LM2591 National Semiconductor, LM2591 Datasheet - Page 15

• Manufacturer Part Number: LM2591
• Description: SIMPLE SWITCHER Power Converter 150 kHz 1A Step-Down Voltage Regulator
• Manufacturer: National Semiconductor

Application Information
UNDERVOLTAGE LOCKOUT
Some applications require the regulator to remain off until
the input voltage reaches a predetermined voltage. An und-
ervoltage lockout feature applied to a buck regulator is
shown in Figure 8, while Figure 9 and Figure 10 applies the
same feature to an inverting circuit. The circuit in Figure 9
features a constant threshold voltage for turn on and turn off
(zener voltage plus approximately one volt). If hysteresis is
FIGURE 8. Undervoltage Lockout for Buck Regulator
(Continued)
FIGURE 7. Delayed Startup
15
needed, the circuit in Figure 10 has a turn ON voltage which
is different than the turn OFF voltage. The amount of hyster-
esis is approximately equal to the value of the output volt-
age. If zener voltages greater than 25V are used, an addi-
tional 47 kΩ resistor is needed from the ON /OFF pin to the
ground pin to stay within the 25V maximum limit of the ON
/OFF pin.
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