lm2588sx-adj National Semiconductor Corporation, lm2588sx-adj Datasheet - Page 26

• Manufacturer Part Number: lm2588sx-adj
• Description: Simple Switcher 5a Flyback Regulator With
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Hints
FLYBACK REGULATOR INPUT CAPACITORS
A flyback regulator draws discontinuous pulses of current
from the input supply. Therefore, there are two input capaci-
tors needed in a flyback regulator — one for energy storage
and one for filtering (see Figure 46). Both are required due to
the inherent operation of a flyback regulator. To keep a
stable or constant voltage supply to the LM2588, a storage
capacitor (≥100 µF) is required. If the input source is a
recitified DC supply and/or the application has a wide tem-
perature range, the required rms current rating of the capaci-
tor might be very large. This means a larger value of capaci-
tance or a higher voltage rating will be needed for the input
capacitor. The storage capacitor will also attenuate noise
which may interfere with other circuits connected to the
same input supply voltage.
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
where V
and is typically 0.5V for Schottky diodes and 0.8V for ultra-
fast recovery diodes. In certain circuits, there exists a volt-
age spike, V
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 in Figure 4
F
is the forward biased voltage of the output diode,
V
SW(OFF)
LL
, superimposed on top of the steady-state
= V
IN
IN
(Max) + (V
(Max):
(Continued)
OUT
+V
F
)/N
OUT
FIGURE 46. Flyback Regulator
, and the
IN
and
26
and other flyback regulator circuits throughout the
datasheet). The schematic in Figure 46 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 LM2588 IC as well. When used in a flyback
regulator, the voltage at the Switch pin (pin 5) 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 46. 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 10Ω 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 5 and 4
(ground), also shown in Figure 46. 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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