lm2588sx-adj National Semiconductor Corporation, lm2588sx-adj Datasheet - Page 24

• Manufacturer Part Number: lm2588sx-adj
• Description: Simple Switcher 5a Flyback Regulator With
• Manufacturer: National Semiconductor Corporation

www.national.com
Typical Boost Regulator Applications
*The LM2588 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
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
operation, 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
device 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).
IN
FIGURE 41. Shutdown Operation
and 40 µA to the ON /OFF pin). To turn the
FIGURE 40. +24V to +48V Boost Regulator
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(Continued)
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
frequency 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
selection and compensation component values.
FIGURE 42. Frequency Setting Resistor Guide
FIGURE 43. Frequency Synchronization
R
SET
Open
200
47
33
22
(kΩ)
Frequency (kHz)
100
125
150
175
200
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