US3012/3012A
Layout Considerations
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
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
Rev. 1.0
5/6/98
4-11
UNISEM [ UNISEM ]
