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REF192 Datasheet(PDF) 21 Page - Analog Devices |
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REF192 Datasheet(HTML) 21 Page - Analog Devices |
21 / 24 page ![]() REV. E REF19x Series –21– U1 REF195 C1 0.1 F VS, VBAT COMMON VOUT COMMON 5.000V C3 1 F U2 REF195 R5 100k R6 100 C2 0.1 F 6 4 2 3 U3 AD820 R4 900k C4 0.1 F R2 100k R3 10M R1 1.1M +VBAT +VS Q1 2N3904 7 Figure 11. A Fail-Safe 5 V Reference Kelvin Connections In many portable instrumentation applications where PC board cost and area go hand-in-hand, circuit interconnects are very often of dimensionally minimum width. These narrow lines can cause large voltage drops if the voltage reference is required to provide load currents to various functions. In fact, a cir- cuit’s interconnects can exhibit a typical line resistance of 0.45 m Ω/square (1 oz. Cu, for example). In those applications where these devices are configured as low dropout voltage regu- lators, these wiring voltage drops can become a large source of error. To circumvent this problem, force and sense connections can be made to the reference through the use of an operational amplifier, as shown in Figure 10. This method provides a means by which the effects of wiring resistance voltage drops can be eliminated. Load currents flowing through wiring resistance produce an I-R error (ILOAD RWIRE) at the load. However, the Kelvin connection overcomes the problem by including the wiring resistance within the forcing loop of the op amp. Since the op amp senses the load voltage, op amp loop control forces the output to compensate for the wiring error and to produce the correct voltage at the load. Depending on the reference device chosen, operational amplifiers that can be used in this application are the OP295, the OP291, and the OP183/OP283. 1 F REF19x VIN GND VOUT VIN 100k RL RLW SLEEP VIN 2 3 1 A1 RLW +VOUT SENSE +VOUT FORCE A1 = 1/2 OP295 1/2 OP292 1/2 OP283 Figure 10. A Low Dropout, Kelvin Connected Voltage Reference A Fail-Safe 5 V Reference Some critical applications require a reference voltage to be main- tained constant, even with a loss of primary power. The low standby power of the REF19x series and the switched output capability allow a “fail-safe” reference configuration to be implemented rather easily. This reference maintains a tight output voltage tolerance for either a primary power source (ac line derived) or a standby (battery derived) power source, automatically switching between the two as the power conditions change. The circuit in Figure 11 illustrates the concept, which borrows from the switched output idea of Figure 8, again using the REF19x device family output “wire-OR” capability. In this case, since a constant 5 V reference voltage is desired for all conditions, two REF195 devices are used for U1 and U2, with their ON/OFF switching controlled by the presence or absence of the primary dc supply source, VS. VBAT is a 6 V battery backup source that sup- plies power to the load only when VS fails. For normal (VS present) power conditions, VBAT sees only the 15 µA (max) standby current drain of U1 in its OFF state. In operation, it is assumed that for all conditions either U1 or U2 is ON and a 5 V reference output is available. With this voltage constant, a scaled down version is applied to the comparator IC U3, providing a fixed 0.5 V input to the (–) input for all power conditions. The R1–R2 divider provides a signal to the U3 (+) input proportional to VS, which switches U3 and U1/U2 dependent upon the absolute level of VS. Op amp U3 is configured here as a comparator with hysteresis, which provides for clean, noise free output switching. This hysteresis is important to eliminate rapid switching at the threshold due to VS ripple. Further, the device chosen is the AD820, a rail-to-rail output device that provides HI and LO output states within a few mV of VS and ground for accurate thresholds and compatible drive for U2 for all VS condi- tions. R3 provides positive feedback for circuit hysteresis, changing the threshold at the (+) input as a function of U3’s output. |
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