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ADR381 Arkusz danych(PDF) 10 Page - Analog Devices |
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ADR381 Arkusz danych(HTML) 10 Page - Analog Devices |
10 / 12 page REV. A –10– ADR380/ADR381 THEORY OF OPERATION Band gap references are the high performance solution for low supply voltage and low power voltage reference applications, and the ADR380/ADR381 are no exception. But the unique- ness of this product lies in its architecture. By observing Figure 1, the ideal zero TC band gap voltage is referenced to the output, not to ground. The band gap cell consists of the PNP pair Q51 and Q52, running at unequal current densities. The difference in VBE results in a voltage with a positive TC which is amplified up by the ratio of 2 × R58/R54. This PTAT voltage, combined with VBEs of Q51 and Q52, produce the stable band gap voltage. Reduction in the band gap curvature is performed by the ratio of the two resistors R44 and R59. Precision laser trimming and other patented circuit techniques are used to further enhance the drift performance. GND VOUT VIN Q1 R59 R54 Q51 R60 R61 R48 R49 R44 R58 R53 Q52 – + Figure 1. Simplified Schematic Device Power Dissipation Considerations The ADR380/ADR381 are capable of delivering load currents to 5 mA with an input voltage that ranges from 2.8 V (ADR381 only) to 15 V. When this device is used in applications with large input voltages, care should be taken to avoid exceeding the specified maximum power dissipation or junction temperature that could result in premature device failure. The following formula should be used to calculate a device’s maximum junc- tion temperature or dissipation: P TT D A A = J J – θ where: PD is the device power dissipation, TJ and TA are junction and ambient temperatures, respectively, and θJ A is the device package thermal resistance. Input Capacitor Input capacitor is not required on the ADR380/ADR381. There is no limit for the value of the capacitor used on the input, but a capacitor on the input will improve transient response in appli- cations where the load current suddenly increases. Output Capacitor The ADR380/ADR381 do not need an output capacitor for stability under any load condition. An output capacitor, typically 0.1 µF, will take out any very low level noise voltage, and will not affect the operation of the part. The only parameter that will degrade by putting an output capacitor here is turn-on time. (This will vary depending on the size of the capacitor.) Load transient response is also improved with an output capacitor. A capacitor will act as a source of stored energy for a sudden in- crease in load current. APPLICATIONS Stacking Reference ICs for Arbitrary Outputs Some applications may require two reference voltage sources which are a combined sum of standard outputs. The following circuit shows how this stacked output reference can be imple- mented: GND VOUT VIN C1 0.1 F U2 ADR380/ ADR381 3 GND VOUT VIN U1 ADR380/ ADR381 C3 0.1 F C2 1 F C4 1 F 3 R1 3.9k VOUT2 VOUT1 2 2 1 1 VIN Figure 2. Stacking Voltage References with the ADR380/ADR381 Two ADR380s or ADR381s are used; the outputs of the indi- vidual references are simply cascaded to reduce the supply current. Such configuration provides two output voltages— VOUT1 and VOUT2. VOUT1 is the terminal voltage of U1, while VOUT2 is the sum of this voltage and the terminal voltage of U2. U1 and U2 can be chosen for the two different voltages that supply the required outputs. While this concept is simple, a precaution is in order. Since the lower reference circuit must sink a small bias current from U2, plus the base current from the series PNP output transistor in U2, the external load of either U1 or R1 must provide a path for this current. If the U1 minimum load is not well-defined, the resistor R1 should be used, set to a value that will conservatively pass 600 µA of current with the applicable V OUT1 across it. Note that the two U1 and U2 reference circuits are locally treated as macrocells, each having its own bypasses at input and output for optimum stability. Both U1 and U2 in this circuit can source dc currents up to their full rating. The minimum input voltage, VS, is determined by the sum of the outputs, VOUT2, plus the 300 mV dropout voltage of U2. A Negative Precision Reference Without Precision Resistors In many current-output CMOS DAC applications where the output signal voltage must be of the same polarity as the refer- ence voltage, it is often required to reconfigure a current-switching DAC into a voltage-switching DAC through the use of a 1.25 V reference, an op amp, and a pair of resistors. Using a current- switching DAC directly requires an additional operational amplifier at the output to reinvert the signal. A negative voltage |
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