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OP279GS Folha de dados(PDF) 9 Page - Analog Devices |
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OP279GS Folha de dados(HTML) 9 Page - Analog Devices |
9 / 16 page OP179/OP279 –9– REV. G ON-OFF settling time of the circuit, R2 can be reduced to 50 k Ω 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 k Ω resistor. One caveat with this approach should be mentioned: although rail-to-rail output amplifiers work best in the application, these operational ampli- fiers 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. R4 10 1/2 OP279 +5V –10V R3 1k C2 1 F C1 1 F R2 100k U1 REF195 GND R5 10k R1 10k 2N3904 4 6 2 3 SHUTDOWN TTL/CMOS +5V –VREF U1 REF192 REF193 REF196 REF194 VOUT (V) 2.5 3.0 3.3 4.5 Figure 8. 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 9 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. C2 0.1 F R2 10k 1% U2 1/2 OP279 VOUT1 = 3.3V @ 30mA R5 1 C5 10 F/25V TANTALUM R1 10k 1% C1 0.1 F VS 5V VOUT2= 3.3V C4 1 F 6 2 3 4 VOUT COMMON C3 0.1 F VC ON/OFF CONTROL INPUT CMOS HI (OR OPEN) = ON LO = OFF VS COMMON R3 (SEE TEXT) R4 3.3k U1 REF196 Figure 9. A High Output Current Reference/Regulator 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 VOUT1, 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. To scale VOUT2 to another (higher) output level, the optional resistor R3 (shown dotted) is added, causing the new VOUT1 to become: VV R R OUT1 OUT2 =× + 1 2 3 As an example, for a VOUT1 = 4.5 V, and VOUT2 = 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 k Ω:22.5 kΩ. Note that for the lowest VOUT1 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 reference/ 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 10 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. |
Nº de peça semelhante - OP279GS |
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Descrição semelhante - OP279GS |
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