IL2575-XX Datasheet, PDF (11/17 Pages) IK Semicon Co., Ltd – 1.0 A, 15 V, Step-Down Switching Regulator

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