廬
ADD CURRENT LIMIT TO THE BUF634
By David Jones and Mark Stitt
Modern buffer amplifiers, such as the BUF634, are often
used with op amps to drive high-current demand loads such
as cables or electro-mechanical devices. BUF634 features
internal overload protection to prevent its destruction under
fault conditions. Internal current limit protects circuitry and
wirebonds from excessive current. Internal over-temp shut
down protects the device from excessive temperature rise by
shutting down the output drive before catastrophic chip
temperatures are reached (approximately 165擄C).
The internal BUF634 current limit can allow output current
to exceed 500mA. In some applications, it is necessary to
limit the output current to a lower level to protect the load.
External current limit can be added to the BUF634 with the
addition of a current-sense resistor and a few other compo-
nents.
Figure 1 shows the basic circuit for BUF634 external current
limit. The BUF634 (A
3
) is connected in the feedback loop of
A
2
forming a precision closed-loop buffer鈥攖he output volt-
age of A
3
is equal to the input of A
2
. A current-sense resistor,
R
CL
, is connected from the buffer to the circuit output, V
OUT
.
The buffer and resistor are enclosed in the feedback loop of
A
1
with a diode connected across the A
2
, A
3
buffer. Output
current produces a voltage drop across R
CL
. Until the voltage
across R
CL
approaches the forward voltage drop of D
1
, the
overall loop amplifier, A
1
, maintains V
OUT
precisely equal
to V
IN
. As the voltage across R
CL
increases, drive current
from A
1
is diverted through D
1
to the output and current
limit is achieved. Output current limit is the sum of the
current limit of A
1
plus the forward voltage drop across D
1
divided by R
CL
. For bipolar current limit, back-to-back
diodes can be used as shown in Figure 2.
R
1
C
1
D
1
R
1
10k鈩?/div>
C
1
100pF
D
2
D
1
1/2
OPA2277
V
IN
A
1
A
2
1/2
OPA2277
A
3
BUF634
BW
R
CL
10鈩?/div>
V
OUT
Overall I
LIMIT
鈮?/div>
A
1
I
LIMIT
+ 0.7V/10鈩?
V鈥?/div>
FIGURE 2. Adding a second diode to the Figure 1 circuit
provides bipolar current limit. With the values
shown, current limit is approximately
鹵100mA.
For stability, the bandwidth of A
2
must be less than approxi-
mately one-fourth the bandwidth of A
3
, and R
1
C
1
/(2蟺) must
be less than approximately one-fourth the bandwidth of A
2
.
With its bandwidth control pin unconnected, BUF634 band-
width is 20MHz typ. Connecting the bandwidth control pin
to V鈥?increases bandwidth to approximately 160MHz.
In some cases, it may also be necessary to compensate the
A
2
鈥?A
3
loop as shown in Figure 3. In this case, R
2
C
2
/(2蟺)
must be less than approximately one-fourth the bandwidth of
A
3
and R
1
C
1
must be less than (R
2
C
2
)/4.
R
1
D
2
D
1
1/2
OPA2277
V
IN
A
1
A
2
1/2
OPA2277
A
3
BUF634
BW
R
CL
V
OUT
C
1
C
2
R
2
A3
V鈥?/div>
FIGURE 1. Current limit can be added to the BUF634 with
the addition of a current-sense resistor and a
few other components. Output current produces
a voltage drop across the current limit resistor,
R
CL
. As the voltage across R
CL
increases, drive
current from A
1
is diverted through D
1
to the
output and current limit is achieved.
漏
1/2
OPA2277
V
IN
A1
1/2
OPA2277
A2
R
C1
10鈩?/div>
V
OUT
BUF634
BW
V鈥?/div>
FIGURE 3. In some applications, it may be necessary to add
compensation around the A
2
, A
3
loop.
AB-137
1
Printed in U.S.A. January, 1999
1999 Burr-Brown Corporation
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