Datasheet

LM2586
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SNVS121D MAY 1996REVISED APRIL 2013
Figure 57. Flyback Regulator
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 appearing at the switch, when it is off, is set by the
transformer turns ratio, N, the output voltage, V
OUT
, and the maximum input voltage, V
IN
(Max):
V
SW(OFF)
= V
IN
(Max) + (V
OUT
+V
F
)/N
where
V
F
is the forward biased voltage of the output diode, and is typically 0.5V for Schottky diodes and 0.8V for
ultra-fast recovery diodes (3)
In certain circuits, there exists a voltage spike, V
LL
, superimposed on top of the steady-state voltage (see
Figure 23, 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
suppressor in series with a diode is inserted across the transformer primary (as shown in the circuit in Figure 22
and other flyback regulator circuits throughout the datasheet). The schematic in Figure 57 shows another method
of clamping 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 voltage 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 5). 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 LM2586 IC as well.
When used in a flyback regulator, the voltage at the Switch pin (pin 5) 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 57. 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 and 1 k, 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 5 and 4
(ground), also shown in Figure 57. This prevents the voltage at pin 5 from dropping below 0.4V. The reverse
voltage rating of the diode must be greater than the switch off voltage.
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