LM2594 datasheet(23/28 Pages) NSC

23 / 28 page

Application Information (Continued)

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.

DELAYED STARTUP

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 R

2 pulls the ON /OFF pin low, thus allowing the cir-

cuit to start switching. Resistor R

1 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 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 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 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.

DS012439-36

FIGURE 21. Delayed Startup

DS012439-37

FIGURE 22. Undervoltage Lockout

for Buck Regulator

DS012439-38

This circuit has an ON/OFF threshold of approximately 13V.

FIGURE 23. Undervoltage Lockout for Inverting Regulator

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