Browse Prior Art Database

Off Chip Driver With a Reduced Output Voltage

IP.com Disclosure Number: IPCOM000102021D
Original Publication Date: 1990-Oct-01
Included in the Prior Art Database: 2005-Mar-17
Document File: 2 page(s) / 55K

Publishing Venue

IBM

Related People

Wissel, L: AUTHOR

Abstract

A clamped driver circuit is shown which allows a 5-volt chip to interface directly to a 3-volt chip without compromising performance.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 69% of the total text.

Off Chip Driver With a Reduced Output Voltage

       A clamped driver circuit is shown which allows a 5-volt
chip to interface directly to a 3-volt chip without compromising
performance.

      A 5-volt chip can have off chip driver (OCD) up-levels too high
to interface with a 3-volt chip even if the 5-volt OCD has an NFET
pull- up, which inherently provides an output level less than 5
volts. Refer ring to Fig. 1, the up-level of the 5-volt chip will be
Vdd - VT . With tolerance, this can exceed the limit for a 3-volt
system, which is set by the allowable forward biasing of the drain to
N-well junction of the common I/O of the receiving chip. When
clamping circuitry is added to the 5-volt OCD, these chips can
interface directly to a 3-volt chip.

      The clamping circuitry consists of two small NFETs, T1 and T3,
shown in Fig. 2. When the OCD is active high, node "1" is pulled high
by T2, and node "2" is pulled high by T4 . T1 conducts in opposition
to T2 . Since T1 is much smaller than T2, the voltage on node "1"
is reduced only a few tenths of a volt. Node "2" drops accordingly a
few tenths of a volt. In addition, T3 conducts in opposition to T4 .
Despite the size mismatch (T3 is a small device and T4 is a very
large output device), T3 is able to pull node "2" down a few
additional tenths of a volt because T4 is at the threshold of
conduction, and its turn-on characteristics are not sharp.

      The resultant output voltage of the clamping circuit is re...