US3004,US3005
Layout Considerations
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 the 4 layer board, dedicate on layer to GND, an-
other layer as the power layer for the 5V, 3.3V, Vcore,
1.5V and if it is possible for the 2.5V.
Connect all grounds to the ground plane using direct
vias to the ground plane.
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:
Use large low inductance/low impedance plane to con-
nect the following connections either using component
side or the solder side.
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.
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
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 12 and C6
next to the 5V, pin 5.
g) C9 to Q4 drain
h) C12 to the Q4 source
Connect the rest of the components using the shortest
connection possible
6) Place the controller IC such that the pwm output
drives, pins 9 and 11 are relatively short distance from
gates of Q1 and Q2.
7) Place resistor dividers, R7 & R8 close to pin 3, R12
& R13 (note 1) close to pin 14 and R14 and R15 (note 1)
close to pin 20.
Note 1: Although, the PWM controller does not require
R12-15 resistors, and the feedback pins 3 and 14 can
be directly connected to their respective outputs, they
can be used to set the outputs slightly higher to ac-
count for any output drop at the load due to the trace
resistance.
8) Place R11, C15, Q3 and C11 close to each other and
do the same with R9, C14, Q4 and C12. Note: It is
better to place the linear regulator components close to
the IC and then run a trace from the output of each regu-
lator to its respective load such as 2.5V to the clock and
1.5V for GTL + termination. However, if this is not pos-
sible then the trace from the linear drive output pins,
pins 2 and 20 must be routed away from any high
frequency data signals.
It is critical, to place high frequency ceramic ca-
pacitors close to the clock chip and termination
resistors to provide local bypassing.
9) Place timing capacitor C1 close to pin1 and soft start
capacitor C2 close to pin 13
Rev. 1.2
12/8/00
4-14
UNISEM [ UNISEM ]
