IL2595-xx
Application Information
FIGURE 2. Delayed Startup
FIGURE 3. Undervoltage Lockout
for Buck Regulator
DELAYED STARTUP
The circuit in Figure 2 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 R2 pulls the ON /OFF pin low, thus allowing the circuit to
start switching. Resistor R1 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 3, while Figure 4 and Figure 5 applies the
same feature to an inverting circuit. The circuit in Figure 4 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 5 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 kW resistor is needed from the ON /OFF pin to the ground pin
to stay within the 25V maximum limit of the ON /OFF pin.
This circuit has an ON/OFF threshold of approximately 13V.
FIGURE 4. Undervoltage Lockout for Inverting Regulator
INVERTING REGULATOR
The circuit in Figure 6 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 example uses the IL2595-5.0 to generate a −5V output, but other output voltages are possible by selecting other
output voltage versions, including the adjustable version.
Since this regulator topology can produce an output voltage that is either greater than or less than the input voltage, the
maximum output current greatly depends on both the input and output voltage. The curve shown in Figure 7provides a
guide as to the amount of output load current possible for the different input and output voltage conditions.
The maximum voltage appearing across the regulator is the absolute sum of the input and output voltage, and this must
be limited to a maximum of 40V. For example, when converting +20V to −12V, the regulator would see 32V between the
input pin and ground pin. The IL2595 has a maximum input voltage spec of 40V.
7
Rev. 00
IKSEMICON [ IK SEMICON CO., LTD ]
