E/DMOS inverter static analysis and detailed explanation

1. Discussion on output voltage

Two operating points on the output characteristic curve of the inverter, point A (on state) corresponds to the output low level, point B (off state) corresponds to the output high level. Now discuss the size of the output high and low levels and their relationship with device parameters.

(1) Output high level

&nbsp ; When the inverter is cut off, the output voltage is close to , then the load management , so meet, the load tube works in the unsaturated zone, and its current is:

Because:

The above formula is rewritten as:

Due to the end of the input tube, , through the load management is also 0, so And get:

When the E/DMOS inverter is off, the output high level is indeed equal to the power supply voltage.

(2) Output low level

When When the inverter is turned on, the output low level , but not Not equal to . At this time, the of the load management meets the requirements of , the load tube works in the saturation zone, and its current is:

And the input tube , meet , the input tube works in the unsaturated zone, The current is:

Because of

The above formula is rewritten as:

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by , Can be solved:

This is when the inverter is turned on, the output low level The expression. It is related to device parameters and process parameters Related. For example, the larger the , the larger the The smaller, then The closer it is to 0.

From the above analysis, we can see that Inverter output high level close Power supply voltage, output low level is close to 0, so the output The voltage amplitude is large and the power supply voltage is fully utilized, which is a major advantage of E/D MOS circuits.

2. Transmission characteristics and noise tolerance

(1) Transmission characteristic curve

 Figure 2-35 shows the transmission characteristic curve of the E/DMOS inverter. The parameters shown above have the same meaning as those in the E/EMOS inverter. same.

According to different working conditions, the transmission characteristic curve can be divided into three different areas. In order to make the discussion clear, let's first explain the working areas of the load tube and the input tube in these three areas. As you all know, the boundary between the saturated and unsaturated regions of the MOS transistor is:

For the load tube of the E/DMOS inverter, because:

Substituting the above formula, the boundary between the saturated and unsaturated regions of the load tube of the E/D MOS inverter is:

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If (Above the dividing line), the load tube is working in the unsaturated zone, if (Below the dividing line), the load tube is working in the saturation zone.

For the input tube, the dividing line between the saturated zone and the unsaturated zone is:

When When (upper left side of the dividing line), the input tube works in the saturation zone; when (the lower right side of the dividing line), the input tube works in the unsaturated zone.

According to the above division, the working conditions of the two tubes of the inverters in the three districts in Figure 2-35 are relatively clear. In the first zone, the load tube works in the unsaturated zone, and the input tube works in the saturated zone; in the II zone, both the load tube and the input tube work In the saturation zone; in the third zone, the load tube works in the saturation zone, and the input tube works in the unsaturated zone.

Now, discuss the relationship between the output voltage and the input voltage of the inverter in the three regions.

Zone I; when, The input tube is cut off, and the current through the input tube, The inverter output is high, Which is the AB section of the characteristic curve. whenSlightly larger than, The output voltage begins to drop, but still meetsand、The condition is that the load tube works in the unsaturated zone and the input tube works in the saturated zone. The current formulas of the two tubes are:

Depend on，The equation can be obtained:

Solving this equation, we can get:

when大于Not long after, meet:

, You can use the series to expand the approximate formula, Simplifying the formula (2-52) into:

It can be seen that withIncreases, the channel of the input tube increases, and the channel resistance decreases, so,Decline quickly, which is characteristicpart.

Zone II,further increase, make the input tube and load tube work in the saturation region, the currents are:

By the equationSolutions have to:

This is a straight line parallel to the vertical axis, which is the characteristic curvepart.

Is the input voltage from the short-cut lead of the inverter, this voltage depends on，and if the device geometry size is fixed, it mainly depends on the process parameters。

In zone III, the input tube is in the unsaturated zone, and the load tube is in the saturated zone. The currents are:

Depend on, Get the equation:

To solve this equation, we get:

Depend onWhen, the radical can be expanded by series, which can be approximated and simplified as:

It can be seen from the formula (2-56) that with the input voltageincreases, the output voltage slowly decreases until it approaches zero; andthe smaller,the faster the decline, this is the DE segment of the characteristic curve.

From the above discussion, we can see that the transmission of E/DMOS inverters and device parameters and process parameters There is a close relationship.

Figure 2-36(a) means different effects on the transmission characteristics, as seen from the figure, take can get good transmission characteristics.

Figure 2-36(b) means Different effects on transferability, as seen in the figure, The impact of transmission characteristics is particularly significant, Heal Larger, the worse the transmission characteristics, the higher the output low level.

In addition, the The same as the threshold voltage of the enhanced load tube , To be affected by source modulation, therefore, in the design, is an important factor that needs to be carefully considered.

(2) Noise tolerance

 If the noise tolerance is required, the expression of the door closing level and the door opening level must be obtained first. From the picture, 2-35, you can see that is the minimum required Output high level, its corresponding input voltage is the closing level. Use and Replace the and can obtain the closing level:

Similarly, you can see that is the specified maximum output low level, and its corresponding input voltage is the door opening level, willand Respectively replace the and , you get the door opening level:

So, according to the definition, the input low-level noise tolerance can be written as:

The output high-level noise tolerance is:

The following is an example, so that everyone has a quantitative concept of the size of the noise.

If there is an E/D MOS inverter, its , which stipulates . Try to find its noise tolerance.

Calculated from the formula (2-57):

Calculated from the formula (2-58):

So:

It can be seen that the noise tolerance can reach more than 35% of the power supply voltage. Large DC noise tolerance and strong anti-interference ability are another feature of E/DMOS inverters.

It should also be emphasized that the DC characteristics of E/DMOS strongly depend on the pinch-off voltage of the load tube, such as output low level and the anti-interference performance are all the same as is directly related, therefore, Become an important factor in design. In order to ensure the low-level output requirements and reduce the chip area, it is not necessary to increase the W/L ratio of the input tube, but to reduce method to achieve.

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