Datasheet

Application Information (Continued)

lNVERTING REGULATOR

The circuit in

Figure 10

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.

This example uses the LM2593HV-5 to generate a −5V

output, but other output voltages are possible by selecting

other output voltage versions, including the adjustable ver-

sion. 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.

To determine how much load current is possible before the

internal device current limit is reached (and power limiting

occurs), the system must be evaluated as a buck-boost

configuration rather than as a buck. The peak switch current

in Amperes, for such a configuration is given as:

where L is in µH and f is in Hz. The maximum possible load

current I

LOAD

is limited by the requirement that I

PEAK

≤ I

CLIM

While checking for this, take I

CLIM

to be the lowest possible

current limit value (min across tolerance and temperature is

2.3A for the LM2593HV). Also to account for inductor toler-

ances, we should take the min value of Inductance for L in

the equation above (typically 20% less than the nominal

value). Further, the above equation disregards the drop

across the Switch and the diode. This is equivalent to as-

10133342

FIGURE 9. Typical Circuit Using Shutdown /Soft-start and Error Flag Features

10133343

FIGURE 10. Inverting −5V Regulator With Shutdown and Soft-start

LM2593HV