OP196/OP296/OP496
REV.
–14–
1/2
OP296
4
1
3
2
8
1000pF
44.2k
1%
30.9k
1%
AD589
43k
1.235V
MJE 350
100 F
VIN
5V TO 3.2V
IL < 50mA
VO
Figure 8. 3 V Low Dropout Voltage Regulator
Figure 9 shows the regulator’s recovery characteristics when its
output underwent a 20 mA to 50 mA step current change.
10
0%
100
90
2V
50s
10mV
50mA
30mA
OUTPUT
STEP
CURRENT
CONTROL
WAVEFORM
Figure 9. Output Step Load Current Recovery
Buffering a DAC Output
Multichannel TrimDACs such as the AD8801/AD8803, are
widely used for digital nulling and similar applications. These
DACs have rail-to-rail output swings, with a nominal output
resistance of 5 k
. If a lower output impedance is required, an
OP296 amplifier can be added. Two examples are shown in
to reduce the output resistance of DAC A. The OP296 provides
rail-to-rail output drive while operating down to a 3 V supply
and requiring only 50
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
VH
VL
VDD
VREFH
GND
VREFL
VH
VL
VH
VL
Figure 10. Buffering a TrimDAC OutputTPC
The next two DACs, B and C, sum their outputs into the other
OP296 amplifier. In this circuit DAC C provides the coarse
output voltage setting and DAC B is used for fine adjustment.
The insertion of R1 in series with DAC B attenuates its contri-
bution 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
design effort is focused on ensuring a pass transistor’s long-term
reliability over a wide range of load current conditions. As a result,
monitoring and limiting device power dissipation is of prime
importance in these designs. The circuit illustrated
in Figure 11is 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 supply
with crowbar protection. This 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 feedback
element in the circuit to convert the op amp’s differential input
voltage into a current. This current is then applied to R2 to gen-
erate a voltage that is a linear representation of the load current.
The transfer equation for the current monitor is given by:
Monitor Output
= R2 ×
R
SENSE
R1
× 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
S
G
D
M1
3N163
MONITOR
OUTPUT
R2
2.49k
R1
100
RSENSE
0.1
IL
5V
Figure 11. A High-Side Load Current Monitor
A Single-Supply RTD Amplifier
produce a bridge driver for an RTD amplifier while operating
from a single 5 V supply. The circuit takes advantage of the
OP496’s wide output swing to generate a bridge excitation
voltage 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 A current source.
This current divides evenly and flows through both halves of
the bridge. Thus, 100
A flows through the RTD to generate
an output voltage which is proportional to its resistance. For
improved accuracy, a 3-wire RTD is recommended to balance
the line resistance in both 100
legs of the bridge.
TrimDAC is a registered trademark of Analog Devices Inc.
E