Application Information (Continued)
DS012439-39
This circuit has hysteresis
=
=
Regulator starts switching at V
13V
8V
IN
Regulator stops switching at V
IN
FIGURE 24. Undervoltage Lockout with Hysteresis for Inverting Regulator
DS012439-40
C
C
—
68 µF/25V Tant. Sprague 595D
120 µF/35V Elec. Panasonic HFQ
22 µF/20V Tant. Sprague 595D
39 µF/16V Elec. Panasonic HFQ
IN
—
OUT
FIGURE 25. Inverting −5V Regulator with Delayed Startup
This example uses the LM2594-5 to generate a −5V output,
but other output voltages are possible by selecting other out-
put 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 maxi-
mum 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 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 LM2594 has a maximum input
voltage spec of 40V (60V for the LM2594HV).
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.
DS012439-41
FIGURE 26. Inverting Regulator Typical Load Current
Because of differences in the operation of the inverting regu-
lator, the standard design procedure is not used to select the
inductor value. In the majority of designs, a 100 µH, 1A in-
ductor is the best choice. Capacitor selection can also be
narrowed down to just a few values. Using the values shown
in Figure 25 will provide good results in the majority of invert-
ing designs.
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.
This type of inverting regulator can require relatively large
amounts of input current when starting up, even with light
loads. Input currents as high as the LM2594 current limit (ap-
prox 0.8A) are needed for at least 2 ms or more, until the out-
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