DELAYED STARTUP
Application Information (Continued)
The circuit in Figure 21 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 volt-
age 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 cir-
cuit 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.
The curves shown in Figure 19 and Figure 20 show the
LM2594 junction temperature rise above ambient tempera-
ture with a 500 mA load for various input and output volt-
ages. This data was taken with the circuit operating as a
buck switcher with all components mounted on a PC board
to simulate the junction temperature under actual operating
conditions. This curve is typical, and can be used for a quick
check on the maximum junction temperature for various con-
ditions, but keep in mind that there are many factors that can
affect the junction temperature.
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
DS012439-36
Some applications require the regulator to remain off until
the input voltage reaches a predetermined voltage. An und-
ervoltage lockout feature applied to a buck regulator is
shown in Figure 22, while Figure 23 and Figure 24 applies
the same feature to an inverting circuit. The circuit in Figure
23 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 24 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 ad-
ditional 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.
FIGURE 21. Delayed Startup
DS012439-37
FIGURE 22. Undervoltage Lockout
for Buck Regulator
INVERTING REGULATOR
The circuit in Figure 25 converts a positive input voltage to a
negative output voltage with a common ground. The circuit
operates by bootstrapping the regulators ground pin to the
negative output voltage, then grounding the feedback pin,
the regulator senses the inverted output voltage and regu-
lates it.
DS012439-38
This circuit has an ON/OFF threshold of approximately 13V.
FIGURE 23. Undervoltage Lockout for Inverting Regulator
23
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