LM2587 datasheet(21/26 Pages) NSC

21 / 26 page

Application Hints

PROGRAMMING OUTPUT VOLTAGE

(SELECTING R

1 AND R2)

Referring to the adjustable regulator in

Figure 41, the output

voltage is programmed by the resistors R

1 and R2 by the fol-

lowing formula:

OUT = VREF (1+R1/R2)

where V

REF = 1.23V

Resistors R

1 and R2 divide the output voltage down so that

it can be compared with the 1.23V internal reference. With

2 between 1k and 5k, R1 is:

1 = R2 (VOUT/VREF − 1)

where V

REF = 1.23V

For best temperature coefficient and stability with time, use

1% metal film resistors.

SHORT CIRCUIT CONDITION

Due to the inherent nature of boost regulators, when the out-

put is shorted (see

Figure 41), current flows directly from the

input, through the inductor and the diode, to the output, by-

passing the switch. The current limit of the switch

does not

limit the output current for the entire circuit. To protect the

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 42), using the stan-

dard transformers, the LM2587 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 de-

vice.

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 42). Both are required due to

the inherent operation of a flyback regulator. To keep a

stable or constant voltage supply to the LM2587, 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 of the input

capacitor. The storage capacitor will also attenuate noise

which may interfere with other circuits connected to the

same input supply voltage.

DS012316-26

FIGURE 41. Boost Regulator

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