Datasheet

LM22670

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SNVS584O –SEPTEMBER 2008–REVISED MARCH 2013

Circuit Board Layout

Board layout is critical for the proper operation of switching power supplies. First, the ground plane area must be

sufficient for thermal dissipation purposes. Second, appropriate guidelines must be followed to reduce the effects

of switching noise. Switch mode converters are very fast switching devices. In such cases, the rapid increase of

input current combined with the parasitic trace inductance generates unwanted L di/dt noise spikes. The

magnitude of this noise tends to increase as the output current increases. This noise may turn into

electromagnetic interference (EMI) and can also cause problems in device performance. Therefore, care must be

taken in layout to minimize the effect of this switching noise.

The most important layout rule is to keep the AC current loops as small as possible. Figure 23 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 LM22670, the bypass capacitor, the Schottky diode, R

FBB

, 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 SNVA054 for more information.

Figure 23. 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

LM22670 regulator. The easiest method to determine the power dissipation within the LM22670 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.

Product Folder Links: LM22670