LM22679TJ-5.0 NSC [National Semiconductor], LM22679TJ-5.0 Datasheet - Page 11

• Manufacturer Part Number: LM22679TJ-5.0
• Manufacturer: NSC [National Semiconductor]

Thermal Considerations
The two highest power dissipating components are the re-
circulating diode and the LM22679 regulator IC. The easiest
method to determine the power dissipation within the
FIGURE 5. Current Flow in a Buck Application
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LM22679 is to measure the total conversion losses (Pin –
Pout) then subtract the power losses in the Schottky diode
and output inductor. An approximation for the Schottky diode
loss is:
An approximation for the output inductor power is:
where R is the DC resistance of the inductor and the 1.1 factor
is an approximation for the AC losses. The regulator has an
exposed thermal pad to aid power dissipation. Adding several
vias under the device to the ground plane will greatly reduce
the regulator junction temperature. Selecting a diode with an
exposed pad will aid the power dissipation of the diode. The
most significant variables that affect the power dissipated by
the LM22679 are the output current, input voltage and oper-
ating frequency. The power dissipated while operating near
the maximum output current and maximum input voltage can
be appreciable. The junction-to-ambient thermal resistance of
the LM22679 will vary with the application. The most signifi-
cant variables are the area of copper in the PC board, the
number of vias under the IC exposed pad and the amount of
forced air cooling provided. The integrity of the solder con-
nection from the IC exposed pad to the PC board is critical.
Excessive voids will greatly diminish the thermal dissipation
capacity. The junction-to-ambient thermal resistance of the
LM22679 TO-263 THIN package is specified in the electrical
characteristics table under the applicable conditions. For
more information regarding the TO-263 THIN package, refer
to Application Note AN-1797 at www.national.com.
P = (1 - D) x I
P = I
OUT
2
x R x 1.1,
OUT
x V
D
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