Electricity: power supply or regulation systems – Self-regulating – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2001-03-22
2003-03-04
Nguyen, Matthew V (Department: 2838)
Electricity: power supply or regulation systems
Self-regulating
Using a three or more terminal semiconductive device as the...
Reexamination Certificate
active
06528980
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the field of low-power integrated circuits. More particularly, the present invention relates to a low-power startup circuit for use with bias current generators for integrated circuits.
BACKGROUND ART
Within the communications industry, there is an ever increasing need for higher performance portable devices having long battery lives. For example, handheld personal information devices (e.g., palmtop computers), cell phones, pagers, and the like, are processing data at faster rates, performing more sophisticated functions, and storing larger amounts of data, while simultaneously functioning for increased periods of time on internal battery power. For example, it is not uncommon for modern cell phone devices to operate continuously in standby mode for several days on end.
Low-power integrated circuits are critical to extend functioning on internal battery power for such handheld devices. To extend battery life, many handheld devices are designed to enter a standby mode when there full functionality is not required by the user. For example, a cell phone is designed to enter a standby mode when it is not being used in a voice conversation. The cell phone can “wake up” from standby when a call is received or when the user desires to place a new call. Similarly, many personal information devices are designed to enter standby mode after some duration of non-use from the user, and wake up when the user activates some function, accesses some data (e.g., clicks a GUI icon) etc. While in standby mode, modern battery power devices are designed to require minimal amounts of power, thereby extending their battery lives.
Well-designed standby mechanisms can greatly extend the functional life of a portable battery power device. Accordingly, the design of integrated circuits that implement standby modes, wake up modes, and full functionality is an area of great interest to the electronics industry. The design of an optimal standby mechanism can be challenging. For example, not only does the standby circuitry have to draw minimal amounts of current while in standby, the standby circuitry has to reliably wake up the device upon some external event, such as, in the case of a cell phone, receiving an incoming phone call. Specific circuits have been designed to ensure the overall device reliably wakes up after being in standby. Such circuits are referred to as startup circuits. Startup circuits are used in powering up devices from a power off condition in addition to waking up devices from sleep modes. For example, the startup circuit must ensure a device reliably powers on from an off state in a predictable fashion, into a known operating state.
Prior art
FIG. 1A
,
FIG. 1B
, and
FIG. 1C
show schematic diagrams of prior art of startup circuits.
FIG. 1A
shows a startup circuit
10
. Startup circuit
10
relies on a very large resistor
11
(e.g., 50 mega-ohms) to reduce the magnitude of the startup current Istart
1
. The startup current Istart
1
is used to start the bias generator circuit
12
. Istart
1
flows continuously and needs to be a constant magnitude. This startup current is what enables the bias generator
12
to wake up the overall device. For example, the startup current allows bias generator
12
to generate a biasing current for the rest of the device (not shown) that in turn, allows the startup of circuit elements such as VCOs, PLLs, output drivers, and the like. If Istart
1
is too low, the bias generator
12
cannot wake up the rest of the circuit. If Istart
1
is too high, the standby power consumption will be too high, and thus, battery life will be too short. Startup circuit
10
uses the very large resistor
11
to tailor the magnitude of the startup current Istart
1
.
There exists a problem with startup circuit
10
however, in that it is very difficult to fabricate very large resistors such as the 50 mega-ohm resistor
11
using VLSI fabrication techniques. With sub-micron fabrication techniques, large resistors require an excessive amount of die surface area. In addition, it is difficult to reliably ensure the correct magnitude of the resistor.
Prior art
FIG. 1B
shows startup circuit
20
and prior art
FIG. 1C
shows startup circuit
30
. Startup circuits
20
and
30
attempt to reduce the problem of the large resistor
11
from startup circuit
10
by chaining together transistors to reduce the need for such a large resistor. In startup circuit
20
, a set of diode connected transistors
21
-
23
are included to reduce the voltage drop experienced by the resistor
25
. In this manner, the transistors
21
-
23
multiply the resistance of resistor
25
. In startup circuit
30
, a string of very long channel transistors are used to completely replace the large resistor
11
of startup circuit
10
. Startup circuits
20
and
30
are both functional, however, they both produce undesirable characteristics in their startup currents Istart
2
and Istart
3
.
Prior art
FIG. 2
shows a graph depicting the magnitude of the startup currents Istart
1
, Istart
2
, and Istart
3
with respect to the voltage level of the battery (e.g., Vdd). As shown in
FIG. 2
, line
40
shows an ideal case for a startup current. In the ideal case (line
40
), the startup current is constant with respect to the voltage level of the battery. Thus, as the battery slowly drains over time, the start up current remains at the optimal level (e.g., 100 nA). None of the startup currents is close to ideal, however, Istart
1
from startup circuit
10
is more desirable since it increases linearly with battery power, while Istart
2
and Istart
3
are sharply non-linear. Thus, there exists the problem where startup circuits
20
and
30
suffer from poor low battery operation, while startup circuit
10
relies upon the difficult to fabricate large resistor
11
.
Thus, what is required is a startup circuit which maintains a more constant, non-varying startup current over a range of power supply voltage level, in comparison to the prior art. What is required is a startup circuit than maintains a constant startup current that can be readily fabricated using modern VLSI fabrication techniques. In addition, what is required is a startup circuit that will reliably produce the required amount of startup current in order to reliably wake up an integrated circuit. The present invention provides a novel solution to the above requirements.
SUMMARY OF THE INVENTION
The present invention is a startup circuit which maintains a more constant, non-varying startup current over a range of power supply voltage level, in comparison to the prior art. The startup circuit of the present invention maintains a constant startup current that can be readily fabricated using modern VLSI fabrication techniques. In addition, the startup circuit of present invention will reliably produce the required amount of startup current in order to reliably wake up an integrated circuit.
In one embodiment, the present invention is implemented as a startup circuit having a wide channel transistor for producing a subthreshold leakage current. A current mirror is coupled to the wide channel resistor and is configured to receive the subthreshold leakage current and produce a startup current therefrom. The subthreshold leakage current produced by the wide channel transistor is constant with respect to a power supply voltage. The subthreshold leakage current produced by the wide channel transistor is constant with respect to a power supply voltage. The current mirror includes a first transistor diode connected to the wide channel transistor and a second transistor having a gate connected to a gate of the first transistor. The gate of the wide channel transistor is coupled to the gates of the first transistor and second transistor of the current mirror.
In so doing, the startup circuit of the present invention maintains a more constant, non-varying startup current over a range of power supply voltage level, and will reliably produce the required amount of startup current in order to reliably wake up an int
National Semiconductor Corporation
Nguyen Matthew V
Wagner , Murabito & Hao LLP
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