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AD780BNZ Datasheet(PDF) 5 Page - Analog Devices |
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AD780BNZ Datasheet(HTML) 5 Page - Analog Devices |
5 / 12 page Data Sheet AD780 Rev. F | Page 5 of 12 THEORY OF OPERATION Band gap 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 band gap voltage. In the AD780, the band gap cell contains two NPN transistors (Q6 and Q7) that differ in emitter area by 12×. The difference in their Vbes produces a PTAT current in R5. This, in turn, produces a PTAT voltage across R4 that, when combined 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. R16 R10 +VIN R4 R5 Q6 Q7 R11 R14 R13 R15 NC VOUT TRIM GND O/P SELECT 2.5V – NC 3.0V – GND TEMP NC NC = NO CONNECT AD780 7 2 1 3 4 8 5 6 Figure 4. Schematic Diagram The output voltage of the AD780 is determined by the configuration 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 2-terminal shunt regulator, providing a −2.5 V or −3.0 V reference voltage output without external components. The PTAT voltage is also used to provide the user with a thermometer output voltage (at Pin 3) that increases at a rate of approximately 2 mV/°C. The AD780’s NC (Pin 7) is a 20 kΩ resistor to +VIN that is used solely for production test purposes. Users who are currently using the LT1019 self-heater pin (Pin 7) must take into account the different load on the heater supply. |
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