IL2576-xx

Application Information

INVERTING REGULATOR

Figure 5

shows a IL2576-12 in a buck-boost configuration to generate a negative 12V output from a positive input

voltage. This circuit bootstraps the regulator’s ground pin to the negative output voltage, then by grounding the feedback pin, the

regulator senses the inverted output voltage and regulates it to −12V.

For an input voltage of 12V or more, the maximum available output current in this configuration is approximately 700 mA.

At lighter loads, the minimum input voltage required drops to approximately 4.7V.

The switch currents in this buck-boost configuration are higher than in the standard buck-mode design, thus lowering the

available output current. Also, the start-up input current of the buck-boost converter is higher than the standard buck-mode

regulator, and this may overload an input power source with a current limit less than 5A. Using a delayed turn-on or an

undervoltage lockout circuit (described in the next section) would allow the input voltage to rise to a high enough level before the

switcher would be allowed to turn on.

Because of the structural differences between the buck and the buck-boost regulator topologies, the buck regulator

design procedure section can not be used to to select the inductor or the output capacitor. The recommended range of inductor

values for the buck-boost design is between 68 µH and 220 µH, and the output capacitor values must be larger than what is

normally required for buck designs. Low input voltages or high output currents require a large value output capacitor (in the

thousands of micro Farads).

The peak inductor current, which is the same as the peak switch current, can be calculated from the following formula:

worst case. Select an inductor that is rated for the peak current anticipated.

FIGURE 5. Inverting Buck-Boost Develops −12V

Also, the maximum voltage appearing across the regulator is the absolute sum of the input and output voltage. For a

−12V output, the maximum input voltage for the IL2576 is +28V.

NEGATIVE BOOST REGULATOR

Another variation on the buck-boost topology is the negative boost configuration. The circuit in Figure 6 accepts an input

voltage ranging from −5V to −12V and provides a regulated −12V output. Input voltages greater than −12V will cause the output

to rise above −12V, but will not damage the regulator.

Typical Load Current

Note: Heat sink may be required.

FIGURE 6. Negative Boost

Rev. 02