tures 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 hyster-
esis is approximately equal to the value of the output volt-
age. If zener voltages greater than 25V are used, an addi-
tional 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.
Application Information (Continued)
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 regulator’s ground pin to the
negative output voltage, then grounding the feedback pin,
the regulator senses the inverted output voltage and regu-
lates it.
01258336
FIGURE 21. Delayed Startup
01258337
01258338
This circuit has an ON/OFF threshold of approximately 13V.
FIGURE 22. Undervoltage Lockout
for Buck Regulator
FIGURE 23. Undervoltage Lockout
for Inverting Regulator
DELAYED STARTUP
This example uses the LM2596-5.0 to generate a −5V out-
put, 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 26 provides a
guide as to the amount of output load current possible for the
different input and output voltage conditions.
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
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.
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 convert-
ing +20V to −12V, the regulator would see 32V between the
input pin and ground pin. The LM2596 has a maximum input
voltage spec of 40V.
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.
Additional diodes are required in this regulator configuration.
Diode D1 is used to isolate input voltage ripple or noise from
coupling through the CIN capacitor to the output, under light
or no load conditions. Also, this diode isolation changes the
topology to closley resemble a buck configuration thus pro-
viding good closed loop stability. A Schottky diode is recom-
mended for low input voltages, (because of its lower voltage
drop) but for higher input voltages, a fast recovery diode
could be used.
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 24 applies the same
feature to an inverting circuit. The circuit in Figure 23 fea-
Without diode D3, when the input voltage is first applied, the
charging current of CIN can pull the output positive by sev-
eral volts for a short period of time. Adding D3 prevents the
output from going positive by more than a diode voltage.
25
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