LM2586 datasheet(25/30 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

multiple 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 prob-

lems.

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

following 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

output is shorted (see Figure 42), current flows directly from

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

bypassing 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

SET(k

Ω)

Frequency (kHz)

Open

100

200

125

150

175

200

FIGURE 39. Frequency Setting Resistor Guide

01251659

FIGURE 40. Frequency Synchronization

01251669

FIGURE 41. Waveforms of a Synchronized

12V Boost Regulator

01251661

FIGURE 42. Boost Regulator

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