LM2588 datasheet(23/29 Pages) NSC

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LM2588 SPECIAL FEATURES

SHUTDOWN CONTROL

A feature of the LM2588 is its ability to be shut down using

the ON /OFF pin (pin 1). This feature conserves input power

by turning off the device when it is not in use. For proper op-

eration, an isolation diode is required (as shown in

Figure

41).

The device will shut down when 3V or greater is applied on

the ON /OFF pin, sourcing current into pin 1. In shut down

mode, the device will draw typically 56 µA of supply current

(16 µA to V

IN and 40 µA to the ON /OFF pin). To turn the de-

vice back on, leave pin 1 floating, using an (isolation) diode,

as shown in

Figure 41 (for normal operation, do not source

or sink current to or from this pin — see the next section).

FREQUENCY ADJUSTMENT

The switching frequency of the LM2588 can be adjusted with

the use of an external resistor. This feature allows the user to

optimize the size of the magnetics and the output capaci-

tor(s) by tailoring the operating frequency. A resistor con-

nected from pin 1 (the Freq. Adj. pin) to ground will set the

switching frequency from 100 kHz to 200 kHz (maximum).

As shown in

Figure 41, the pin can be used to adjust the fre-

quency while still providing the shut down function. A curve in

the Performance Characteristics Section graphs the resistor

value to the corresponding switching frequency. The table in

Figure 42 shows resistor values corresponding to commonly

used frequencies.

However, changing the LM2588’s operating frequency from

its nominal value of 100 kHz will change the magnetics se-

lection and compensation component values.

FREQUENCY SYNCHRONIZATION

Another feature of the LM2588 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 LM2588 to an external oscillator (see

Figure 43 and

44).

Use of this feature enables the LM2588 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 44 shows a LM2588 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 45, 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 45 ), 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

DS012420-54

FIGURE 41. Shutdown Operation

SET(kΩ)

Frequency (kHz)

Open

100

200

125

150

175

200

FIGURE 42. Frequency Setting Resistor Guide

DS012420-55

FIGURE 43. Frequency Synchronization

DS012420-64

FIGURE 44. Waveforms of a Synchronized

12V Boost Regulator

LM2588 Datasheet(PDF) 23 Page - National Semiconductor (TI)

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