Electronic digital logic circuitry – Interface – Logic level shifting
Reexamination Certificate
2001-06-28
2002-09-17
Tokar, Michael (Department: 2819)
Electronic digital logic circuitry
Interface
Logic level shifting
C326S081000, C326S083000
Reexamination Certificate
active
06452418
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of electronic circuits, and in particular to a level shifter with independent ground potentials, and reduced electromagnetic emissions (EME).
2. Description of Related Art
Level shifters are commonly used to provide communication of signals from a system having a first range of voltage potential to another system having a different range of voltage potential. For example, level shifters are commonly used in automotive applications, because typical automobile systems operate at 0-18 volts, whereas common integrated circuits typically operate at 0-3 volts, or 0-5 volts. An asserted “logic-high” three or five volt output from an integrated circuit may be insufficient to be interpreted as a logic-high in a twelve volt system. A level shifter provides the appropriate mapping of voltage levels from one system to another, to ensure the proper communication of logic values.
FIG. 1
illustrates an example schematic of a conventional level shifter
100
that is configured to provide a logic-high voltage level of Vd
2
at its output (Out) corresponding to a logic-high voltage level of Vd
1
at its input (In). The logic-low levels of both signal (In, Out) is zero volts. Each of these voltage potentials are referenced with regard to a common ground potential.
When a logic-high (Vd
1
) input signal is received at the In node, the transistor N
1
is turned on. At the same time, the inverter
110
turns transistor N
2
off. With transistor N
1
turned on, and N
2
turned off, transistor P
2
is turned on, thereby pulling the Out node to Vd
2
. In this manner, an input voltage level of Vd
1
is converted to an output voltage level of Vd
2
.
When a logic-low (ground potential) input signal is received, transistor N
1
is turned off, and the inverter
110
turns transistor N
2
on. With transistor N
1
turned off, transistor P
2
is turned off, and transistor N
2
pulls the Out node to ground potential. In this manner, an input signal at ground potential provides an output signal that is also at ground potential.
Note that the level shifter of
FIG. 1
relies on a common ground potential for proper operation. If the ground of the input system (inverter
110
) differs from the ground potential of the output system (N
1
, P
1
, N
2
, P
2
), ground currents will flow. If the difference between ground potentials is significant, the level shifter
100
may fail to operate, or may operate intermittently.
FIG. 2
illustrates an example schematic of a level shifter
200
that is configured to convert signals from floating sources
210
,
220
into signals for driving a bus between two potentials, V+, V−, as presented in U.S. Pat. No. 6,154,061, “CAN BUS DRIVER WITH SYMMETRICAL DIFFERENTIAL OUTPUT SIGNALS”, issued Nov. 28, 2000 to Hendrik Boezen et al, and incorporated by reference herein. Transistor P
2
and diode D
1
are configured to drive a bus load Lb to a potential of V+, and transistors N
2
and diode D
2
are configured to drive a bus load La to a potential of V− and therefore substantially independent of potentials V+ and V− for both logic-high and logic-low signal levels. The level shifter
200
provides equal and opposite currents to the loads La, Lb, thereby avoiding ground currents, and ground bounce as the bus switches state. The output transistors P
2
and N
2
, however, are hard-driven to each corresponding rail voltage, V+ and V−, at each bus state transition, causing substantial electromagnetic emissions (EME).
BRIEF SUMMARY OF THE INVENTION
It is an object of this invention to provide a level shifter that allows for two substantially independent ground voltage potentials. It is a further object of this invention to provide a level shifter that has reduced electromagnetic emissions (EME), compared to the conventional level shifters of
FIGS. 1 and 2
.
These objects, and others, are achieved by providing a symmetric differential driving signal relative to one set of voltage potentials to a pair of current sources that drive an output node to another set of voltage potentials. The differential driving signals from the driving system are equal and opposite to each other, thereby avoiding stray current flow between the driving system and the driven current mirrors. The transistors that provide the driving signal are continuously biased, using a weak bias in one logic state and stronger bias in the other state, to avoid hard-switching transients.
REFERENCES:
patent: 6255857 (2001-07-01), Iliasevitch
patent: 6154061 (2001-11-01), Boezen et al.
patent: 6339355 (2002-01-01), Yamauchi et al.
Bredius Martijn
De Langen Klaas-Jan
Singh Balwinder
Koninklijke Philips Electronics , N.V.
Le Don Phu
Schmitt Michael
Tokar Michael
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