LM1575, LM2575-N, LM2575HV
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SNVS106E –MAY 1999–REVISED APRIL 2013
The peak inductor current, which is the same as the peak switch current, can be calculated from the following
formula:
where
•
fosc = 52 kHz.
(15)
Under normal continuous inductor current operating conditions, the minimum VIN represents the worst case.
Select an inductor that is rated for the peak current anticipated.
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 LM2575 is +28V, or +48V for the LM2575HV.
The Switchers Made Simple (version 3.3) design software can be used to determine the feasibility of regulator
designs using different topologies, different input-output parameters, different components, and so on.
Figure 32. Inverting Buck-Boost Develops −12V
NEGATIVE BOOST REGULATOR
Another variation on the buck-boost topology is the negative boost configuration. The circuit in Figure 33 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.
Because of the boosting function of this type of regulator, the switch current is relatively high, especially at low
input voltages. Output load current limitations are a result of the maximum current rating of the switch. Also,
boost regulators can not provide current limiting load protection in the event of a shorted load, so some other
means (such as a fuse) may be necessary.
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