LM2586 datasheet(24/28 Pages) NSC

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Application Hints (Continued)

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:

OUT ≈ NxVIN xD/(1−D)

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),

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 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

IN and CIN shown 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

SAT is the switch saturation voltage and can be

found in the Characteristic Curves.

DS012516-63

FIGURE 44. Input Line Filter

DS012516-64

FIGURE 45. Circuit Board Layout

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