Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Particular stable state circuit
Reexamination Certificate
2000-07-31
2002-06-11
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Particular stable state circuit
C327S143000, C327S538000, C327S545000, C323S312000, C323S315000
Reexamination Certificate
active
06404252
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the design of electronic circuits that are useful in low power, low current electronic systems. In particular, the present invention relates to electronic circuits that utilize start up circuits, the start up circuits consuming no stand by current upon the completion of a start up sequence.
2. Description of the Prior Art
Typical microelectronic systems have various electronic components that often share one or more common biasing circuits. Examples of circuits that have common biasing arrangements include operational amplifiers, comparators, and other analog components such as level detectors.
MOS type transistors function as voltage controlled devices where the conduction channel is activated by applying a voltage field across the conduction channel. Because MOS devices are field effect devices with an insulated gate structure, current does not flow through the devices' gate terminals. Since MOS transistors only consume power when biased in their active region, MOS transistors are useful in low power circuits. For example, a typical CMOS digital inverter circuit does not use any power unless it is transitioning from one logic state to another. MOS transistors are thus widely used in digital electronic systems for their reduced power consumption characteristics.
Analog circuits tend to use more power than digital circuits in part due to the active nature of the circuits. Linear amplifiers, comparators and other analog circuits normally require biasing circuits for proper operation. Unlike many digital circuits, analog circuits tend to consume “stand-by” current. For this reason, it is often desired to consolidate biasing circuits from various analog circuits into one common biasing circuit in order to reduce overall system power.
A simple biasing arrangement using MOS devices includes a diode-connected transistor that is series connected to a resistor, the combination connected across the power supply. The diode-connected transistor will conduct current as soon as the power supply levels exceed the threshold voltage of the transistor. Once the power supply has reached its full potential, the current through the diode-connected device is inversely proportional to the resistor value. The gate connection of the diode connected device functions as a biasing voltage for other transistors. The gate connections of the other transistors are connected to the common biasing voltage in such a way that the other transistors will also conduct a current level that is inversely proportional to the resistor in the bias circuit.
The diode/resistor combination discussed above is not well suited for ultra low power microchip applications. In order to keep the overall power consumption down, the resistor must have a relatively high value (>1M&OHgr;). For Example, when the resistor in the bias circuit has a value of 5M&OHgr;, the diode-connected device will consume roughly 860 nA on a 5 Volt supply (4.3 v/5M&OHgr;=860 nA). Thus, where the microchip power budget is on the order of 1 or 2 &mgr;A, almost an entire &mgr;A of current will be used up on the biasing arrangement alone (approximately 40% of the power budget). The diode/resistor arrangement suffers from poor regulation over varied power supply voltages. In addition, the simple diode/resistor circuit tends to have poor regulation performance over varying temperature ranges.
Another example of a conventional biasing arrangement is known as a “V
t
generator”. Many modern CMOS processes are based on P-type doped substrates. Parasitic PNP transistors are inherently formed in the substrate of p-type CMOS circuits, with a fixed collector connected to the substrate. The parasitic PNP devices can be configured as diodes. A “V
t
generator” uses the temperature dependent characteristics of diode connected parasitic PNP devices to generate a voltage proportional to absolute temperature (VPTAT). A circuit is arranged using current mirrors with diode connected devices in such a way as to form a “vptat-loop”, which generates a voltage drop across a resistor. The voltage drop is normally very small (V
t
is on the order of 26 mV).
The VPTAT generator biasing arrangement requires a smaller resistor value as compared to the diode/resistor circuit previously discussed. The current density of each parasitic diode connected device in a VPTAT generator is proportional to the current mirror ratio and the area of the diode. The voltage across the resistor is given by V
R
=V
t
·ln((I
C1
·I
S2
/(I
C2
·I
S1
)), where I
C1
, I
C2
are the currents in the mirrors and I
S1
, I
S2
are proportional to the emitter areas (A
1
, A
2
) of the diodes. The voltage across the resistor is also given as V
R
=I
C2
·R. Thus, R=V
R
/I
C2
=(V
t
/I
C2
)·ln((I
C1
·I
S2
)/(I
C2
·I
S1
)). For example, if the ratio of currents in the mirror are 1:1 (I
C1
:I
C2
), the bias current is 860 nA, the ratio of the diode emitter areas is 4:1 (I
S2
:I
S1
), and the V
t
is 26 mV, then the resistor value which is required in the VPTAT generator is given by: R=(26 mV/860 nA)*ln(4)=42K&OHgr;. This is substantially less than the 5M&OHgr; resistor which is required in the simple diode/resistor biasing circuit previously discussed. In addition, the VPTAT generator provides a supply voltage independent bias current which consumes less die area than the diode/resistor arrangement. However, the VPTAT generator produces a bias current that is dependent upon temperature.
A further conventional biasing arrangement counteracts temperature effects by using a so-called “band-gap” reference circuit. Band-gap reference circuits use the inherent characteristics of bipolar transistors (often connected as diode devices by shorting the collector and base together) to compensate for detrimental temperature effects. The energy band-gap of Silicon is on the order of 1.2 V, and is independent from temperature and power supply variations. Bipolar transistors have a negative temperature drift with respect to the base-emitter voltage (V
be
decreases as operating temperature increases). However, the thermal voltage of a bipolar transistor has a positive temperature drift (V
t
=kT/q, thus V
t
increases as temperature increases). Thus, the negative temperature drift in a bipolar transistor base-emitter voltage is counteracted by the positive temperature drift in the thermal voltage (V
t
).
One typical problem associated with band-gap type reference circuits, as well as other electronic circuits, is that there is a possibility that during the power up sequence, the transistors will find a state where they will not turn on. VPTAT and bandgap reference circuits are circuits that are configured as feedback circuits. When the voltage gain in the feedback loop is too high, an unstable operating condition results. Typically the gain in the feedback loop must be maintained at a lower level to permit stable operation. As discussed previously, it is likely that the reference circuit is being used as a biasing reference for other circuits. To ensure proper functionality of circuits that share the common biasing connection, it is crucial that the reference circuit properly starts-up when the power is turned on and simultaneously finds a stable operating condition.
One way to ensure start-up of a circuit is to form a conduction path from one supply through a transistor in the problematic circuit. This in turn causes the problematic circuit to enter a known state of active operation. For example, a resistor can be connected between one of the respective power supplies and a diode connected device in the reference circuit. The resistor and diode device form the conduction path in the reference circuit. The diode device will begin to conduct and in turn the circuit will begin normal operation.
A problem with the above described resistor conduction path approach is that the resistor will remain connected to the reference circuit even though the reference circuit has reached normal operating
Callahan Timothy P.
Hertzberg Brett A.
Luu An T.
National Semiconductor Corporation
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