LM2588 datasheet(24/29 Pages) NSC

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Application Hints (Continued)

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

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.

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

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

IN and

ground as close as possible to the device.

DS012420-56

FIGURE 45. Boost Regulator

DS012420-57

FIGURE 46. Flyback Regulator

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