FMS6141_06 Datasheet, PDF (8/11 Pages) Fairchild Semiconductor – Low-Cost, Single-Channel 4th-Order Standard Definition Video Filter Driver

Layout Considerations
General layout and supply bypassing play a major role
in high-frequency performance and thermal
characteristics. Fairchild offers a demonstration board
for the FMS6141 to guide layout and aid device
evaluation. The demo board is a four-layer board with
full power and ground planes. Following this layout
configuration provides optimum performance and
thermal characteristics for the device. For the best
results, follow the steps and recommended routing rules
listed below.
Recommended Routing/Layout Rules
ƒ Do not run analog and digital signals in parallel.
ƒ Use separate analog and digital power planes to
supply power.
ƒ Traces should run on top of the ground plane at all
times.
ƒ No trace should run over ground/power splits.
ƒ Avoid routing at 90-degree angles.
ƒ Minimize clock and video data trace length
differences.
ƒ Include 10µF and 0.1µF ceramic power supply bypass
capacitors.
ƒ Place the 0.1µF capacitor within 0.1 inches of the
device power pin.
ƒ Place the 10µF capacitor within 0.75 inches of the
device power pin.
ƒ For multilayer boards, use a large ground plane to
help dissipate heat.
ƒ For two-layer boards, use a ground plane that extends
beyond the device body by at least 0.5 inches on all
sides. Include a metal paddle under the device on the
top layer.
ƒ Minimize all trace lengths to reduce series inductance.
Thermal Considerations
Since the interior of most systems, such as set-top
boxes, TVs, and DVD players are at +70ºC;
consideration must be given to providing an adequate
heat sink for the device package for maximum heat
dissipation. When designing a system board, determine
how much power each device dissipates. Ensure that
devices of high power are not placed in the same
location, such as directly above (top plane) and below
(bottom plane) each other on the PCB.
PCB Thermal Layout Considerations
ƒ Understand the system power requirements and
environmental conditions.
ƒ Maximize thermal performance of the PCB.
ƒ Consider using 70µm of copper for high-power
designs.
ƒ Make the PCB as thin as possible by reducing FR4
thickness.
ƒ Use vias in power pad to tie adjacent layers together.
ƒ Remember that baseline temperature is a function of
board area, not copper thickness.
ƒ Modeling techniques can provide a first-order
approximation.
Power Dissipation
Consider the FMS6141’s output drive configuration
when calculating overall power dissipation. Care must
be taken not to exceed the maximum die junction
temperature. The following example can be used to
calculate the FMS6141’s power dissipation and internal
temperature rise.
T_J = T_A + P_CHANNEL Θ_JA
where P_{CHANNEL =} V_CC • I_CH + (V_O2/R_L)
V_O = 2V_IN + 0.280V
I_CH = I_CC + (V_O/R_L)
V_IN = RMS value of input signal
I_CC = 7mA
V_S = 5V
R_L = channel load resistance
The FMS6141 is specified to operate with output
currents typically less than 50mA, which is more than
sufficient for a dual (75Ω) video load. The internal
amplifiers of the FMS6141 are current limited to a
maximum of 100mA and can withstand a brief-duration
short-circuit condition, but this capability is not guaranteed.
© 2006 Fairchild Semiconductor Corporation
FMS6141 Rev. 1.0.1
8
www.fairchildsemi.com