Note 24: The LM2586 will require a heat sink in these applications. The size of the heat sink will depend on the maximum ambient temperature. To calculate the

thermal resistance of the IC and the size of the heat sink needed, see the “Heat Sink/Thermal Considerations” section in the Application Hints.

Application Hints

LM2586 SPECIAL FEATURES

SHUTDOWN CONTROL

A feature of the LM2586 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

38).

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

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

as shown in

Figure 38 (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 LM2586 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 38, 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 39 shows resistor values corresponding to commonly

used frequencies.

However, changing the LM2586’s operating frequency from

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

lection and compensation component values.

DS012516-57

FIGURE 37. +24V to +48V Boost Regulator

DS012516-58

FIGURE 38. Shutdown Operation

R

Frequency (kHz)

Open

100

200

125

47

150

33

175

22

200

FIGURE 39. Frequency Setting Resistor Guide

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