US3012/3012A

4-11

Rev. 1.0

5/6/98

Layout Considerations

Switching regulators require careful attention to the lay-

out of the components, specifically power components

since they switch large currents. These switching com-

ponents can create large amount of voltage spikes and

high frequency harmonics if some of the critical compo-

nents are far away from each other and are connected

with inductive traces. The following is a guideline of how

to place the critical components and the connections

between them in order to minimize the above issues.

Start the layout by first placing the power components:

1)

Place the input capacitors C3 and the high side

mosfet ,Q1 as close to each other as possible

2)

Place the synchronous mosfet,Q2 and the Q1 as

close to each other as possible with the intention that

the source of Q1 and drain of the Q2 has the shortest

length.

3) Place the snubber R4 & C7 between Q1 & Q2.

4) Place the output inductor ,L2 and the output capaci-

tors ,C10 between the mosfet and the load with output

capacitors distributed along the slot 1 and close to it.

5) Place the bypass capacitors, C4 and C6 right next to

12V and 5V pins. C4 next to the 12V, pin 13 and C6

next to the 5V, pin 6.

6) Place the IC such that the pwm output drives, pins

12 and 9 are relatively short distance from gates of Q1

and Q2.

7) If the ouput voltage is to be adjusted, place resistor

dividers, R10 & R11 close to the feedback pin.

Note 1: Although, the device does not require resistor

dividers and the feedback pin can be directly connected

to the output, they can be used to set the outputs slightly

higher to account for any output drop at the load due to

the trace resistance. See the application note.

8) Place timing capacitor C8 close to pin1 and soft start

capacitor C2 close to pin 16.

Component connections:

Note : It is extremely important that no data bus

should be passing through the switching regulator

section specifically close to the fast transition nodes

such as PWM drives or the inductor voltage.

Using 4 layer board, dedicate on layer to GND, another

layer as the power layer for the 5V, 3.3V and Vcore.

Connect all grounds to the ground plane using di-

rect vias to the ground plane.

Use large low inductance/low impedance plane to con-

nect the following connections either using component

side or the solder side.

a) C3 to Q1 Drain

b) Q1 Source to Q2 Drain

c) Q2 drain to L2

d) L2 to the output capacitors, C10

e) C10 to the slot 1

f) Input filter L1 to the C3

Connect the rest of the components using the shortest

connection possible