Datasheet

LM2587

SNVS115D –APRIL 2000–REVISED APRIL 2013

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The other method to reduce or eliminate the “ringing” is to insert a Schottky diode clamp between pins 4 and 3

(ground), also shown in Figure 56. This prevents the voltage at pin 4 from dropping below −0.4V. The reverse

voltage rating of the diode must be greater than the switch off voltage.

Figure 57. Input Line Filter

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

≈ N × V

× D/(1 − D) (4)

The duty cycle of a flyback regulator is determined by the following equation:

(5)

Theoretically, the maximum output voltage can be as large as desired—just keep increasing the turns ratio of the

transformer. However, there exists some physical limitations that prevent the turns ratio, and thus the output

voltage, from increasing to infinity. The physical limitations are capacitances 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. The circuit in Figure 57 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 the schematic 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:

(6)

where V

SAT

is the switch saturation voltage and can be found in the Characteristic Curves.

Product Folder Links: LM2587