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
procedure has to be used to select the inductor L1 or the output capacitor C
out
.
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:
Under normal continuous inductor current operating conditions, the worst case occurs when V
in
is
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
IKSEMICON [ IK SEMICON CO., LTD ]
