LM2576, LM2576HV
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SNVS107C –JUNE 1999–REVISED APRIL 2013
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 LM2576 is +28V, or +48V for the LM2576HV.
The Switchers Made Simple (version 3.0) design software can be used to determine the feasibility of regulator
designs using different topologies, different input-output parameters, different components, and so on.
NEGATIVE BOOST REGULATOR
Another variation on the buck-boost topology is the negative boost configuration. The circuit in Figure 29 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.
+
Feedback
C
OUT
V
IN
1
4
2200 mF
LM2576-12
Output
LOW ESR
2
3
5
1N5820
C
+
IN
GND
ON/OFF
100 mF
V
OUT
= -12V
100 mH
-V
IN
-5V to -12V
Typical Load Current
400 mA for VIN = −5.2V
750 mA for VIN = −7V
Heat sink may be required.
Figure 29. Negative Boost
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.
UNDERVOLTAGE LOCKOUT
In some applications it is desirable to keep the regulator off until the input voltage reaches a certain threshold. An
undervoltage lockout circuit which accomplishes this task is shown in Figure 30, while Figure 31 shows the same
circuit applied to a buck-boost configuration. These circuits keep the regulator off until the input voltage reaches
a predetermined level.
VTH ≈ VZ1 + 2VBE(Q1)
Complete circuit not shown.
Figure 30. Undervoltage Lockout for Buck Circuit
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