LM2588S-3.3 National Semiconductor, LM2588S-3.3 Datasheet - Page 24

• Manufacturer Part Number: LM2588S-3.3
• Description: SIMPLE SWITCHER 5A Flyback Regulator with
• Manufacturer: National Semiconductor

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Application Hints
load and prevent damage to the switch, the current must be
externally limited, either by the input supply or at the output
with an external current limit circuit. The external limit should
be set to the maximum switch current of the device, which is
5A.
In a flyback regulator application ( Figure 46 ), using the stan-
dard transformers, the LM2588 will survive a short circuit to
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 reciti-
fied DC supply and/or the application has a wide tempera-
ture range, the required rms current rating of the capacitor
might be very large. This means a larger value of capaci-
tance or a higher voltage rating will be needed for the input
(Continued)
FIGURE 46. Flyback Regulator
FIGURE 45. Boost Regulator
24
the main output. When the output voltage drops to 80% of its
nominal value, the frequency will drop to 25 kHz. With a
lower frequency, off times are larger. With the longer off
times, the transformer can release all of its stored energy be-
fore the switch turns back on. Hence, the switch turns on ini-
tially with zero current at its collector. In this condition, the
switch current limit will limit the peak current, saving the
device.
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.
DS012420-56
DS012420-57
IN
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