Datasheet

LM22675

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SNVS591K –SEPTEMBER 2008–REVISED MARCH 2013

The most important layout rule is to keep the AC current loops as small as possible. Figure 17 shows the current

flow in a buck converter. The top schematic shows a dotted line which represents the current flow during the FET

switch on-state. The middle schematic shows the current flow during the FET switch off-state.

The bottom schematic shows the currents referred to as AC currents. These AC currents are the most critical

since they are changing in a very short time period. The dotted lines of the bottom schematic are the traces to

keep as short and wide as possible. This will also yield a small loop area reducing the loop inductance. To avoid

functional problems due to layout, review the PCB layout example. Best results are achieved if the placement of

the LM22675, the bypass capacitor, the Schottky diode, R

FFB

, R

FBT

and the inductor are placed as shown in the

example. Note that, in the layout shown, R1 = R

FBB

and R2 = R

FBT

. It is also recommended to use 2oz copper

boards or heavier to help thermal dissipation and to reduce the parasitic inductances of board traces. See

application note AN-1229 SNVA054for more information.

Figure 17. 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

LM22675 regulator. The easiest method to determine the power dissipation within the LM22675 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 LM22675 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 continuous ground plane on the top or 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 LM22675 SO

PowerPAD-8 package is specified in the Electrical Characteristics table. See application note AN-2020 SNVA419

for more information.

Product Folder Links: LM22675