lm2587sx-5.0

Manufacturer Part Numberlm2587sx-5.0
DescriptionSimple Switcher 5a Flyback Regulator
ManufacturerNational Semiconductor Corporation
lm2587sx-5.0 datasheet
 
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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
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 trans-
former turns ratio, N, the output voltage, V
maximum input voltage, V
(Max):
IN
V
= V
(Max) + (V
SW(OFF)
IN
OUT
where V
is the forward biased voltage of the output diode,
F
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
, superimposed on top of the steady-state
LL
voltage (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 suppressor in series with a diode is inserted across
the transformer 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 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 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
FIGURE 42. Flyback Regulator
switch pin is caused by the output diode capacitance and the
transformer leakage inductance forming a resonant circuit at
and
the secondary(ies). The resonant circuit generates the “ring-
IN
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
, and the
OUT
range in value between 10Ω and 1 kΩ, and the capacitor will
vary from 0.001 µF to 0.1 µF. Adding a snubber will (slightly)
+V
)/N
F
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 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.
23
01231627
01231628
FIGURE 43. Input Line Filter
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