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

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 k

Ω 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

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Rev. 03