14
Rev. 1.7
07/16/02
IRU3004
www.irf.com
Input Filter
It is recommended to place an inductor between the
system 5V supply and the input capacitors of the switch-
ing regulator to isolate the 5V supply from the switching
noise that occurs during the turn on and off of the switch-
ing components. Typically an inductor in the range of 1
to 3mH will be sufficient in this type of application.
Switcher External Shutdown
The best way to shutdown the switcher is to pull down
on the soft-start pin using an external small signal tran-
sistor such as 2N3904 or 2N7002 small signal MOSFET.
This allows slow ramp up of the output, the same as the
power up.
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 12 and C6
next to the 5V, pin 5.
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 (see note) close to pin 14 and R14 and R15
(see note) close to pin 20.
Note: 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
account 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 compo-
nents close to the IC and then run a trace from the
output of each regulator to its respective load such
as 2.5V to the clock and 1.5V for GTL + termination.
However, if this is not possible 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 capaci-
tors close to the clock chip and termination resistors
to provide local bypassing.
9) Place timing capacitor C1 close to pin 1 and soft
start capacitor C2 close to pin 13.
Component connections:
Note: It is extremely important that no data bus should
be passing through the switching regulator section spe-
cifically close to the fast transition nodes such as PWM
drives or the inductor voltage.
Using the 4 layer board, dedicate on layer to ground,
another 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.
Use large low inductance/low impedance plane to con-
nect the following connections either using component
side or the solder side:
Connect the rest of the components using the shortest
connection possible.
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
g) C9 to Q4 drain
h) C12 to the Q4 source
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Data and specifications subject to change without notice. 02/01
