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LM2586 Datasheet(PDF) 21 Page - National Semiconductor (TI) |
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LM2586 Datasheet(HTML) 21 Page - National Semiconductor (TI) |
21 / 28 page Typical Boost Regulator Applications (Continued) 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 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 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 SET(kΩ) Frequency (kHz) Open 100 200 125 47 150 33 175 22 200 FIGURE 39. Frequency Setting Resistor Guide www.national.com 21 |
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