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REF192ES-REEL7 Datasheet(PDF) 23 Page - Analog Devices |
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REF192ES-REEL7 Datasheet(HTML) 23 Page - Analog Devices |
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23 / 28 page  REF19x Series Rev. I | Page 23 of 28 In applications where these devices are configured as low dropout voltage regulators, 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 29. 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. Because 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, OP292, and OP183. 2 3 1 +VOUT SENSE +VOUT FORCE RLW 1µF 100kΩ RL REF19x VIN GND VOUT A1 = 1/2 OP295 1/2 OP292 OP183 VIN 6 2 4 SLEEP VIN RLW A1 3 Figure 29. A Low Dropout, Kelvin-Connected Voltage Reference FAIL-SAFE 5 V REFERENCE Some critical applications require a reference voltage to be maintained at a constant voltage, 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 30 illustrates this concept, which borrows from the switched output idea of Figure 28, 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 supplies power to the load only when VS fails. For normal (VS present) power conditions, VBAT sees only the 15 μA (maximum) 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 negative input for all power conditions. The R1 to R2 divider provides a signal to the U3 positive input proportionally to VS, which switches U3 and U1/U2, dependent upon the absolute level of VS. In Figure 30, Op Amp U3 is configured as a comparator with hysteresis, which provides clean, noise-free output switching. This hysteresis is important to eliminate rapid switching at the threshold due to VS ripple. Furthermore, the device chosen is the AD820, a rail-to-rail output device. This device provides HI and LO output states within a few mV of VS, ground for accurate thresholds, and compatible drive for U2 for all VS conditions. R3 provides positive feedback for circuit hysteresis, changing the threshold at the positive input as a function of the output of U3. R5 100kΩ R6 100Ω R3 10MΩ U3 AD820 C4 0.1µF R2 100kΩ R1 1.1MΩ Q1 2N3904 +VBAT VS, VBAT COMMON VOUT COMMON C2 0.1µF C1 0.1µF 5.000V C3 1µF U1 REF195 (SEE TABLE) 2 3 4 U2 REF195 (SEE TABLE) 2 3 4 + 6 6 +VS R4 900kΩ 4 2 7 6 3 + – Figure 30. A Fail-Safe 5 V Reference |
Similar Part No. - REF192ES-REEL7 |
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Similar Description - REF192ES-REEL7 |
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