CN111970004A - Bootstrap switch structure without influencing service life of device - Google Patents

Abstract

The invention relates to a bootstrap switch structure that does not affect the life of the device, and can be applied to the front end of an analog signal sampling circuit. In high-speed analog-to-digital converters, a bootstrap switch circuit with constant on-resistance is required due to the linearity requirement of the sampling signal. The traditional bootstrap switch structure will make the gate lining voltage of the switch tube double the power supply voltage, thereby greatly reducing the Reducing the life of the device and directly connecting the source to the substrate will cause leakage to affect the signal sampling, and the use of high-voltage devices will affect the linearity of the sampled signal. The bootstrap switch is composed of a switch tube with a substrate gate circuit and a bootstrap voltage generating circuit; the invention can control the gate liner voltage of the switch tube in the bootstrap switch within the power supply voltage, and ensure the linearity of the switch output signal. It does not affect the life of the switch tube at the same time.

Description

A Bootstrap Switch Structure That Does Not Affect Device Life

technical field

The invention is a special switching circuit, plays a vital role in an analog-to-digital converter, and belongs to the field of analog circuit design.

Background technique

Analog to digital converter (ADC) is an indispensable component in the digital-to-analog conversion system. It is used in broadcasting stations, radars, submarine equipment, audio processing, video processing, wireless communication, data collection. , medical imaging, digital equipment, industrial automation and other application scenarios, analog-to-digital converters are widely used.

There is a sample and hold circuit at the front end of the analog-to-digital converter, which can be equivalent to a switched capacitor structure. Its function is to sample the input signal, discretely process the input analog signal in the time domain, and ensure that the processed discrete signal must be continuous; the larger the input bandwidth of the analog-to-digital converter, the higher the requirement to introduce nonlinearity to this switch; the traditional bootstrap switch (as shown in Figure 1) can reduce the nonlinearity introduced during sampling, but its switch tube The gate voltage is the power supply +Vin, so that the gate liner voltage is greater than the power supply voltage, which causes the device life problem. If the structure of the bootstrap switch is not changed, the methods and disadvantages that can be used are as follows: 1. The switch tube uses high-voltage devices, and the disadvantage is that high-voltage devices are used. The conduction in the low-voltage domain is not good, and additional nonlinear errors are introduced; 2. The switch tube uses a deep N-well device and connects its substrate to the source terminal. The disadvantage is that it will cause leakage after sampling, which will introduce larger errors .

The bootstrap switch structure involved in the present invention can be applied to an analog-to-digital converter, which ensures the linearity of the output signal of the switch and does not affect the life of the switch tube.

SUMMARY OF THE INVENTION

(1) Purpose of the invention

Since the resistance of the CMOS switch will change with the change of the input signal, when the front end of the analog-to-digital converter applied to the wideband signal input adopts the CMOS switch, its dynamic performance will decrease with the increase of the input signal frequency, that is, the CMOS switch introduces and A nonlinearity that is positively related to the frequency of the input signal. Using a conventional bootstrap switch can reduce switching nonlinearity, but introduces lifetime issues for the constant-voltage device. Therefore, based on the analysis of the above application environment, I have invented a bootstrap switch structure with a simple structure, which ensures the linearity of the switch input signal and does not affect the life of the low-voltage device.

(2) Technical solution

As shown in Figure 2, the bootstrap voltage generation circuit is composed of 4 switch modules S1, S2, S3, S4 and capacitor C1. One end of S1 and S3 is connected to the upper plate of capacitor C1, and one end of S2 and S4 is connected to the capacitor. The lower plate of 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 substrate gating circuit is composed of a switch tube Ndnw, a switch module S5 and a substrate gating circuit, wherein: the switch tube Ndnw adopts a deep N-well NMOS tube, and its source and drain terminals are respectively connected to the sampling input signal terminal (Vin ) and the switch output terminal (Vout), the gate of which is connected to one end of S3 and S5, and the other end of S5 is grounded to control the switch tube to be turned off; the substrate gating circuit is composed of two switch modules S6 and S7. One end is connected to the switch tube substrate, the other end of S6 is grounded, and the other end of S7 is connected to the input signal port (Vin).

This bootstrap switch is controlled by a two-phase non-overlapping clock, and their effective phases are defined here as Φ1 and Φ2. The working principle is as follows:

Φ1 is valid: As shown in Figure 3, this stage is the hold phase, switches S1 and S2 are turned on, switches S3 and S4 are turned off, and the voltage difference across capacitor C1 is charged to the power supply voltage; switch S5 is turned on, and the gate of Ndnw is grounded. The switch is turned off; the switch S6 is turned on, and the switch S7 is turned off. At this time, the substrate of the switch tube Ndnw is grounded to prevent the substrate from leaking to the Vin terminal;

Φ2 is valid: As shown in Figure 4, this stage is the sampling phase, the switches S1, S2, S5 are turned off, and the switches S3 and S4 are turned on. From the law of charge conservation, it can be known that the voltage difference across C1 is the power supply voltage, which is the switch tube Ndnw. The gate-source voltage is the power supply voltage and remains unchanged; the switch S7 is turned on, the switch S6 is turned off, and the source lining voltage of the switch tube is the same level, so: 1. The voltage difference between any two ends of the switch tube does not exceed the power supply and will not affect the switch. Tube life; 2. The threshold voltage of the switch tube is constant, and the change of the on-resistance is further reduced.

Description of drawings

Figure 1 Traditional bootstrap switch structure

Fig. 2 Bootstrap switch structure of the present invention

Figure 3. The holding phase working state of the bootstrap switch

Figure 4. The working state of the sampling phase of the bootstrap switch

Detailed ways

First, change Fig. 1 to the traditional bootstrap switch structure, change it to the structure of Fig. 2, and then replace the ideal switches S1, S2, S3, S4, S5, S6 and S7 in Fig. 2 with actual MOS switches, switch S1 In the selection of the size of ~S7 and the type of switch tube, it is necessary to ensure that the switch can be turned on normally, and the leakage current at the cut-off time is required to be as small as possible, because one end of the capacitor C1 is in a floating state during the sampling phase, and the leakage current will affect the sampling; finally introduce The non-overlapping clocks of the two phases respectively control the switches S1 to S7 so that their working states are the holding phase and the sampling phase as shown in FIG. 3 and FIG. 4 , respectively.

Note that a certain compromise should be considered in the selection of the value of capacitor C1: if C1 is too small, due to the existence of the parasitic capacitance of the Ndnw gate of the switch, the gate-source voltage of the switch will not reach the power supply voltage, which will cause the switch to fail to turn on. Good; when C1 is too large, the switch start-up time becomes longer; C1 can be selected to be equal to 10 times the parasitic capacitance of the switch gate.

To sum up, the present invention realizes the improvement of the traditional bootstrap switch through the above technical solutions, so that the voltage difference between any two ports of the switch tube in the traditional bootstrap switch does not exceed the power supply voltage, thereby increasing the life of the normal pressure switch tube, and at the same time, the The circuit structure is simple, portable and practical.

Claims (2)

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.

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