LM2586 datasheet(22/28 Pages) NSC

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Application Hints (Continued)

FREQUENCY SYNCHRONIZATION

Another feature of the LM2586 is the ability to synchronize

the switching frequency to an external source, using the

sync pin (pin 6). This feature allows the user to parallel mul-

tiple devices to deliver more output power.

A negative falling pulse applied to the sync pin will synchro-

nize the LM2586 to an external oscillator (see

Figures 40,

41).

Use of this feature enables the LM2586 to be synchronized

to an external oscillator, such as a system clock. This opera-

tion allows multiple power supplies to operate at the same

frequency,

thus

eliminating

frequency-related

noise

problems.

The scope photo in

Figure 41 shows a LM2586 12V Boost

Regulator synchronized to a 200 kHz signal. There is a 700

ns delay between the falling edge of the sync signal and the

turning on of the switch.

PROGRAMMING OUTPUT VOLTAGE

(SELECTING R1 AND R2)

Referring to the adjustable regulator in

Figure 42, the output

voltage is programmed by the resistors R1 and R2 by the fol-

lowing formula:

OUT = VREF (1 + R1/R2)

where V

REF = 1.23V

Resistors R1 and R2 divide the output voltage down so that

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

R2 between 1k and 5k, R1 is:

R1 = R2 (V

OUT/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 42), 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

3A.

In a flyback regulator application (

Figure 43), using the stan-

dard transformers, the LM2586 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 43). Both are required due to

the inherent operation of a flyback regulator. To keep a

DS012516-59

FIGURE 40. Frequency Synchronization

DS012516-69

FIGURE 41. Waveforms of a Synchronized

12V Boost Regulator

DS012516-61

FIGURE 42. Boost Regulator

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