Datasheet AD780 (Analog Devices) - 5

Data Sheet AD780 THEORY OF OPERATION
Band gap references are the high performance solution for low The output voltage of the AD780 is determined by the supply voltage and low power voltage reference applications. In configuration of Resistors R13, R14, and R15 in the amplifier’s this technique, a voltage with a positive temperature coefficient feedback loop. This sets the output to either 2.5 V or 3.0 V, is combined with the negative coefficient of a transistor’s Vbe to depending on whether R15 (Pin 8) is grounded or not connected. produce a constant band gap voltage. A unique feature of the AD780 is the low headroom design of In the AD780, the band gap cell contains two NPN transistors the high gain amplifier, which produces a precision 3 V output (Q6 and Q7) that differ in emitter area by 12×. The difference in from an input voltage as low as 4.5 V (or 2.5 V from a 4.0 V their Vbes produces a PTAT current in R5. This, in turn, produces a input). The amplifier design also al ows the part to work with PTAT voltage across R4 that, when combined with the Vbe of +VIN = VOUT when current is forced into the output terminal. Q7, produces a voltage (Vbg) that does not vary with temperature. This allows the AD780 to work as a 2-terminal shunt regulator, Precision laser trimming of the resistors and other patented circuit providing a −2.5 V or −3.0 V reference voltage output without techniques are used to further enhance the drift performance. external components.
+VIN DNC
The PTAT voltage is also used to provide the user with a
2 7
thermometer output voltage (at Pin 3) that increases at a rate of
AD780
approximately 2 mV/°C.
R10 R11
The DNC (Pin 7) of the AD780 is a 20 kΩ resistor to +VIN that
DNC 1
is used solely for production test purposes. Users who are currently
6 VOUT R13
using the LT1019 self-heater pin (Pin 7) must take into account
Q6
the different load on the heater supply.
Q7 R16 R5 5 TRIM R14 TEMP 3 R15 R4 4 8 GND O/P SELECT 2.5V – NC 3.0V – GND
004
DNC = DO NOT CONNECT TO THIS PIN
00841- Figure 4. Schematic Diagram Rev. I | Page 5 of 12 Document Outline Features Functional Block Diagram General Description Product Highlights Table of Contents Revision History Specifications Absolute Maximum Ratings Thermal Resistance Notes ESD Caution Theory of Operation Applying the AD780 Noise Performance Noise Comparison Temperature Performance Temperature Output Pin Temperature Transducer Circuit Supply Current Over Temperature Turn-On Time Dynamic Performance Line Regulation Precision Reference for High Resolution 5 V Data Converters 4.5 V Reference from 5 V Supply Negative (–2.5 V) Reference Outline Dimensions Ordering Guide