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AD780AR датащи(PDF) 4 Page - Analog Devices |
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AD780AR датащи(HTML) 4 Page - Analog Devices |
4 / 10 page AD780 REV. B –4– THEORY OF OPERATION Bandgap references are the high performance solution for low supply voltage and low power voltage reference applications. In this technique a voltage with a positive temperature coefficient is combined with the negative coefficient of a transistor’s Vbe to produce a constant bandgap voltage. In the AD780, the bandgap cell contains two npn transistors (Q6 and Q7) which differ in emitter area by 12 . The differ- ence in their Vbe’s produces a PTAT current in R5. This in turn produces a PTAT voltage across R4, which when com- bined with the Vbe of Q7, produces a voltage Vbg that does not vary with temperature. Precision laser trimming of the resistors and other patented circuit techniques are used to further enhance the drift performance. NC TEMP +VIN VOUT TRIM GND O/P SELECT 2.5V - NC 3.0V - GND Q6 Q7 R10 R11 R5 R4 R14 R15 R16 R13 NC = NO CONNECT NC AD780 Figure 1. Schematic Diagram The output voltage of the AD780 is determined by the configu- ration of resistors R13, R14 and R15 in the amplifier’s feedback loop. This sets the output to either 2.5 V or 3.0 V depending on whether R15 (Pin 8) is grounded or not connected. A unique feature of the AD780 is the low headroom design of the high gain amplifier which produces a precision 3 V output from an input voltage as low as 4.5 V (or 2.5 V from a 4.0 V input). The amplifier design also allows the part to work with VIN = VOUT when current is forced into the output terminal. This allows the AD780 to work as a two terminal shunt regula- tor providing a –2.5 V or –3.0 V reference voltage output with- out external components. The PTAT voltage is also used to provide the user with a ther- mometer output voltage (at Pin 3) which increases at a rate of approximately 2 mV/ °C. The AD780’s NC Pin 7 is a 20 k Ω resistor to V+ which is used solely for production test purposes. Users who are currently us- ing the LT1019 self-heater pin (Pin 7) must take into account the different load on the heater supply. APPLYING THE AD780 The AD780 can be used without any external components to achieve specified performance. If power is supplied to Pin 2 and Pin 4 is grounded, Pin 6 provides a 2.5 V or 3.0 V output de- pending on whether Pin 8 is left unconnected or grounded. A bypass capacitor of 1 µF (VIN to GND) should be used if the load capacitance in the application is expected to be greater than 1 nF. The AD780 in 2.5 V mode typically draws 700 µA of Iq at 5 V. This increases by ~2 µA/V up to 36 V. NC TEMP +VIN VOUT TRIM GND O/P SELECT 2.5V – NC 3.0V – GND NC RNULL R POT. 1 F AD780 NC = NO CONNECT Figure 2. Optional Fine Trim Circuit Initial error can be nulled using a single 25 k Ω potentiometer connected between VOUT, Trim and GND. This is a coarse trim with an adjustment range of ±4% and is only included here for compatibility purposes with other references. A fine trim can be implemented by inserting a large value resistor (e.g. 1–5 M Ω) in series with the wiper of the potentiometer. See Figure 2 above. The trim range, expressed as a fraction of the output, is simply greater than or equal to 2.1 k Ω/RNULL for either the 2.5 V or 3.0 V mode. The external null resistor affects the overall temperature coeffi- cient by a factor equal to the percentage of VOUT nulled. For example a 1 mV (.03%) shift in the output caused by the trim circuit, with a 100 ppm/ °C null resistor will add less than 0.06 ppm/ °C to the output drift (0.03% 200 ppm/ °C, since the resistors internal to the AD780 also have temperature coeffi- cients of less than 100 ppm/ °C). |
Аналогичный номер детали - AD780AR |
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Аналогичное описание - AD780AR |
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