OPA687 Burr-Brown, OPA687 Datasheet - Page 15

• Manufacturer Part Number: OPA687
• Description: Wideband / Ultra-Low Noise / Voltage Feedback OPERATIONAL AMPLIFIER With Power Down
• Manufacturer: Burr-Brown

DISABLE OPERATION
The OPA687 provides an optional disable feature that may
be used to reduce system power. If the DIS control pin is left
unconnected, the OPA687 will operate normally. To dis-
able, the control pin must be asserted low. Figure 9 shows
a simplified internal circuit for the disable control feature.
FIGURE 9. Simplified Disabled Control Circuit.
In normal operation, base current to Q1 is provided through
the 110k
15k
turn on the two diodes in Q1’s emitter. As V
low, additional current is pulled through the 15k
eventually turning on these two diodes ( 100 A). At this
point, any further current pulled out of V
those diodes holding the emitter-based voltage of Q1 at
approximately zero volts. This shuts off the collector current
out of Q1, turning the amplifier off. The supply current in
the disable mode are only those required to operate the
circuit of Figure 9.
THERMAL ANALYSIS
The OPA687 will not require heatsinking or airflow in most
applications. Maximum desired junction temperature will
set the maximum allowed internal power dissipation as
described below. In no case should the maximum junction
temperature be allowed to exceed 175 C.
Operating junction temperature (T
quiescent power (P
the output stage (P
power is simply the specified no-load supply current times
the total supply voltage across the part. P
the required output signal and load but would, for a grounded
resistive load, be at a maximum when the output is fixed at
a voltage equal to 1/2 either supply voltage (for equal bipolar
supplies). Under this condition P
includes feedback network loading. This is the absolute
highest power that can be dissipated for a given R
actual applications will dissipate less power in the output
stage.
JA
. The total internal power dissipation (P
resistor sets up a voltage drop that is inadequate to
V
DIS
resistor while the emitter current through the
DQ
DL
15k
25k
) and additional power dissipated in
) to deliver load power. Quiescent
Control
+V
I
S
S
Q1
DL
J
= V
) is given by T
S
–V
2
/(4 • R
DL
S
110k
DIS
D
will depend on
) is the sum of
goes through
DIS
L
) where R
is pulled
A
resistor,
+ P
L
. All
D
L
•
15
Note that it is the power in the output stage and not in the
load that determines internal power dissipation.
As an absolute worst-case example, compute the maximum
T
Figure 1 operating at the maximum specified ambient tem-
perature of +85 C and driving a grounded 100
P
Maximum T
All actual applications will operate at a lower junction
temperature than the 127 C computed above. Compute your
actual output stage power to get an accurate estimate of
maximum junction temperature, or use the results shown
here as an absolute maximum.
BOARD LAYOUT
Achieving optimum performance with a high frequency
amplifier like the OPA687 requires careful attention to
board layout parasitics and external component types. Rec-
ommendations that will optimize performance include:
a) Minimize parasitic capacitance to any AC ground for
all of the signal I/O pins. Parasitic capacitance on the
output and inverting input pins can cause instability: on the
non-inverting input, it can react with the source impedance
to cause unintentional bandlimiting. To reduce unwanted
capacitance, a window around the signal I/O pins should be
opened in all of the ground and power planes around those
pins. Otherwise, ground and power planes should be unbro-
ken elsewhere on the board.
b) Minimize the distance (< 0.25") from the power
supply pins to high frequency 0.1 F decoupling capaci-
tors. At the device pins, the ground and power plane layout
should not be in close proximity to the signal I/O pins. Avoid
narrow power and ground traces to minimize inductance
between the pins and the decoupling capacitors. The power
supply connections should always be decoupled with these
capacitors. Larger (2.2 F to 6.8 F) decoupling capacitors,
effective at lower frequency, should also be used on the
main supply pins. These may be placed somewhat farther
from the device and may be shared among several devices in
the same area of the PC board.
c) Careful selection and placement of external compo-
nents will preserve the high frequency performance of
the OPA687. Resistors should be a very low reactance type.
Surface-mount resistors work best and allow a tighter over-
all layout. Metal-film and carbon composition, axially-leaded
resistors can also provide good high frequency performance.
Again, keep their leads and PC board trace length as short as
possible. Never use wirewound type resistors in a high
frequency application. Since the output pin and inverting
input pin are the most sensitive to parasitic capacitance,
always position the feedback and series output resistor, if
any, as close as possible to the output pin. Other network
components, such as non-inverting input termination resis-
tors, should also be placed close to the package. Where
double-side component mounting is allowed, place the feed-
D
J
using an OPA687N (SOT23-6 package) in the circuit of
= 10V (20.5mA) + 5
J
= +85 C + (0.28W • 150 C/W) = 127 C
2
/(4 • (100
OPA687
|| 789 )) = 275mW
load.
®