Browse Prior Art Database

Differential Interface Between Two Different Power Supply Voltages

IP.com Disclosure Number: IPCOM000109043D
Original Publication Date: 1992-May-01
Included in the Prior Art Database: 2005-Mar-23
Document File: 1 page(s) / 50K

Publishing Venue

IBM

Related People

Gillingham, RD: AUTHOR [+3]

Abstract

A method for passing a high-speed differential signal between circuits powered by two different power supply voltages on the same chip is disclosed. The power supply voltages are not required to track over the normal operating range, and power sequencing concerns are eliminated.

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Differential Interface Between Two Different Power Supply Voltages

      A method for passing a high-speed differential signal between
circuits powered by two different power supply voltages on the same
chip is disclosed.  The power supply voltages are not required to
track over the normal operating range, and power sequencing concerns
are eliminated.

      The figure shows the circuitry used for the VDD1 to VDD2
conversion.  A differential signal referenced off VDD1 is applied
between PIN, NIN. This signal is level shifted by devices 0, 1, 2, 3,
4 and 5.  The resistor dividers formed by 6,8 and 7,9 serve to reduce
the power supply sensitivity of the VDD1 signal and make it track the
ground connection, which is common to the two supplies.  The signal
is then transferred through diodes 10 and 11, which are always on, to
the bases of 16 and 17.  Resistors 12 and 13 supply base current to
16 and 17, while biasing 10 and 11 in the on state.  The current
source comprised of 18, 19 and 20 provides current for the
differential driver, whose outputs are terminated to the VDD2 supply.

      If either the VDD1 or VDD2 supply is grounded, the circuit must
not allow saturation of any devices; this avoids substrate currents
and a latchup condition.  If the VDD1 supply is high while the VDD2
supply is low, diodes 10 and 11 prohibit current from entering the
VDD2 portion of the circuit, thus protecting the devices.  If the
VDD2 supply is high and the VDD1 supply is l...