LM2591HV datasheet(16/20 Pages) NSC

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Application Information (Continued)

lNVERTING REGULATOR

The circuit in Figure 11 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 LM2591HV-5.0 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 IPEAK

≤ I

CLIM.

While checking for this, take I

CLIM to be the lowest possible

current limit value (min across tolerance and temperature is

1.2A for the LM2591HV). 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-

suming 100% efficiency, which is never so. Therefore expect

PEAK to be an additional 10-20% higher than calculated from

the above equation.

The reader is also referred to Application Note AN-1157 for

examples based on positive to negative configuration.

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 60V. For example, when convert-

ing +20V to −12V, the regulator would see 32V between the

input pin and ground pin. The LM2591HV has a maximum

input voltage spec of 60V.

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.

10129384

This circuit has an ON/OFF threshold of approximately 13V.

FIGURE 9. Undervoltage Lockout for Inverting Regulator

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