IL2596-xx
Application Information
Figure 3. Delayed Startup
Figure 4. Undervoltage Lockout for Buck Regulator
DELAYED STARTUP
The circuit in Figure 3 uses the 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. Once the input voltage reaches its final
value and the capacitor stops charging, and resistor R
2
pulls the ON /OFF pin low, thus allowing the circuit to start switching.
Resistor R
1
is included to limit the maximum voltage applied to the ON /OFF pin (maximum of 25V), 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 startup 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.
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 4, while Figure 5 and 6 applies the same feature to
an inverting circuit. The circuit in Figure 5 features a constant threshold voltage for turn on and turn off (zener voltage plus
approximately one volt). If hysteresis is needed, the circuit in Figure 6 has a turn ON voltage which is different than the turn OFF
voltage. The amount of hysteresis is approximately equal to the value of the output voltage. If zener voltages greater than 25V
are used, an additional 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.
INVERTING REGULATOR
The circuit in Figure 7 converts a positive input voltage to a negative output voltage with a common ground. The circuit
operates by bootstrapping the regulator’s ground pin to the negative output voltage, then grounding the feedback pin, the
regulator senses the inverted output voltage and regulates it.
This circuit has an ON/OFF threshold of approximately 13V.
Figure 5. Undervoltage Lockout for Inverting Regulator
Rev. 03 7
IKSEMICON [ IK SEMICON CO., LTD ]
