LM2597 datasheet(29/34 Pages) NSC

29 / 34 page

Application Information (Continued)

lNVERTING REGULATOR

The circuit in

Figure 29 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.

This example uses the LM2597-5 to generate a −5V output,

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 30 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. In this example, when

converting +20V to −5V, the regulator would see 25V be-

tween the input pin and ground pin. The LM2597 has a

maximum input voltage rating of 40V (60V for the

LM2597HV).

DS012440-47

FIGURE 28. Typical Circuit Using Shutdown /Soft-start and Error Flag Features

DS012440-48

FIGURE 29. Inverting −5V Regulator With Shutdown and Soft-start

LM2597 Datasheet(PDF) 29 Page - National Semiconductor (TI)