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ADR380 Datasheet(PDF) 11 Page - Analog Devices |
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ADR380 Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page ADR380/ADR381 Rev. C | Page 11 of 16 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. However, the uniqueness of this product lies in its architecture. As shown in Figure 26, 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 that is amplified by the ratio of 2 × R58/R54. This PTAT voltage, combined with the VBE 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 26. 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, take care to avoid exceeding the specified maximum power dissipation or junction temperature that may result in premature device failure. Use the following formula to calculate a device’s maximum junction temperature or dissipation: JA A J D T T P θ − = where: PD is the device power dissipation, TJ and TA are junction and ambient temperatures, respectively. θJA is the device package thermal resistance. INPUT CAPACITOR An 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 improves transient response in applications where the load current suddenly increases. OUTPUT CAPACITOR The ADR380/ADR381 do not need an output capacitor for stability under any load condition. Using an output capacitor, typically 0.1 μF, removes any very low level noise voltage and does not affect the operation of the part. The only parameter that degrades by applying an output capacitor is turn-on time. (This varies depending on the size of the capacitor.) Load transient response is also improved with an output capacitor, which acts as a source of stored energy for a sudden increase in load current. |
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