Datasheet
THERMAL ANALYSIS
T =T +P ´ q
J D JA
A
(6)
P =10V(36mA)+5 /(4 100 )=422.5mW´ W
D
2
(7)
MaximumT =+85 C+(0.443W 80 C/W)=120.5 C° ´ ° °
J
BOARD LAYOUT
VCA821
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....................................................................................................................................... SBOS407B – DECEMBER 2007 – REVISED DECEMBER 2008
band limiting. To reduce unwanted capacitance, a
window around the signal I/O pins should be opened
The VCA821 does not require heatsinking or airflow
in all of the ground and power planes around those
in most applications. The maximum desired junction
pins. Otherwise, ground and power planes should be
temperature sets the maximum allowed internal
unbroken elsewhere on the board. Place a small
power dissipation as described in this section. In no
series resistance (greater than 25 Ω ) with the input pin
case should the maximum junction temperature be
connected to ground to help decouple package
allowed to exceed +150 ° C.
parasitics.
Operating junction temperature (T
J
) is given by
b) Minimize the distance (less than 0.25 inches, or
Equation 6 :
6.3mm) from the power-supply pins to high-frequency
0.1 µ F decoupling capacitors. At the device pins, the
ground and power plane layout should not be in close
The total internal power dissipation (P
D
) is the sum of
proximity to the signal I/O pins. Avoid narrow power
quiescent power (P
DQ
) and additional power
and ground traces to minimize inductance between
dissipated in the output stage (P
DL
) to deliver load
the pins and the decoupling capacitors. The
power. Quiescent power is simply the specified
power-supply connections should always be
no-load supply current times the total supply voltage
decoupled with these capacitors. Larger (2.2 µ F to
across the part. P
DL
depends on the required output
6.8 µ F) decoupling capacitors, effective at lower
signal and load; for a grounded resistive load,
frequencies, should also be used on the main supply
however, it is at a maximum when the output is fixed
pins. These capacitors may be placed somewhat
at a voltage equal to one-half of either supply voltage
farther from the device and may be shared among
(for equal bipolar supplies). Under this worst-case
several devices in the same area of the PCB.
condition, P
DL
= V
S
2
/(4 × R
L
), where R
L
is the
resistive load. c) Careful selection and placement of external
components preserve the high-frequency
Note that it is the power in the output stage and not in
performance of the VCA821. Resistors should be a
the load that determines internal power dissipation.
very low reactance type. Surface-mount resistors
As a worst-case example, compute the maximum T
J
work best and allow a tighter overall layout. Metal-film
using a VCA821ID (SO-14 package) in the circuit of
and carbon composition, axially-leaded resistors can
Figure 74 operating at maximum gain and at the
also provide good high-frequency performance.
maximum specified ambient temperature of +85 ° C.
Again, keep the leads and PCB trace length as short
as possible. Never use wire-wound type resistors in a
high-frequency application. Because the output pin is
the most sensitive to parasitic capacitance, always
(8)
position the series output resistor, if any, as close as
possible to the output pin. Other network
This maximum operating junction temperature is well
components, such as inverting or non-inverting input
below most system level targets. Most applications
termination resistors, should also be placed close to
should be lower because an absolute worst-case
the package.
output stage power was assumed in this calculation
of V
CC
/2, which is beyond the output voltage range for
d) Connections to other wideband devices on the
the VCA821.
board may be made with short direct traces or
through onboard transmission lines. For short
connections, consider the trace and the input to the
next device as a lumped capacitive load. Relatively
Achieving optimum performance with a
wide traces (50mils to 100mils, or 1.27mm to
high-frequency amplifier such as the VCA821
2.54mm) should be used, preferably with ground and
requires careful attention to printed circuit board
power planes opened up around them.
(PCB) layout parasitics and external component
types. Recommendations to optimize performance
e) Socketing a high-speed part like the VCA821 is
include:
not recommended. The additional lead length and
pin-to-pin capacitance introduced by the socket can
a) Minimize parasitic capacitance to any ac ground
create an extremely troublesome parasitic network,
for all of the signal I/O pins. This recommendation
which can make it almost impossible to achieve a
includes the ground pin (pin 2). Parasitic capacitance
smooth, stable frequency response. Best results are
on the output can cause instability: on both the
obtained by soldering the VCA821 onto the board.
inverting input and the noninverting input, it can react
with the source impedance to cause unintentional
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