
OP196/OP296/OP496
REV. A
–14–
Figure 45 shows the regulator’s recovery characteristics when its
output underwent a 20 mA to 50 mA step current change.
10
0%
100
90
2V
50μs
10mV
50mA
30mA
OUTPUT
STEP
CURRENT
CONTROL
WAVEFORM
Figure 45. Output Step Load Current Recovery
Buffering a DAC Output
Multichannel T rimDACs (T rimDAC) such as the AD8801/
AD8803, are widely used for digital nulling and similar applica-
tions. T hese DACs have rail-to-rail output swings, with a nomi-
nal output resistance of 5 k
. If a lower output impedance is
required, an OP296 amplifier can be added. T wo examples are
shown in Figure 45. One amplifier of an OP296 is used as a
simple buffer to reduce the output resistance of DAC A. T he
OP296 provides rail-to-rail output drive while operating down
to a 3 V supply and requiring only 50
μ
A of supply current.
+5V
OP296
SIMPLE BUFFER
0V TO +5V
+4.983V
+1.1mV
R1
100k
SUMMER CIRCUIT
WITH FINE TRIM
ADJUSTMENT
DIGITAL INTERFACING
OMITTED FOR CLARITY
AD8801/
AD8803
V
H
V
L
V
DD
V
REFH
GND
V
REFL
V
H
V
L
V
H
V
L
Figure 46. Buffering a TrimDAC Output
T he next two DACs, B and C, sum their outputs into the other
OP296 amplifier. In this circuit DAC C provides the coarse out-
put voltage setting and DAC B is used for fine adjustment. T he
insertion of R1 in series with DAC B attenuates its contribution
to the voltage sum node at the DAC C output.
A High-Side Current Monitor
In the design of power supply control circuits, a great deal of de-
sign effort is focused on ensuring a pass transistor’s long-term
reliability over a wide range of load current conditions. As a re-
sult, monitoring and limiting device power dissipation is of
prime importance in these designs. T he circuit illustrated in Fig-
ure 47 is an example of a +5 V, single-supply high-side current
monitor that can be incorporated into the design of a voltage
regulator with fold-back current limiting or a high current power
T rimDAC is a registered trademark of Analog Devices Inc.
supply with crowbar protection. T his design uses an OP296’s
rail-to-rail input voltage range to sense the voltage drop across a
0.1
current shunt. A p-channel MOSFET is used as the feed-
back element in the circuit to convert the op amp’s differential
input voltage into a current. T his current is then applied to R2
to generate a voltage that is a linear representation of the load
current. T he transfer equation for the current monitor is given
by:
Monitor Output
=
R
2
×
R
SENSE
R
1
×
I
L
For the element values shown, the Monitor Output’s transfer
characteristic is 2.5 V/A.
8
1
2
3
4
1/2
OP296
+5V
+5V
S
G
D
M1
3N163
MONITOR
OUTPUT
R2
2.49k
R1
100
R
0.1
I
L
+5V
Figure 47. A High-Side Load Current Monitor
A Single-Supply RT D Amplifier
T he circuit in Figure 48 uses three op amps on the OP496 to
produce a bridge driver for an RT D amplifier while operating
from a single +5 V supply. T he circuit takes advantage of the
OP496’s wide output swing to generate a bridge excitation volt-
age of 3.9 V. An AD589 provides a 1.235 V reference for the
bridge current. Op amp A1 drives the bridge to maintain 1.235
V across the parallel combination of the 6.19 k
and 2.55 M
resistors, which generates a 200
μ
A current source. T his current
divides evenly and flows through both halves of the bridge.
T hus, 100
μ
A flows through the RT D to generate an output
voltage which is proportional to its resistance. For improved
accuracy, a 3-wire RT D is recommended to balance the line
resistance in both 100
legs of the bridge.
V
OUT
+5V
A3
A2
A1
100k
0.1μF
1/4
OP496
1/4
OP496
100k
GAIN = 259
200
10-TURNS
26.7k
26.7k
100
6.17k
37.4k
+5V
100
RTD
2.55M
AD589
1/4
OP496
NOTE:
ALL RESISTORS 1% OR BETTER
392
392
20k
Figure 48. A Single Supply RTD Amplifier