LM2597HVM-12 National Semiconductor, LM2597HVM-12 Datasheet - Page 27

• Manufacturer Part Number: LM2597HVM-12
• Description: SIMPLE SWITCHER Power Converter 150 kHz 0.5A Step-Down Voltage Regulator/ with Features
• Manufacturer: National Semiconductor

Application Information
SHUTDOWN /SOFT-START
The circuit shown in Figure 27 is a standard buck regulator
with 24V in, 12V out, 100 mA load, and using a 0.068 µF
Soft-start capacitor. The photo in Figure 25 and Figure 26
show the effects of Soft-start on the output voltage, the input
current, with, and without a Soft-start capacitor. Figure 25
also shows the error flag output going high when the output
voltage reaches 95% of the nominal output voltage. The re-
duced input current required at startup is very evident when
comparing the two photos. The Soft-start feature reduces the
startup current from 700 mA down to 160 mA, and delays
and slows down the output voltage rise time.
FIGURE 24. Effects of Bias Supply Feature on 12V
Regulator Efficiency
FIGURE 27. Typical Circuit Using Shutdown /Soft-start and Error Flag Features
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27
This reduction in start up current is useful in situations where
the input power source is limited in the amount of current it
can deliver. In some applications Soft-start can be used to
replace undervoltage lockout or delayed startup functions.
If a very slow output voltage ramp is desired, the Soft-start
capacitor can be made much larger. Many seconds or even
minutes are possible.
If only the shutdown feature is needed, the Soft-start capaci-
tor can be eliminated.
FIGURE 26. Output Voltage, Input Current, at Start-Up,
FIGURE 25. Output Voltage, Input Current, Error Flag
Signal, at Start-Up, WITH Soft-start
WITHOUT Soft-start
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