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Temperature-Compensated Off-Chip Driver

IP.com Disclosure Number: IPCOM000036800D
Original Publication Date: 1989-Oct-01
Included in the Prior Art Database: 2005-Jan-29
Document File: 2 page(s) / 34K

Publishing Venue

IBM

Related People

Hoffman, JA: AUTHOR [+4]

Abstract

In all FET chip designs, the off-chip driver (OCD) performance is limited by the package inductance. Thus, OCD switching speed must be controlled at the fastest design point (-55oC) in order to prevent V=Ldi/dt noise sufficient to cause logic errors. Unfortunately, the slowest design point occurs at 125oC. To prevent logic errors at -55oC, the total number of OCDs allowed to switch at one time is limited by the high di/dt. A design which reduces di/dt will normally also increase the propagation delay.

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Temperature-Compensated Off-Chip Driver

In all FET chip designs, the off-chip driver (OCD) performance is limited by the package inductance. Thus, OCD switching speed must be controlled at the fastest design point (-55oC) in order to prevent V=Ldi/dt noise sufficient to cause logic errors. Unfortunately, the slowest design point occurs at 125oC. To prevent logic errors at -55oC, the total number of OCDs allowed to switch at one time is limited by the high di/dt. A design which reduces di/dt will normally also increase the propagation delay.

This invention uses polysilicon resistors that have a large inverse temperature coefficient of resistance, i.e., the highest resistance occurs at the lowest temperature.

Fig. 1 shows one possible implementation. For the circuit shown in Fig. 1, the resistance from the "+" supply is greatest at -55oC. The resistors R1 and R2 slow the leading edge of transition very slightly, but the derivative of the current waveform is significantly

(Image Omitted)

reduced. This is due to two mechanisms, the "RC" damping and the IR negative feedback into the source of T3 and T5. This voltage feedback only occurs during the initial rush of current and is maximum at maximum switching current (V=IR). At high temperature, the RC damping is now RC/4, as the resistance of the R1 and R2 is approximately 1/4. However, this minute damping (RC/4) is insignificant compared to the total circuit delay at 125oC.

Fig. 2A shows the circuit delay and switching...