LM2594 datasheet(24/28 Pages) NSC

24 / 28 page

Application Information (Continued)

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 C

IN 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.

Without diode D3, when the input voltage is first applied, the

charging current of C

IN 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.

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

Figure 25 will provide good results in the majority of invert-

ing designs.

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-

DS012439-39

This circuit has hysteresis

Regulator starts switching at VIN = 13V

Regulator stops switching at VIN = 8V

FIGURE 24. Undervoltage Lockout with Hysteresis for Inverting Regulator

DS012439-40

CIN — 68 µF/25V Tant. Sprague 595D

120 µF/35V Elec. Panasonic HFQ

COUT — 22 µF/20V Tant. Sprague 595D

39 µF/16V Elec. Panasonic HFQ

FIGURE 25. Inverting −5V Regulator with Delayed Startup

DS012439-41

FIGURE 26. Inverting Regulator Typical Load Current

LM2594 Datasheet(PDF) 24 Page - National Semiconductor (TI)

LM2594 Datasheet(HTML) 24 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL