THERMAL RESISTANCE vs
CIRCUIT BOARD COPPER AREA
60
50
40
30
20
10
Circuit Board Copper Area
BUF634F
Surface Mount Package
1oz copper
BUF634F
Surface Mount Package
0
1
2
3
4
5
Copper Area (inches2)
FIGURE 3. Thermal Resistance vs Circuit Board Copper Area.
POWER DISSIPATION
the quiescent current to approximately 15mA. Intermediate
bandwidths can be set by connecting a resistor in series with
the bandwidth control pin—see typical curve "Quiescent
Current vs Resistance" for resistor selection. Characteristics
of the bandwidth control pin can be seen in the simplified
circuit diagram, Figure 1.
Power dissipation depends on power supply voltage, signal
and load conditions. With DC signals, power dissipation is
equal to the product of output current times the voltage
across the conducting output transistor, VS – VO. Power
dissipation can be minimized by using the lowest possible
power supply voltage necessary to assure the required output
voltage swing.
The rated output current and slew rate are not affected by the
bandwidth control, but the current limit value changes slightly.
Output voltage swing is somewhat improved in the wide
bandwidth mode. The increased quiescent current when in
wide bandwidth mode produces greater power dissipation
during low output current conditions. This quiescent power
is equal to the total supply voltage, (V+) + |(V–)|, times the
quiescent current.
For resistive loads, the maximum power dissipation occurs
at a DC output voltage of one-half the power supply voltage.
Dissipation with AC signals is lower. Application Bulletin
AB-039 explains how to calculate or measure power dissi-
pation with unusual signals and loads.
Any tendency to activate the thermal protection circuit
indicates excessive power dissipation or an inadequate heat
sink. For reliable operation, junction temperature should be
limited to 150°C, maximum. To estimate the margin of
safety in a complete design, increase the ambient tempera-
ture until the thermal protection is triggered. The thermal
protection should trigger more than 45°C above the maxi-
mum expected ambient condition of your application.
BOOSTING OP AMP OUTPUT CURRENT
The BUF634 can be connected inside the feedback loop of
most op amps to increase output current—see Figure 4.
When connected inside the feedback loop, the BUF634’s
offset voltage and other errors are corrected by the feedback
of the op amp.
To assure that the op amp remains stable, the BUF634’s
phase shift must remain small throughout the loop gain of
the circuit. For a G=+1 op amp circuit, the BUF634 must
contribute little additional phase shift (approximately 20° or
less) at the unity-gain frequency of the op amp. Phase shift
is affected by various operating conditions that may affect
stability of the op amp—see typical Gain and Phase curves.
INPUT CHARACTERISTICS
Internal circuitry is protected with a diode clamp connected
from the input to output of the BUF634—see Figure 1. If the
output is unable to follow the input within approximately 3V
(such as with an output short-circuit), the input will conduct
increased current from the input source. This is limited by
the internal 200Ω resistor. If the input source can be dam-
aged by this increase in load current, an additional resistor
can be connected in series with the input.
Most general-purpose or precision op amps remain unity-
gain stable with the BUF634 connected inside the feedback
loop as shown. Large capacitive loads may require the
BUF634 to be connected for wide bandwidth for stable
operation. High speed or fast-settling op amps generally
require the wide bandwidth mode to remain stable and to
assure good dynamic performance. To check for stability
with an op amp, look for oscillations or excessive ringing on
signal pulses with the intended load and worst case condi-
tions that affect phase response of the buffer.
BANDWIDTH CONTROL PIN
The –3dB bandwidth of the BUF634 is approximately 30MHz
in the low quiescent current mode (1.5mA typical). To select
this mode, leave the bandwidth control pin open (no connec-
tion).
Bandwidth can be extended to approximately 180MHz by
connecting the bandwidth control pin to V–. This increases
®
8
BUF634
BB [ BURR-BROWN CORPORATION ]
