LM2585

SNVS120F – APRIL 2000 – REVISED APRIL 2013

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If poor circuit layout techniques are used (see CIRCUIT LAYOUT GUIDELINES), negative voltage transients may

appear on the Switch pin (pin 4). Applying a negative voltage (with respect to the IC's ground) to any monolithic

IC pin causes erratic and unpredictable operation of that IC. This holds true for the LM2585 IC as well. When

used in a flyback regulator, the voltage at the Switch pin (pin 4) can go negative when the switch turns on. The

“ringing” voltage at the switch pin is caused by the output diode capacitance and the transformer leakage

inductance forming a resonant circuit at the secondary(ies). The resonant circuit generates the “ringing” voltage,

which gets reflected back through the transformer to the switch pin. There are two common methods to avoid this

problem. One is to add an RC snubber around the output rectifier(s), as in Figure 53. The values of the resistor

and the capacitor must be chosen so that the voltage at the Switch pin does not drop below

−0.4V. The resistor

may range in value between 10

Ω and 1 kΩ, and the capacitor will vary from 0.001 μF to 0.1 μF. Adding a

snubber will (slightly) reduce the efficiency of the overall circuit.

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 53. 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.

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:

(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 LM2585

switch, the output diode(s), and the transformer—such as reverse recovery time of the output diode (mentioned

above).

Figure 54. Input Line Filter

NOISY INPUT LINE CONDITION

A small, low-pass RC filter should be used at the input pin of the LM2585 if the input voltage has an unusual

large amount of transient noise, such as with an input switch that bounces. The circuit in Figure 54 demonstrates

the layout of the filter, with the capacitor placed from the input pin to ground and the resistor placed between the

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).

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