OP496_15 Datasheet, PDF (14/19 Pages) Analog Devices – Micropower RRIO Operational Amplifiers

OP196/OP296/OP496
V_IN
5V TO 3.2V
MJE 350
8
3
1/2
1 _OP296
4
2
1000pF
43k⍀
I_L < 50mA
44.2k⍀
1%
V_O
100␮F
30.9k⍀
1%
1.235V
AD589
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.
2V
STEP ^50mA
CURRENT
100
90
CONTROL
WAVEFORM _30mA
OUTPUT 10
0%
10mV
50µs
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 kΩ. If a lower output impedance is required, an
OP296 amplifier can be added. Two examples are shown in
Figure 10. One amplifier of an OP296 is used as a simple buffer
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 µA of supply current.
5V
V_REFH V_DD
V_H
V_L
OP296
V_H
V_L
R1
V_H
100k⍀
V_L
AD8801/
AD8803
V_REFL GND
DIGITAL INTERFACING
OMITTED FOR CLARITY
SIMPLE BUFFER
0V TO 5V
+4.983V
+1.1mV
SUMMER CIRCUIT
WITH FINE TRIM
ADJUSTMENT
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 11
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 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 Ω
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.
R_SENSE
0.1⍀
5V
R1
100⍀
S
M1
G
MONITOR
OUTPUT
3N163
D
R2
2.49k⍀
I_L
5V
5V
3
8
1/2
OP296 ¹
2
4
Figure 11. A High-Side Load Current Monitor
A Single-Supply RTD Amplifier
The circuit in Figure 12 uses three op amps on the OP496 to
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 kΩ and
2.55 MΩ resistors, which generates a 200 µA current source.
This current divides evenly and flows through both halves of
the bridge. Thus, 100 µA 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 Ω legs of the bridge.
TrimDAC is a registered trademark of Analog Devices Inc.
–14–
REV. E