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# On-Chip Thermal Shut Down Circuit

IP.com Disclosure Number: IPCOM000036076D
Original Publication Date: 1989-Sep-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 2 page(s) / 35K

IBM

## Related People

DeMoor, MK: AUTHOR [+2]

## Abstract

A linear ASIC library typically contains voltage references and comparators as standard cells. These cells in concert with a unique NTC (Negative Temperature Coefficient) cell provide overtemperature detection which is used for thermal shutdown of an integrated system. The block diagram in Fig. 1 shows how the NTC cell is used for thermal shutdown.

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On-Chip Thermal Shut Down Circuit

A linear ASIC library typically contains voltage references and comparators as standard cells. These cells in concert with a unique NTC (Negative Temperature Coefficient) cell provide overtemperature detection which is used for thermal shutdown of an integrated system. The block diagram in Fig. 1 shows how the NTC cell is used for thermal shutdown.

When the output VOUT of the NTC cell drops below VTH, a thermal shutdown signal is generated. Both the NTC cell and the comparison threshold VTH are referenced to a temperature stable voltage reference VREF. Therefore, accurate temperature sensing is achieved.

The NTC cell is given by the circuit shown in Fig. 2.

The difference in VBE's between Q1 and Q2 is equal to the voltage across R1 and is determined by the ratio of collector currents in Q1 and Q2 as follows: VR1 = VBE = kT/q ln (IC1/IC2)

(Image Omitted)

whereK = 1.38 x 10 -23J/@K, g = 1.6 x 10 -19 Coulomb, T = temperature in @K.

The ratio IC1/IC2 is set by R3 and R1, typically 10:1 current ratio. Since IE2 = IC2 (for a ~ 1), the voltage across R2 is given by: VR2 = W VBE R2/R1, Finally, the output voltage VOUT is given by: VOUT = VREF - R2/R1 kT/q ln (IC1/IC2), Since VREF is independent of temperature, the output voltage has a negative temperature coefficient of: TEMPCO = R2/R1 k/q In (IC1/IC2), which can be set for the desired application by choosing R2, R1 and IC1/IC2 appropriately. Since integrated resistors match very well an...