LM2587 datasheet(22/26 Pages) NSC

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Application Hints (Continued)

In addition, a small bypass capacitor is required due to the

noise generated by the input current pulses. To eliminate the

noise, insert a 1.0 µF ceramic capacitor between V

IN and

ground as close as possible to the device.

SWITCH VOLTAGE LIMITS

In a flyback regulator, the maximum steady-state voltage ap-

pearing at the switch, when it is off, is set by the transformer

turns ratio, N, the output voltage, V

OUT, and the maximum in-

put voltage, V

IN (Max):

SW(OFF) = VIN (Max) + (VOUT +VF)/N

where V

F is the forward biased voltage of the output diode,

and is 0.5V for Schottky diodes and 0.8V for ultra-fast recov-

ery diodes (typically). In certain circuits, there exists a volt-

age spike, V

LL, superimposed on top of the steady-state volt-

age (see

Figure 5, waveform A). Usually, this voltage spike is

caused by the transformer leakage inductance and/or the

output rectifier recovery time. To “clamp” the voltage at the

switch from exceeding its maximum value, a transient sup-

pressor in series with a diode is inserted across the trans-

former primary (as shown in the circuit on the front page and

other flyback regulator circuits throughout the datasheet).

The schematic in

Figure 42 shows another method of clamp-

ing the switch voltage. A single voltage transient suppressor

(the SA51A) is inserted at the switch pin. This method

clamps the total voltage across the switch, not just the volt-

age across the primary.

If poor circuit layout techniques are used (see the “Circuit

Layout Guideline” section), 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 LM2587 IC as well. When used in a flyback

regulator, the voltage at the Switch pin (pin 4) can go nega-

tive 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 “ring-

ing” voltage, which gets reflected back through the trans-

former 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 42. 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 and1kΩ, 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 in-

sert a Schottky diode clamp between pins 4 and 3 (ground),

also shown in

Figure 42. 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:

OUT ≈ NxVIN xD/(1−D)

DS012316-27

FIGURE 42. Flyback Regulator

DS012316-28

FIGURE 43. Input Line Filter

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