lm2597hvn-5.0 National Semiconductor Corporation, lm2597hvn-5.0 Datasheet - Page 28

• Manufacturer Part Number: lm2597hvn-5.0
• Description: Simple Switcher Power Converter 150 Khz 0.5a Step-down Voltage Regulator, With Features
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
To use the bias supply feature with output voltages between
4V and 15V, wire the bias pin to the regulated output. Since
the V
part cannot be used this way. When the V
the intemal regulator circuitry is powered from the input
voltage.
SHUTDOWN /SOFT-START
The circuit shown in Figure 28 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 26 and Figure 27
show the effects of Soft-start on the output voltage, the input
FIGURE 25. Effects of Bias Supply Feature on 12V
FIGURE 24. Effects of Bias Supply Feature on 5V
BS
pin requires a minimum of 4V to operate, the 3.3V
Regulator Efficiency
Regulator Efficiency
DS012440-43
DS012440-45
(Continued)
BS
pin is left open,
28
current, with, and without a Soft-start capacitor. Figure 26
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 re-
duces the startup current from 700 mA down to 160 mA, and
delays and slows down the output voltage rise time.
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 27. Output Voltage, Input Current, at Start-Up,
FIGURE 26. Output Voltage, Input Current, Error Flag
Signal, at Start-Up, WITH Soft-start
WITHOUT Soft-start
DS012440-46
DS012440-44