LM2574, LM2574HV
SNVS104C –JUNE 1999–REVISED APRIL 2013
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For an input voltage of 8V or more, the maximum available output current in this configuration is approximately
100 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
0.6A. 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 LM2574
Series Buck Regulator Design Procedure can not be used 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:
where
• f
osc
= 52 kHz. Under normal continuous inductor current operating conditions,
• the minimum V
IN
represents the worst case. Select an inductor that is rated for the peak current anticipated.
(6)
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 LM2574 is +28V, or +48V for the LM2574HV.
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, etc.
NEGATIVE BOOST REGULATOR
Another variation on the buck-boost topology is the negative boost configuration. The circuit in Figure 32 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.
Note: Pin numbers are for 8-pin PDIP package.
Figure 32. 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.
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