Datasheet

6
®
OPA633
APPLICATIONS INFORMATION
As with any high frequency circuitry, good circuit layout
technique must be used to achieve optimum performance.
Power supply connections must be bypassed with high
frequency capacitors. Many applications benefit from the
use of two capacitors on each power supply—a ceramic
capacitor for good high frequency decoupling and a tanta-
lum type for lower frequencies. They should be located as
close as possible to the buffer’s power supply pins. A large
ground plane is used to minimize high frequency ground
drops and stray coupling.
Pin 6 connects to the substrate of the integrated circuit and
should be connected to ground. In principle it could also be
connected to +V
S
or –V
S
, but ground is preferable. The
additional lead length and capacitance associated with sock-
ets may cause problems in applications requiring the highest
fidelity of high speed pulses.
Depending on the nature of the input source impedance, a
series input resistor may be required for best stability. This
behavior is influenced somewhat by the load impedance
(including any reactive effects). A value of 50Ω to 200Ω is
typical. This resistor should be located close to the OPA633’s
input pin to avoid stray capacitance at the input which could
reduce bandwidth (see Gain and Phase versus Frequency
curve).
OVERLOAD CONDITIONS
The input and output circuitry of the OPA633 are not
protected from overload. When the input signal and load
characteristics are within the devices’s capabilities, no pro-
tection circuitry is required. Exceeding device limits can
result in permanent damage.
The OPA633’s small package and high output current capa-
bility can lead to overheating. The internal junction tempera-
ture should not be allowed to exceed 150°C. Although
failure is unlikely to occur until junction temperature
exceeds 200°C, reliability of the part will be degraded
significantly at such high temperatures. Since significant
heat transfer takes place through the package leads, wide
printed circuit traces to all leads will improve heat sinking.
Sockets reduce heat transfer significantly and are not recom-
mended.
Junction temperature rise is proportional to internal power
dissipation. This can be reduced by using the minimum
supply voltage necessary to produce the required output
voltage swing. For instance, 1V video signals can be easily
handled with ±5V power supplies thus minimizing the
internal power dissipation.
Output overloads or short circuits can result in permanent
damage by causing excessive output current. The 50Ω or
75Ω series output resistor used to match line impedance
will, in most cases, provide adequate protection. When this
resistor is not used, the device can be protected by limiting
the power supply current. See “Protection Circuits.”
Excessive input levels at high frequency can cause increased
internal dissipation and permanent damage. See the safe
input voltage versus frequency curves. When used to buffer
an op amp’s output, the input to the OPA633 is limited, in
most cases, by the op amp. When high frequency inputs can
exceed safe levels, the device must be protected by limiting
the power supply current.
PROTECTION CIRCUITS
The OPA633 can be protected from damage due to exces-
sive currents by the simple addition of resistors in series with
the power supply pins (Figure 5a). While this limits output
current, it also limits voltage swing with low impedance
loads. This reduction in voltage swing is minimal for AC or
high crest factor signals since only the average current from
the power supply causes a voltage drop across the series
resistor. Short duration load-current peaks are
supplied by the bypass capacitors.
The circuit of Figure 5b overcomes the limitations of the
previous circuit with DC loads. It allows nearly full output
voltage swing up to its current limit of approximately 140mA.
Both circuits require good high frequency capacitors (e.g.,
tantalum) to bypass the buffer’s power supply connections.
CAPACITIVE LOADS
The OPA633 is designed to safely drive capacitive loads up
to 0.01µF. It must be understood, however, that rapidly
changing voltages demand large output load currents:
I
LOAD
= C
LOAD
Thus, a signal slew rate of 1000V/µs and load capacitance of
0.01µF demands a load current of 10A. Clearly maximum
slew rates cannot be combined with large capacitive loads.
Load current should be kept less than 100mA continuous
(200mA peak) by limiting the rate of change of the input
signal or reducing the load capacitance.
USE INSIDE A FEEDBACK LOOP
The OPA633 may be used inside the feedback path of an op
amp such as the OPA602. Higher output current is achieved
without degradation in accuracy. This approach may actu-
ally improve performance in precision applications by re-
moving load-dependent dissipation from a precision op amp.
All vestiges of load-dependent offset voltage and tempera-
ture drift can be eliminated with this technique. Since the
buffer is placed within the feedback loop of the op amp, its
DC errors will have a negligible effect on overall accuracy.
Any DC errors contributed by the buffer are divided by the
loop gain of the op amp.
The low phase shift of the OPA633 allows its use inside the
feedback loop of a wide variety of op amps. To assure
stability, the buffer must not add significant phase shift to
the loop at the gain crossing frequency of the circuit—the
frequency at which the open loop gain of the op amp is equal
to the closed loop gain of the application. The OPA633 has
a typical phase shift of less than 10° up to 70MHz, thus
making it useful even with wideband op amps.
dV
dt