Circuit Board Layout

Board layout is critical for 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 con-

verters are very fast switching devices. In such devices, 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 parasitic spike noise may turn into

electromagnetic interference (EMI) and can also cause prob-

lems 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 6 shows the current flow of a buck

converter. The top schematic shows a dotted line which rep-

resents 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 current

is changing in very short time periods. The dotted lines of the

bottom schematic are the traces to keep as short as possible.

This will also yield a small loop area reducing the loop induc-

tance. To avoid functional problems due to layout, review the

PCB layout example. Providing 5A of output current in a very

low thermal resistance package such as the TO-263 THIN is

challenging considering the trace inductances involved. Best

results are achieved if the placement of the LM22677, the by-

pass capacitor, the Schottky diode and the inductor are

placed as shown in the example. It is also recommended to

use 2oz copper boards or thicker to help thermal dissipation

and to reduce the parasitic inductances of board traces.

It is very important to ensure that the exposed DAP on the

TO-263 THIN package is soldered to the ground area of the

PCB to reduce the AC trace length between the bypass ca-

pacitor ground and the ground connection to the LM22677.

Not soldering the DAP to the board may result in erroneous

operation due to excessive noise on the board.

30074124

FIGURE 6. Current Flow in a Buck Application

Thermal Considerations

The two highest power dissipating components are the re-

circulating diode and the LM22677 regulator IC. The easiest

method to determine the power dissipation within the

LM22677 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 LM22677 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 LM22677 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

LM22677 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.

www.national.com

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