LM2597, LM2597HV
SNVS119C –MARCH 1998–REVISED APRIL 2013
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Figure 43. Effects of Bias Supply Feature on 5V
Regulator Efficiency
Figure 44. Effects of Bias Supply Feature on 12V
Regulator Efficiency
SHUTDOWN /SOFT-START
The circuit shown in Figure 47 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 45 and Figure 46 show the effects of Soft-start on the output voltage,
the input current, with, and without a Soft-start capacitor. Figure 45 also shows the error flag output going high
when the output voltage reaches 95% of the nominal output voltage. The reduced 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 45. Output Voltage, Input Current, Error
Flag Signal, at Start-Up, WITH Soft-start
Figure 46. Output Voltage, Input Current, at Start-
Up, WITHOUT Soft-start
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 capacitor can be eliminated.
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