eorex
EP3101
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.
1
+VIN
VIN
EP3101
C1
0.1uF
EN
GND
+
5
3
CIN
680uF
R1
47K
R2
47K
Figure 1-Delayed Startup
Jul. 2006
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EOREX [ EOREX CORPORATION ]
