OP179 Datasheet, PDF (9/16 Pages) Analog Devices – Rail-to-Rail High Output Current Operational Amplifiers

OP179/OP279
ON-OFF settling time of the circuit, R2 can be reduced to
50 kΩ or less. Although the integrator’s time constant chosen
here is 1 ms, room exists to trade-off circuit bandwidth and
noise by increasing R3 and decreasing C2. The SHUTDOWN
feature is maintained in the circuit with the simple addition of a
PNP transistor and a 10 kΩ resistor. One caveat with this ap-
proach should be mentioned: although rail-to-rail output ampli-
fiers work best in the application, these operational amplifiers
require a finite amount (mV) of headroom when required to
provide any load current. The choice for the circuit’s negative
supply should take this issue into account.
SHUTDOWN
TTL/CMOS
+5V
R5
10k⍀
2N3904 ₂
U1 VOUT (V)
REF192 2.5
REF193 3.0
REF196
REF194
3.3
4.5
C2
1␮F
U1
3 _REF195 6
R1
10k⍀
GND
4 _R2
100k⍀
R3
+5V
1k⍀
1/2
C1
OP279
1␮F
–10V
R4
10⍀
–V_REF
The low dropout performance of this circuit is provided by stage
U2, one-half of an OP179/OP279 connected as a follower/buffer
for the basic reference voltage produced by U1. The low voltage
saturation characteristic of the OP179/OP279 allows up to 30 mA
of load current in the illustrated use, as a 5 V to 3.3 V converter
with high dc accuracy. In fact, the dc output voltage change for
a 30 mA load current delta measures less than 1 mV. This
corresponds to an equivalent output impedance of < 0.03 Ω. In
this application, the stable 3.3 V from U1 is applied to U2
through a noise filter, R1-C1. U2 replicates the U1 voltage
within a few mV, but at a higher current output at V_OUT1, with
the ability to both sink and source output current(s)—unlike
most IC references. R2 and C2 in the feedback path of U2
provide bias compensation for lowest dc error and additional
noise filtering.
Transient performance of the reference/regulator for a 10 mA
step change in load current is also quite good and is determined
largely by the R5-C5 output network. With values as shown, the
transient is about 10 mV peak and settles to within 2 mV in
8 µs, for either polarity. Although room exists for optimizing the
transient response, any changes to the R5-C5 network should be
verified by experiment to preclude the possibility of excessive
ringing with some capacitor types.
Figure 29. A Negative Precision Voltage Reference That
Uses No Precision Resistors Exhibits High Output Current
Drive
A High Output Current, Buffered Reference/Regulator
Many applications require stable voltage outputs relatively close
in potential to an unregulated input source. This “low dropout”
type of reference/regulator is readily implemented with a rail-to-
rail output op amp, and is particularly useful when using a
higher current device such as the OP179/OP279. A typical
example is the 3.3 V or 4.5 V reference voltage developed from
a 5 V system source. Generating these voltages requires a three-
terminal reference, such as the REF196 (3.3 V) or the REF194
(4.5 V), both of which feature low power, with sourcing outputs
of 30␣ mA or less. Figure 30 shows how such a reference can be
outfitted with an OP179/OP279 buffer for higher currents and/
or voltage levels, plus sink and source load capability.
+V_S
+5V
C1
0.1␮F
U2
1/2 OP279
V_OUT1 =
3.3V @ 30mA
R1
10k⍀
1%
C3
0.1␮F
V_C
ON/OFF
CONTROL
INPUT CMOS HI
(OR OPEN) = ON
LO = OFF
2
6
U1
3 REF196 V_OUT2 =
3.3V
4
V_S
COMMON
R3
(SEE TEXT)
R4
3.3k⍀
C4
1␮F
R2
10k⍀
1%
C2
0.1␮F
C5
10␮F/25V
TANTALUM
R5
1⍀
V_OUT
COMMON
Figure 30. A High Output Current Reference/Regulator
To scale V_OUT2 to another (higher) output level, the optional
resistor R3 (shown dotted) is added, causing the new V_OUT1 to
become:
V_{OUT 1}
= V_OUT
2
×
_{1 +}
R2
R 3 
As an example, for a V_OUT1 = 4.5 V, and V_OUT2 = 2.5 V from a
REF192, the gain required of U2 is 1.8 times, so R2 and R3
would be chosen for a ratio of 0.8:1, or 18 kΩ:22.5 kΩ. Note
that for the lowest V_OUT1 dc error, the parallel combination of
R2 and R3 should be maintained equal to R1 (as here), and the
R2-R3 resistors should be stable, close tolerance metal film
types.
The circuit can be used as shown as either a 5 V to 3.3 V refer-
ence/regulator, or it can be used with ON/OFF control. By
driving Pin 3 of U1 with a logic control signal as noted, the
output is switched ON/OFF. Note that when ON/OFF control
is used, resistor R4 should be used with U1 to speed ON-OFF
switching.
Direct Access Arrangement for Telephone Line Interface
Figure 31 illustrates a +5 V only transmit/receive telephone line
interface for 110 Ω transmission systems. It allows full duplex
transmission of signals on a transformer coupled 110 Ω line in a
differential manner. Amplifier A1 provides gain that can be
adjusted to meet the modem output drive requirements. Both
A1 and A2 are configured to apply the largest possible signal on a
single supply to the transformer. Because of the OP179/
OP279’s high output current drive and low dropout voltage, the
largest signal available on a single +5 V supply is approximately
4.5 V p-p into a 110 Ω transmission system. Amplifier A3 is
configured as a difference amplifier to extract the receive signal
from the transmission line for amplification by A4. A4’s gain
can be adjusted in the same manner as A1’s to meet the modem’s
input signal requirements. Standard resistor values permit the
use of SIP (Single In-line Package) format resistor arrays. Couple
this with the OP179/OP279’s 8-lead SOIC footprint and this
circuit offers a compact, cost-sensitive solution.
REV. F
–9–