
V
N x V
x D/(1 − D)
OUT
IN
The duty cycle of a flyback regulator is determined by the fol-
lowing 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 in-
creasing to infinity. The physical limitations are capacitances
and inductances in the LM2586 switch, the output diode(s),
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and the transformer — such as reverse recovery time of the
FIGURE 44. Input Line Filter
output diode (mentioned above).
NOISY INPUT LINE CONDITION
A small, low-pass RC filter should be used at the input pin of
the LM2586 if the input voltage has an unusually large
amount of transient noise, such as with an input switch that
bounces. The circuit in Figure 44 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
and C
shown in
IN
IN
the schematic are good enough for most applications, but
some readjusting might be required for a particular applica-
tion. If efficiency is a major concern, replace the resistor with
a small inductor (say 10 µH and rated at 200 mA).
STABILITY
All current-mode controlled regulators can suffer from an in-
stability, known as subharmonic oscillation, if they operate
with a duty cycle above 50%. To eliminate subharmonic os-
cillations, a minimum value of inductance is required to en-
sure stability for all boost and flyback regulators. The mini-
mum inductance is given by:
where V
is the switch saturation voltage and can be
SAT
found in the Characteristic Curves.
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FIGURE 45. Circuit Board Layout
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