Thermal Considerations

The two highest power dissipating components are the re-

circulating diode and the LM22671 regulator IC. The easiest

method to determine the power dissipation within the

LM22671 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:

P = (1 - D) x I

An approximation for the output inductor power is:

P = I

OUT

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 LM22671 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 LM22671 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

LM22671 PSOP-8 package is specified in the electrical char-

acteristics table under the applicable conditions.

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