eorex

EP3101

Jul. 2006

www.eorex.com

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Delayed Startup

The circuit in Figure 1 uses the EN 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 EN pin high, keeping the

regulator off. Once the input voltage reaches its final value and the capacitor stops charging, and resistor

R2 pulls the EN pin low, thus allowing the circuit to start switching. Resistor R1 is included to limit the

maximum voltage applied to the EN 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 EN 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.

GND

EN

EP3101

680uF

1

53

+

C1

0.1uF

R2

47K

R1

47K

Figure 1-Delayed Startup