Datasheet

LM22677

SNVS582M –SEPTEMBER 2008–REVISED APRIL 2013

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Figure 22. Current Flow in a Buck Application

Thermal Considerations

The components with the highest power dissipation are the power diode and the power MOSFET internal to the

LM22677 regulator. The easiest method to determine the power dissipation within the LM22677 is to measure

the total conversion losses then subtract the power losses in the diode and inductor. The total conversion loss is

the difference between the input power and the output power. An approximation for the power diode loss is:

(17)

Where V

is the diode voltage drop. An approximation for the inductor power is:

(18)

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 multiple vias under the device to the

ground plane will greatly reduce the regulator junction temperature. Selecting a diode with an exposed pad will

also aid the power dissipation of the diode. The most significant variables that affect the power dissipation of the

regulator are output current, input voltage and operating 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 LM22677 will vary with the application. The most significant 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. A

large continuos ground plane on the top of bottom PCB layer will provide the most effective heat dissipation. The

integrity of the solder connection 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 LM22677 PFM

package is specified in the Electrical Characteristics table. See application note AN-2020 SNVA419 for more

information.

Product Folder Links: LM22677