IL2575-xx

APPLICATION INFORMATION

INVERTING REGULATOR

Figure 15. Inverting Buck–Boost Regulator Using the

IL2575–12 Develops –12 V @ 0.35 A

An inverting buck–boost regulator using the IL2575–12 is shown in Figure 15. This circuit converts a

positive input voltage to a negative output voltage with a common ground by bootstrapping the regulators

ground to the negative output voltage. By grounding the feedback pin, the regulator senses the inverted

output voltage and regulates it.

In this example the IL2575–12 is used to generate a –12 V output. The maximum input voltage in this

case cannot exceed +28 V because the maximum voltage appearing across the regulator is the absolute

sum of the input and output voltages and this must be limited to a maximum of 40 V.

This circuit configuration is able to deliver approximately 0.35 A to the output when the input voltage is

12 V or higher. At lighter loads the minimum input voltage required drops to approximately 4.7 V, because

the buck–boost regulator topology can produce an output voltage that, in its absolute value, is either greater

or less than the input voltage.

Since the switch currents in this buck–boost configuration are higher than in the standard buck converter

topology, the available output current is lower.

This type of buck–boost inverting regulator can also require a larger amount of startup input current,

even for light loads. This may overload an input power source with a current limit less than 1.5 A.

Such an amount of input startup current is needed for at least 2.0 ms or more. The actual time depends

on the output voltage and size of the output capacitor.

Because of the relatively high startup currents required by this inverting regulator topology, the use of a

delayed startup or an undervoltage lockout circuit is recommended.

Using a delayed startup arrangement, the input capacitor can charge up to a higher voltage before the

switch–mode regulator begins to operate.

The high input current needed for startup is now partially supplied by the input capacitor Cin.

Design Recommendations:

The inverting regulator operates in a different manner than the buck converter and so a different design

The output capacitor values must be larger than is normally required for buck converter designs. Low

input voltages or high output currents require a large value output capacitor (in the range of thousands of uF).

The recommended range of inductor values for the inverting converter design is between 68 uH and

220 uH. To select an inductor with an appropriate current rating, the inductor peak current has to be

calculated.

The following formula is used to obtain the peak inductor current:

minimal. Note that the voltage appearing across the regulator is the absolute sum of the input and output

voltage, and must not exceed 40 V.

Rev. 00