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CIRCUIT FOR GENERATING HIGH POSITIVE-TEMPERATURE-COEFFICIENT VOLTAGES

IP.com Disclosure Number: IPCOM000008624D
Original Publication Date: 1998-Mar-01
Included in the Prior Art Database: 2002-Jun-27
Document File: 2 page(s) / 93K

Publishing Venue

Motorola

Related People

John E. (Ted) Hanna: AUTHOR

Abstract

Many semiconductor processes have layers with high sheet resistivity that are used to generate high value resistors. Typically, the resistance of such a layer will be well controlled at room temperature but will have a large positive temperature coeffi- cient of resistance (TCR). If a small current with a low TC is required, either a voltage must be gener- ated which has a high temperature coefficient of voltage (TCV) which matches the TCR of the resistor, or a layer with lower sheet resistivity must be used requiring much more area for the resistor.

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MOTOROLA Technical Developments

CIRCUIT FOR GENERATING HIGH POSITIVE-TEMPERATURE-COEFFICIENT VOLTAGES

by John E. (Ted) Hanna

1.0 BACKGROUND

  Many semiconductor processes have layers with high sheet resistivity that are used to generate high value resistors. Typically, the resistance of such a layer will be well controlled at room temperature but will have a large positive temperature coeffi- cient of resistance (TCR). If a small current with a low TC is required, either a voltage must be gener- ated which has a high temperature coefficient of voltage (TCV) which matches the TCR of the resistor, or a layer with lower sheet resistivity must be used requiring much more area for the resistor.

2.0 THE PROBLEM

  The voltages commonly available on an IC for generating references are generated either by diodes or by proportional to absolute temperature (PTAT) circuits or a combination of these two. Zeners have breakdown voltages higher than today's supply voltages, The voltage vs. temperature of a diode is given by:

upper end of this range, and cannot be compensated for by diodes or PTATs.

3.0 THE SOLUTION

  If a PTAT voltage is applied to one terminal of the resistor and a diode related voltage applied to the other end, the TCV increases not only because the voltages applied to either end have additive TCs in mV/degC, but also because the net voltage across the resistor has decreased, increasing the mV/VldegC (or %/degC). The voltage across the resistor is:

Vr(T) = Vprcct(T1)

,', -k I/go + [Vd(Tl) - Vgo] ,', t 11

i 1 i

where k is the fraction of Vd applied to the bottom of the resistor. The TCV is given by:

-ki

Vd( TI ) - VW

Tl 1

The relative change in voltage across the resistor is:

T...