Datasheet OP179, OP279 (Analog Devices) - 9

OP179/OP279
ON-OFF settling time of the circuit, R2 can be reduced to The low dropout performance of this circuit is provided by stage 50 kΩ or less. Although the integrator’s time constant chosen U2, one-half of an OP179/OP279 connected as a follower/buffer here is 1 ms, room exists to trade off circuit bandwidth and for the basic reference voltage produced by U1. The low voltage noise by increasing R3 and decreasing C2. The SHUTDOWN saturation characteristic of the OP179/OP279 allows up to 30 mA feature is maintained in the circuit with the simple addition of a of load current in the illustrated use, as a 5 V to 3.3 V converter PNP transistor and a 10 kΩ resistor. One caveat with this with high dc accuracy. In fact, the dc output voltage change for approach should be mentioned: although rail-to-rail output a 30 mA load current delta measures less than 1 mV. This amplifiers work best in the application, these operational ampli- corresponds to an equivalent output impedance of < 0.03 Ω. In fiers require a finite amount (mV) of headroom when required this application, the stable 3.3 V from U1 is applied to U2 to provide any load current. The choice for the circuit’s negative through a noise filter, R1-C1. U2 replicates the U1 voltage supply should take this issue into account. within a few mV, but at a higher current output at VOUT1, with the ability to both sink and source output current(s)—unlike
U1 V
most IC references. R2 and C2 in the feedback path of U2
OUT (V) +5V REF192 2.5
provide bias compensation for lowest dc error and additional
REF193 3.0
noise filtering.
R5 REF196 3.3 C2 10k
⍀
REF194 4.5 SHUTDOWN 1
␮
F 2N3904
Transient performance of the reference/regulator for a 10 mA
TTL/CMOS 2 R3
step change in load current is also quite good and is determined
+5V U1 1k
⍀
3 6 R4
largely by the R5-C5 output network. With values as shown, the
REF195 10
⍀
1/2
transient is about 10 mV peak and settles to within 2 mV in 8 µs,
–V C1 OP279 REF GND
for either polarity. Although room exists for optimizing the
1
␮
F 4 R2
transient response, any changes to the R5-C5 network should
R1 –10V 100k
⍀
10k
⍀ be verified by experiment to preclude the possibility of excessive ringing with some capacitor types. Figure 8. A Negative Precision Voltage Reference That To scale VOUT2 to another (higher) output level, the optional Uses No Precision Resistors Exhibits High Output Current resistor R3 (shown dotted) is added, causing the new VOUT1 to Drive become:
A High Output Current, Buffered Reference/Regulator
 R2 Many applications require stable voltage outputs relatively close V = V × 1+ OUT1 OUT2  R3 in potential to an unregulated input source. This “low dropout” type of reference/regulator is readily implemented with a rail-to- As an example, for a VOUT1 = 4.5 V, and VOUT2 = 2.5 V from a rail output op amp, and is particularly useful when using a REF192, the gain required of U2 is 1.8 times, so R2 and R3 higher current device such as the OP179/OP279. A typical would be chosen for a ratio of 0.8:1, or 18 kΩ:22.5 kΩ. Note that example is the 3.3 V or 4.5 V reference voltage developed from for the lowest VOUT1 dc error, the parallel combination of R2 and a 5 V system source. Generating these voltages requires a three- R3 should be maintained equal to R1 (as here), and the R2-R3 terminal reference, such as the REF196 (3.3 V) or the REF194 resistors should be stable, close tolerance metal film types. (4.5 V), both of which feature low power, with sourcing outputs The circuit can be used as shown as either a 5 V to 3.3 V reference/ of 30 mA or less. Figure 9 shows how such a reference can be regulator, or it can be used with ON/OFF control. By driving outfitted with an OP179/OP279 buffer for higher currents and/ Pin 3 of U1 with a logic control signal as noted, the output is or voltage levels, plus sink and source load capability. switched ON/OFF. Note that when ON/OFF control is used, resistor R4 should be used with U1 to speed ON-OFF switching.
VS U2 5V 1/2 OP279 Direct Access Arrangement for Telephone Line Interface C1 VOUT1 =
Figure 10 illustrates a 5 V only transmit/receive telephone line
0.1
␮
F 3.3V @ 30mA
interface for 110 Ω transmission systems. It allows full duplex
R2
transmission of signals on a transformer coupled 110 Ω line in
10k
⍀
R1 1%
a differential manner. Amplifier A1 provides gain that can be
10k
⍀ adjusted to meet the modem output drive requirements. Both
1% C2
A1 and A2 are configured to apply the largest possible signal on a
0.1
␮
F R3 C3
single supply to the transformer. Because of the OP179/OP279’s
(SEE TEXT) C5 0.1
␮
F 2 10
␮
F/25V
high output current drive and low dropout voltage, the largest
6 TANTALUM
signal available on a single 5 V supply is approximately 4.5 V p-p
U1 3 REF196 V R4 OUT2 = V
into a 110 Ω transmission system. Amplifier A3 is configured as
C 3.3k
⍀
3.3V R5 ON/OFF 4 C4
a difference amplifier to extract the receive signal from the
1
⍀
CONTROL 1
␮
F
transmission line for amplification by A4. A4’s gain can be adjusted
INPUT CMOS HI (OR OPEN) = ON
in the same manner as A1’s to meet the modem’s input signal
LO = OFF V
requirements. Standard resistor values permit the use of SIP
OUT VS COMMON COMMON
(Single In-line Package) format resistor arrays. Couple this with the OP179/OP279’s 8-lead SOIC footprint and this circuit Figure 9. A High Output Current Reference/Regulator offers a compact, cost-sensitive solution. REV. G –9–