IL2575-xx

Delayed Startup

There are some applications, like the inverting regulator already mentioned above, which require a

higher amount of startup current. In such cases, if the input power source is limited, this delayed startup

feature becomes very useful. To provide a time delay between the time the input voltage is applied and the

time when the output voltage comes up, the circuit in Figure 20 can be used. As the input voltage is applied,

the capacitor C1 charges up, and the voltage across the resistor R2 falls down. When the voltage on the

ON/OFF pin falls below the threshold value 1.4 V, the regulator starts up. Resistor R1 is included to limit the

maximum voltage applied to the ON/OFF pin, reduces the power supply noise sensitivity, and also limits the

capacitor C1 discharge current, but its use is not mandatory.

When a high 50 Hz or 60 Hz (100 Hz or 120 Hz respectively) ripple voltage exists, a long delay time

can cause some problems by coupling the ripple into the ON/OFF pin, the regulator could be switched

periodically on and off with the line (or double) frequency.

NOTE: This picture does not show the complete circuit.

Figure 20. Delayed Startup Circuitry

Undervoltage Lockout

Some applications require the regulator to remain off until the input voltage reaches a certain threshold

level. Figure 21 shows an undervoltage lockout circuit applied to a buck regulator. A version of this circuit for

buck–boost converter is shown in Figure 22. Resistor R3 pulls the ON/OFF pin high and keeps the regulator

off until the input voltage reaches a predetermined threshold level, which is determined by the following

expression:

NOTE: This picture does not show the complete circuit.

Figure 21. Undervoltage Lockout Circuit for

Buck Converter

Rev. 00