Bootstrapped low-voltage switch

Details Bootstrapped low-voltage switch Bootstrapped low-voltage switch

1998-09-30

2001-04-10

Tran, Toan (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Gating

Signal transmission integrity or spurious noise override

C327S589000

Reexamination Certificate

active

06215348

ABSTRACT:

This invention relates to switches for use in partly-analog integrated circuits. More particularly, this invention relates to low-voltage analog switches operating with constant overdrive.
2 BACKGROUND: DESCRIPTION OF PRIOR ART
Analog switches are used in various electrical circuits such as: multiplexers, sample-and-hold (S/H) circuits, analog-to-digital converters (ADCs), digital-to-analog converters (DACs), and in particular in discrete-time analog systems such as switched-capacitor (SC) and switched-current (SI) circuits.
2.1 An Analog Switch
An analog switch is a controllable two-terminal device as illustrated in FIG.
1
. It will typically have two states: an “on-state” and an “off-state.” In the on-state, the analog switch should short circuit the two switch terminals, labeled V
in,a
and V
in,b
in the Figures. In the off-state, the analog switch should disconnect the two switch terminals. In neither state should the analog-switch circuit load the switch terminals. The state of the analog switch is usually controlled by a third terminal: the control terminal, labeled V
&PHgr;
in the Figures.
An analog switch may require power to operate. In that case, a high (V
dd
) and a low (V
ss
) supply potential must be supplied.
2.2 A MOSFET-Based Switch
MOSFETs (Metal-Oxide-Semiconductor Field-Effect-Transistors), also known as IGFETs (Insulated-Gate Field-Effect-Transistors), are by themselves analog switches. Current can flow between the two switch terminals (the drain and the source terminals) if, and only if, the gate has a sufficiently high potential (for NMOS, i.e. N channel MOSFETS) to form a conductive channel under the gate area.
The stand-alone MOSFET switch is passive, i.e. it does not require external power to perform the switching operation. MOSFET switches are, however, not straightforward to control, because the potential of the control terminal (the gate terminal) must be adjusted relative to the channel potential to obtain linear behavior. A power-consuming circuit is often required to generate an appropriate signal to control the MOSFET-based analog switch.
2.3 Linearizing the MOSFET Switch
A good switch should in its on-state provide conductance that is independent of the signal it is conducting. A MOSFET's drain-to-source conductance is (when the drain-to-source voltage is zero) proportional to the charge accumulated in the channel. As described by C. Enz, K. Krummennacher, and E. Vittoz in
Analog Integrated Circuits and Signal Processing,
vol. 8, no. 1, July 1995, pp. 83-114, the channel charge has a nonlinear dependence on the relative potentials of all 4 MOSFET terminals: gate, drain, source, and bulk. In the switch's on-state, the drain and source terminals should have essentially the same potential. Hence, the problem is described by only three variables: the gate, channel, and bulk potentials. The channel potential is determined by the signal the switch is conducting, i.e. the switch can be controlled only by adjusting the gate and bulk potentials.
A NMOS requires a minimum gate-to-channel voltage (the threshold voltage), to be able to conduct. If the gate-to-channel voltage is raised beyond this level, the switch's conductance will increase. The threshold voltage is somewhat dependent on the bulk-to-channel voltage. This dependence is often referred to as the “body effect.” A PMOS (P-channel MOSFET) has the same behavior, but with the opposite polarity.
To assure that the MOSFET-based analog switch has a reasonably constant (signal-independent) conductance, it is necessary to compensate for the gate-to-channel voltage dependence. Whether or not it is necessary to compensate for the body effect depends on the application, the supply-voltage difference, and other design specifications.
2.3.1 Constant-Overdrive MOSFET Switches
The principle of constant-overdrive MOSFET switches is that the gate-to-drain voltage (or equivalent, the gate-to-source voltage) in the on-state is controlled to a constant value. This technique is described in U.S. Pat. Nos. 3,955,103 and 4,093,874. These circuits are, however, not suitable for implementation as integrated circuits. They require components that are no longer available in standard CMOS technologies, and they need a fairly high supply-voltage difference to operate.
2.3.2 Body-Effect-Compensated MOSFET Switches
The body effect can in many cases be thought off as a second-order effect. However, when the supply-voltage difference (and thereby the feasible overdrive of the MOSFET) is reduced, the body effect will play a more dominant role (due to an only partial depletion of the substrate/body). Some techniques to avoid or compensate for the body effect are known.
A brute-force method would be to use a SOI (silicon-on-insulator) technology. The substrate of such technologies is typically so thin that the body effect is negligible. This option may become relevant for mass-produced systems, but currently it is considered to be an exotic approach.
In a traditional bulk technology, in which the MOSFETs are realized as diffusions in a common substrate, it may be possible to compensate for the body effect. The technique requires that the MOSFET switch is implemented in a separate well. By biasing the well properly, the body effect can be eliminated. This technique is known as the “back-gate” or “switched-tub” technique, and it is taught in U.S. Pat. Nos. 4,473,761 and 4,529,897. An advantage of this technique is that the threshold voltage is held (constant) at its “minimum.” Hence, the technique is well-suited for use in low-voltage systems. An idealized version is discussed in the book “Delta-Sigma Data Converters,” edited by S. Norsworthy, R. Schreier, and G. Temes,
IEEE Press,
1996, FIG. 11.6.
2.3.3 Constant-Overdrive Switches for use in Low-Voltage Systems
The maximum supply-voltage difference permissible for use in last-generation integrated-circuits technologies is scaled down each year. Hence, there is a great demand for an analog switch that can operate with a low supply-voltage difference and that can conduct signals in the rail-to-rail range. The requirement of rail-to-rail operation, combined with the constant-overdrive-MOSFET approach, leads to the conclusion that the gate potential unavoidably must exceed one of the supply rails (for enhancement MOSFETs). Hence, it is necessary to either boost the supply-voltage difference or use a signal-dependent clock booster.
2.4 Prior Art:
FIG. 2
The basic principle of operation of a constant-overdrive switch is shown in FIG. 11.7 in the book “Delta-Sigma Data Converters.” The figure omits many details, so it has been interpreted. The interpretation is shown in FIG.
2
.
The off-state (clock phase {overscore (&PHgr;)}): The gate terminal of the main switch element, NMOS [
34
], is connected to the low supply potential V
ss
via an internal switch [
44
]. This assures that the two switch terminals, V
in,a
and V
in,b
, are not provided a conductive path through the switch [
10
], assuming that their potentials are higher than the low supply potential V
ss
.
The bootstrap capacitor [
32
] is charged via internal switches [
46
] and [
48
], as preparation for use in the on-state. The voltage thereby stored on the bootstrap capacitor will equal the supply-voltage difference
V
sup
=V
dd
−V
ss
The on-state (clock phase &PHgr;): The state of the internal switches [
40
] [
42
] [
44
] [
46
] [
48
] are all alternated, such that the bootstrap capacitor [
32
] is coupled between the first switch terminal (V
in,a
) and the MOSFET's [
34
] gate terminal. The bootstrap capacitor [
32
] does not have a current path to discharge through, so the overdrive of the MOSFET [
34
] will be constant. Hence, the MOSFET [
34
] will provide a current path between the two switch terminals, V
in,a
and V
in,b
, which has an approximately constant conductance.
The bulk terminal of the MOSFET [
34
&rsqb

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