CN111970004A

CN111970004A - Bootstrap switch structure without influencing service life of device

Info

Publication number: CN111970004A
Application number: CN202010806149.4A
Authority: CN
Inventors: 张昱桐; 樊星; 陈艳
Original assignee: Beijing CEC Huada Electronic Design Co Ltd
Current assignee: Beijing CEC Huada Electronic Design Co Ltd
Priority date: 2020-08-12
Filing date: 2020-08-12
Publication date: 2020-11-20

Abstract

The invention relates to a bootstrap switch structure without influencing the service life of a device, which can be applied to the front end of an analog signal sampling circuit. In a high-speed analog-to-digital converter, a bootstrap switch circuit with constant on-resistance is required due to the requirement on the linearity of a sampling signal, the traditional bootstrap switch structure can enable the grid-substrate voltage of a switch tube to reach twice the power supply voltage, so that the service life of a device is greatly reduced, the source substrate is directly connected to cause electric leakage to influence signal sampling, and the high-voltage device is used to influence the linearity of the sampling signal. The bootstrap switch consists of a switch tube with a substrate gating circuit and a bootstrap voltage generating circuit; the invention can control the grid-substrate voltage of the switch tube in the bootstrap switch within the power supply voltage, and does not influence the service life of the switch tube while ensuring the linearity of the output signal of the switch.

Description

Bootstrap switch structure without influencing service life of device

Technical Field

The invention relates to a special switching circuit which plays an important role in an analog-to-digital converter and belongs to the field of analog circuit design.

Background

An Analog-to-digital converter (ADC) is an indispensable component structure in a digital-to-Analog conversion system, and has a widespread application in application scenarios such as broadcasting stations, radars, submarine equipment, audio processing, video processing, wireless (wired) communication, data collection, medical images, digital equipment, industrial automation, and the like.

The front end of the analog-to-digital converter is provided with a sampling hold circuit which can be equivalent to a switched capacitor structure and is used for sampling an input signal, performing discrete processing on the input analog signal on a time domain and ensuring that the processed discrete signal is continuous in amplitude; the larger the input bandwidth of the analog-to-digital converter is, the higher the requirement for introducing nonlinearity to the switch is; the traditional bootstrap switch (as shown in fig. 1) can reduce the nonlinearity introduced during sampling, but the gate voltage of the switching tube is + Vin, so that the gate-substrate voltage is greater than the power supply voltage, which causes the device lifetime problem, and if the structure of the bootstrap switch is not changed, the following methods and disadvantages can be adopted: 1. the switch tube uses a high-voltage device, and has the defects that the high-voltage device is not well conducted in a low-voltage domain, and an additional nonlinear error is introduced; 2. the switch tube uses a deep N-well device and leads the substrate thereof to be connected with the source end, and has the defects of causing electric leakage after the sampling is finished and introducing larger errors.

The bootstrap switch structure can be applied to an analog-to-digital converter, and the service life of a switch tube is not influenced while the linearity of a switch output signal is ensured.

Disclosure of Invention

(1) Objects of the invention

Since the resistance of the CMOS switch varies with the input signal, when the CMOS switch is used in the front end of the analog-to-digital converter for broadband signal input, the dynamic performance thereof decreases with the increase of the frequency of the input signal, that is, the CMOS switch introduces nonlinearity in positive correlation with the frequency of the input signal. The use of a conventional bootstrapped switch can reduce switching non-linearity, but introduces lifetime issues for the normal voltage devices. Therefore, through analysis according to the application environment, the invention provides the bootstrap switch structure which is simple in structure, ensures the linearity of the input signal of the switch, and does not influence the service life of the low-voltage device.

(2) Technical scheme

As shown in fig. 2, the bootstrap voltage generating circuit is composed of 4 switch modules S1, S2, S3, S4 and a capacitor C1, wherein one end of S1, S3 is connected to the upper plate of the capacitor C1, one end of S2, S4 is connected to the lower plate of the capacitor C1, the other end of S1 is connected to the power supply, the other end of S2 is grounded, the other end of S3 is connected to the gate of the switch tube, and the other end of S4 is connected to the input signal port (Vin);

the switch tube with the substrate gating circuit consists of a switch tube Ndnw, a switch module S5 and the substrate gating circuit, wherein: the switching tube Ndnw adopts a deep N-well NMOS tube, the source drain end of the switching tube Ndnw is respectively connected with a sampling input signal end (Vin) and a switch output end (Vout), the grid electrode of the switching tube Ndnw is connected with one end of S3 and S5, the other end of S5 is grounded and used for controlling the switching tube to be cut off; the substrate selection circuit is composed of 2 switch modules S6 and S7, one ends of S6 and S7 are connected with the switch tube substrate, the other end of S6 is grounded, and the other end of S7 is connected with an input signal port (Vin).

The bootstrap switch is controlled by a two-phase non-overlapping clock, and the effective phases of the two-phase non-overlapping clock are defined as phi 1 and phi 2, and the working principle is as follows:

Φ 1 is valid: as shown in fig. 3, in this phase, to maintain the phase, the switches S1 and S2 are turned on, the switches S3 and S4 are turned off, and the voltage difference across the capacitor C1 is charged to the power supply voltage; the switch S5 is turned on, the Ndnw grid is grounded, and the switch is turned off; the switch S6 is turned on, the switch S7 is turned off, and the Ndnw substrate of the switch tube is grounded at the moment, so that the substrate is prevented from leaking to the Vin end;

Φ 2 is valid: as shown in fig. 4, this phase is the sampling phase, the switches S1, S2, S5 are turned off, the switches S3, S4 are turned on, and the voltage difference between the two ends of C1 is the power supply voltage, that is, the gate-source voltage of the switching tube Ndnw is the power supply voltage and remains unchanged; the switch S7 is turned on, the switch S6 is turned off, and the source-substrate voltage of the switch tube is at the same level, so that: 1. the voltage difference between any two ends of the switching tube does not exceed the power supply, so that the service life of the switching tube is not influenced; 2. the threshold voltage of the switching tube is constant, and the change of the on-resistance is further reduced.

Drawings

FIG. 1 conventional bootstrap switch structure

FIG. 2 illustrates the bootstrap switch structure of the present invention

FIG. 3 shows the hold-phase operating state of the bootstrap switch

FIG. 4 sample phase operating conditions of the bootstrap switch

Detailed Description

Firstly, the traditional bootstrap switch structure shown in fig. 1 is changed into the structure shown in fig. 2, then the ideal switches S1, S2, S3, S4, S5, S6 and S7 shown in fig. 2 are replaced by actual MOS switches, the selection of the sizes of the switches S1 to S7 and the types of the switching tubes is to ensure that the switches can be normally turned on, and the leakage current when the switches are turned off is required to be as small as possible, because one end of the capacitor C1 is in a suspended state during sampling phase, the leakage current can affect sampling; finally, two non-overlapping clocks are introduced to respectively control the switches S1-S7, so that the working states of the switches are the holding phase and the sampling phase shown in the figures 3 and 4 respectively.

Note that some compromise is also considered in the choice of the value of the capacitor C1: when C1 is too small, due to the existence of the switch tube Ndnw grid parasitic capacitance, the grid-source voltage of the switch tube can not reach the power supply voltage far, and the switch can not be conducted well; when C1 is too large, the switch activation time becomes long; c1 may be chosen to equal 10 times the parasitic capacitance of the switching transistor gate.

In conclusion, the improvement of the traditional bootstrap switch is realized through the technical scheme, the voltage difference between any two ports of the switch tube in the traditional bootstrap switch is not more than the power supply voltage, and therefore the service life of the normal-voltage switch tube is prolonged.

Claims

1. The bootstrap switch structure without affecting the service life of the device is characterized in that the bootstrap switch consists of a switch tube with a substrate gating circuit and a bootstrap voltage generating circuit, wherein:

the bootstrap voltage generating circuit is composed of 4 switch modules S1, S2, S3, S4 and a capacitor C1, wherein one end of S1 and S3 is connected with the upper electrode plate of the capacitor C1, one end of S2 and S4 is connected with the lower electrode plate of the capacitor C1, the other end of S1 is connected with a power supply, the other end of S2 is grounded, the other end of S3 is connected with the grid of a switch tube, and the other end of S4 is connected with an input signal port (Vin);

the switch tube with the substrate gating circuit consists of a switch tube Ndnw, a switch module S5 and the substrate gating circuit, wherein: the switching tube Ndnw adopts a deep N-well NMOS tube, the source drain end of the switching tube Ndnw is respectively connected with a sampling input signal end (Vin) and a switch output end (Vout), the grid electrode of the switching tube Ndnw is connected with one end of S3 and S5, the other end of S5 is grounded and used for controlling the switching tube to be cut off; the substrate selection circuit is composed of 2 switch modules S6 and S7, one end of S6 and S7 is connected to the substrate of the switch tube Ndnw, the other end of S6 is grounded, and the other end of S7 is connected to the input signal port (Vin).

2. The switch tube with the substrate gating circuit according to claim 1, wherein the switch tube Ndnw is a deep N-well NMOS tube with switch logic on the substrate, the switch S7 is turned on during sampling phase, the switch S6 is turned off, the source-substrate voltage of the switch tube Ndnw is at the same level, the switch S6 is turned on during holding phase, the switch S7 is turned off, and the substrate of the switch tube Ndnw is grounded at this time, so that the gate-substrate voltage of the switch tube Ndnw is less than the power supply voltage during the whole operation time, and the linearity of the output signal of the bootstrap switch tube is ensured without affecting the service life of the switch tube.