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RC5050
PRODUCT SPECIFICATION
16
2.
Place decoupling capacitors (0.1
m
F) as close to the
RC5050 pins as possible. Extra lead length on these
capacitors will negate their ability to suppress noise.
3.
Each VCC and GND pin should have its own via down
to the appropriate plane underneath. This will help to
add isolation between pins.
4.
The CEXT timing capacitor should be surrounded with
a ground trace if possible. The placement of a ground or
power plane underneath the capacitor will also provide
further noise isolation. This will help to shield the oscil-
lator from the noise on the PCB. This capacitor should
be placed as close to pin 1 as possible.
5.
Group the MOSFETs, inductor and Schottky as close
together as possible for the same reasons as #1 above.
Also place the input bulk capacitors as close to the
drains of MOSFETs as possible. In addition, placement
of a 0.1
m
F decoupling cap right on the drain of each
MOSFET will help to suppress some of the high
frequency switching noise on the input of the DC-DC
converter.
6.
The traces that run from the RC5050 IFB (pin 4) and
VFB (pin 5) pins should be run together next to each
other and be Kelvin connected to the sense resistor.
Running these lines together will help in rejecting some
of the common noise that is presented to the RC5050
feedback input. Try as much as possible to run the noisy
switching signals (HIDRV & VCCQP) on one layer and
use the inner layers for power and ground only. If the
top layer is being used to route all of the noisy switching
signals, use the bottom layer to route the analog sensing
signals VFB and IFB.
PC Board Layout Checklist
Bypass Capacitor near Vref pin.
This pin should be adequately bypassed with a 0.1
m
F
capacitor.
Bypass Capacitors for VCC (5V).
A 0.1
m
F should be placed right next to the VCC pin of the
controller.
Bypass Capacitors for Power MOSFET.
A 0.1
m
F cap should be placed at the drain connection of
each power MOSFET.
5V Connection to the controller IC.
Each VCC pin on the IC should be connected to the 5V
power plane through its own via.
Power MOSFET Gate Drive Trace.
– The gate drive trace should be routed on one layer only.
– The controller IC and the power FET should be
oriented in such a way as to minimize the trace length
of the gate drive trace (< 1 inch).
– The gate drive trace routing should stay away from the
quiet analog section of the RC50XX controller IC.
(i.e. keep away from Vref, IFB, VFB, and CEXT.)
Bulk Capacitance.
– The input bulk capacitance needs to be located less
than 1" from the drain of the power MOSFET. We
recommend the following guidelines for the amount of
bulk input capacitance:
For an output load of <10A use 2 X 1500
m
F caps.
For an output load of >10A use 3 X 1500
m
F caps.
– The output bulk capacitors should be located as close to
the CPU socket as possible. We recommend the
following guidelines for the amount of bulk output
capacitance:
For Pentium Pro use 4 X 1500
m
F.
For P55C MMX Pentium/ AMD K6 use 2X 1500
m
F.
For Pentium II use 7 X 1500
m
F.
Figure 7. Examples of Good and Bad MOSFET Layout
Correct layout
8
7
6
2
11
15
14
13
12
3
16
1
5
4
Poor layout
RC5050
“Quiet" Pins
65-5050-10
=
9
10
17
18
19
20
8
7
6
2
11
15
14
13
12
3
16
1
5
4
9
10
17
18
19
20
RC5050