LM2576, LM2576HV
SNVS107C –JUNE 1999–REVISED APRIL 2013
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When using a heat sink, the junction temperature rise can be determined by the following:
ΔTJ = (PD) (θJC + θinterface + θHeat sink
)
(6)
(7)
The operating junction temperature will be:
TJ = TA + ΔTJ
As in Equation 14, if the actual operating junction temperature is greater than the selected safe operating
junction temperature, then a larger heat sink is required (one that has a lower thermal resistance).
Included on the Switcher Made Simple design software is a more precise (non-linear) thermal model that can
be used to determine junction temperature with different input-output parameters or different component values.
It can also calculate the heat sink thermal resistance required to maintain the regulators junction temperature
below the maximum operating temperature.
Additional Applications
INVERTING REGULATOR
Figure 28 shows a LM2576-12 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 NEGATIVE BOOST REGULATOR)
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 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
•
fosc = 52 kHz
(8)
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.
Figure 28. Inverting Buck-Boost Develops −12V
20
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