LM1575, LM2575-N, LM2575HV
SNVS106E –MAY 1999–REVISED APRIL 2013
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The dynamic losses during turn-on and turn-off are negligible if a Schottky type catch diode is used.
When no heat sink is used, the junction temperature rise can be determined by the following:
ΔTJ = (PD) (θJA)
(11)
To arrive at the actual operating junction temperature, add the junction temperature rise to the maximum ambient
temperature.
TJ = ΔTJ + TA
(12)
If the actual operating junction temperature is greater than the selected safe operating junction temperature
determined in step 3, then a heat sink is required.
When using a heat sink, the junction temperature rise can be determined by the following:
ΔTJ = (PD) (θJC + θinterface + θHeat sink
)
(13)
The operating junction temperature will be:
TJ = TA + ΔTJ
(14)
As shown 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).
When using the LM2575 in the plastic CDIP or surface mount SOIC packages, several items about the thermal
properties of the packages should be understood. The majority of the heat is conducted out of the package
through the leads, with a minor portion through the plastic parts of the package. Since the lead frame is solid
copper, heat from the die is readily conducted through the leads to the printed circuit board copper, which is
acting as a heat sink.
For best thermal performance, the ground pins and all the unconnected pins should be soldered to generous
amounts of printed circuit board copper, such as a ground plane. Large areas of copper provide the best transfer
of heat to the surrounding air. Copper on both sides of the board is also helpful in getting the heat away from the
package, even if there is no direct copper contact between the two sides. Thermal resistance numbers as low as
40°C/W for the SOIC package, and 30°C/W for the CDIP package can be realized with a carefully engineered pc
board.
Included on the Switchers 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 32 shows a LM2575-12 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
0.35A. 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
1.5A. Using a delayed turn-on or an undervoltage lockout circuit (described in the NEGATIVE BOOST
REGULATOR 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 buck
regulator design procedure section cannot 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).
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