Datasheet

LM2574, LM2574HV
www.ti.com SNVS104C –JUNE 1999–REVISED APRIL 2013
Because of the 0.5A current rating of the LM2574, the total package power dissipation for this switcher is quite
low, ranging from approximately 0.1W up to 0.75W under varying conditions. In a carefully engineered printed
circuit board, both the P and the NPA package can easily dissipate up to 0.75W, even at ambient temperatures
of 60°C, and still keep the maximum junction temperature below 125°C.
A curve, Figure 17, displaying thermal resistance vs. pc board area for the two packages is shown in Typical
Performance Characteristics of this data sheet.
These thermal resistance numbers are approximate, and there can be many factors that will affect the final
thermal resistance. Some of these factors include board size, shape, thickness, position, location, and board
temperature. Other factors are, the area of printed circuit copper, copper thickness, trace width, multi-layer,
single- or double-sided, and the amount of solder on the board. The effectiveness of the pc board to dissipate
heat also depends on the size, number and spacing of other components on the board. Furthermore, some of
these components, such as the catch diode and inductor will generate some additional heat. Also, the thermal
resistance decreases as the power level increases because of the increased air current activity at the higher
power levels, and the lower surface to air resistance coefficient at higher temperatures.
The data sheet thermal resistance curves and the thermal model in Switchers Made Simple software (version
3.3) can estimate the maximum junction temperature based on operating conditions. ln addition, the junction
temperature can be estimated in actual circuit operation by using the following equation.
T
j
= T
cu
+ (θ
j-cu
× P
D
) (4)
With the switcher operating under worst case conditions and all other components on the board in the intended
enclosure, measure the copper temperature (T
cu
) near the IC. This can be done by temporarily soldering a small
thermocouple to the pc board copper near the IC, or by holding a small thermocouple on the pc board copper
using thermal grease for good thermal conduction.
The thermal resistance (θ
j-cu
) for the two packages is:
θ
j-cu
= 42°C/W for the P-8 package
θ
j-cu
= 52°C/W for the NPA-14 package
The power dissipation (P
D
) for the IC could be measured, or it can be estimated by using the formula:
where
• I
S
is obtained from the typical supply current curve (adjustable version use the supply current vs. duty cycle
curve).
(5)
Additional Applications
INVERTING REGULATOR
Figure 31 shows a LM2574-12 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.
Note: Pin numbers are for the 8-pin PDIP package.
Figure 31. Inverting Buck-Boost Develops −12V
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