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Temperature-Compensated Current Source Using a Bandgap Reference

IP.com Disclosure Number: IPCOM000036420D
Original Publication Date: 1989-Sep-01
Included in the Prior Art Database: 2005-Jan-29
Document File: 2 page(s) / 40K

Publishing Venue

IBM

Related People

Schrader, RE: AUTHOR

Abstract

The subject disclosure provides a temperature-compensated voltage reference in a module that contains a digital-to-analog converter (DAC). The DAC output voltage tolerance, which is primarily a function of the tolerance and tracking of its current sources, must be minimized to reduce step-to-step adjustment tolerance.

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Temperature-Compensated Current Source Using a Bandgap Reference

The subject disclosure provides a temperature-compensated voltage reference in a module that contains a digital-to-analog converter (DAC). The DAC output voltage tolerance, which is primarily a function of the tolerance and tracking of its current sources, must be minimized to reduce step-to-step adjustment tolerance.

The tolerance of current sources can be reduced by driving them from an accurate voltage reference (VREF) that has been compensated to have the same temperature coefficient as the base-emitter voltage of the current source transistors.

Referring to the figure, the DAC under consideration requires ten current sources. After the DAC switching and scaling functions, current from the current sources is converted to output voltage by the DAC transimpedance stage.

A normal current source is made by connecting the base-emitter junction of transistor Q6 in series with resistor R3. A voltage (VREF) is applied to the series circuit. Voltage VREF comprises a bandgap voltage (VBG) and a transistor base-emitter voltage (VQ5). Transistors Q5 and Q6 are the same type of device.

In general, by Ohm's Law, current I3 can be calculated by the following equation. I3 = ALPHA (VQ5 + VGB - VQ6) / R3

Since all module circuit components are part of a single integrated chip, tracking between all like types of devices is satisfactory. Voltages VQ5 and VQ6 have substantially the same temperature coefficient. Thus...