9.3.1 Delayed Start-Up

The circuit in Figure 21 uses the ON/OFF pin to provide a time delay between the time the input voltage is applied and the time the output voltage comes up (only the circuitry pertaining to the delayed start-up is shown). As the input voltage rises, the charging of capacitor C1 pulls the ON/OFF pin high, keeping the regulator off. When the input voltage reaches its final value and the capacitor stops charging, the resistor R₂ pulls the ON/OFF pin low, thus allowing the circuit to start switching. Resistor R₁ is included to limit the maximum voltage applied to the ON/OFF pin (maximum of 25 V), reduces power supply noise sensitivity, and also limits the capacitor, C1, discharge current. When high input ripple voltage exists, avoid long delay time, because this ripple can be coupled into the ON/OFF pin and cause problems.

This delayed start-up feature is useful in situations where the input power source is limited in the amount of current it can deliver. It allows the input voltage to rise to a higher voltage before the regulator starts operating. Buck regulators require less input current at higher input voltages.

Figure 21. Delayed Start-Up

9.3.2 Undervoltage Lockout

Some applications require the regulator to remain off until the input voltage reaches a predetermined voltage. An undervoltage lockout feature applied to a buck regulator is shown in Figure 22, while Figure 23 and Figure 24 applies the same feature to an inverting circuit. The circuit in Figure 23 features a constant threshold voltage for turnon and turnoff (Zener voltage plus approximately one volt). If hysteresis is needed, the circuit in Figure 24 has a turnon voltage which is different than the turnoff voltage. The amount of hysteresis is approximately equal to the value of the output voltage. If Zener voltages greater than 25 V are used, an additional 47-kΩ resistor is needed from the ON/OFF pin to the ground pin to stay within the 25-V maximum limit of the ON/OFF pin.

Figure 22. Undervoltage Lockout for Buck Regulator

This circuit has an ON/OFF threshold of approximately 13 V.

Figure 23. Undervoltage Lockout for Inverting Regulator

9.4.1 Shutdown Mode

The ON/OFF pin provides electrical ON and OFF control for the LM2591HV. When the voltage of this pin is higher than 2 V, the device is shutdown mode. The typical standby current in this mode is 90 μA.

9.4.2 Active Mode

When the ON/OFF pin is left floating or pull below 0.6 V, the device starts switching and the output voltage rises until it reaches a normal regulation voltage.