What Is Reverse Voltage?

Whenever a reverse voltage is applied to a diode, it is called a reverse voltage. Generally there is no numerical definition of reverse voltage. Regardless of the voltage, as long as it is reverse, it is the reverse voltage.

CMOS circuits have the advantages of low cost, low power consumption, and fast speed. Various interface circuits, such as USB, IEEE 1394, RS422 / 485, etc., can be implemented using CMOS technology. The interface circuit usually uses a CMOS power switch as the output buffer circuit. In the actual application environment, the interface circuit is often subject to the impact of reverse voltage. Therefore, a corresponding reverse voltage protection circuit must be designed. When the interface circuit is subject to reverse voltage shock, the interface circuit port voltage is higher than the power supply voltage. At this time, the protection circuit isolates the CMOS power switch from the port surge voltage, thereby ensuring the safety of the interface circuit. A simple and effective protection circuit is proposed. This circuit can automatically select the substrate and gate bias voltage of the CMOS power switch, thereby suppressing reverse current and ensuring device safety. This protection circuit is not connected in series with the CMOS power switch, so the output drive capability and power consumption efficiency of the circuit are not affected. This circuit is applied to an interface circuit chip. It is designed with a 0.6 m standard CMOS digital process. The circuit is made to achieve + 12V reverse voltage protection and achieves good results.

Reverse voltage typical overvoltage protection circuit

The output stage of the ordinary CMOS interface circuit is shown in Figure 1. In the figure,

with

Between the drain of the CMOS switch and the substrate.

Health diode. During normal operation, D1 and D2 are in the reverse cutoff state. However, in actual electronic systems, there are often situations where some components are powered off and other components are normally powered. at this time,

Is 0, and

A positive voltage can be obtained from other components of the electronic system. In this case, the output terminal of the device is subject to reverse voltage, that is, the output terminal voltage is higher than the voltage of the power supply terminal, it will conduct forward current, and a large current will flow, which affects the safety of the device.

figure 1

In order to solve the reverse voltage problem, diodes are usually used for protection, as shown in Figure 2. However, the existence of the diode causes the output swing of the device to decrease, which affects the output drive capability of the device. At the same time, the introduction of the diode also increases the on-resistance of the output and affects the transient characteristics of the output stage. The MOS tube is used instead of the diode, but in order to reduce the on-resistance of the MOS tube, a larger width-to-length ratio MOS tube is used, which occupies a larger layout area and affects the chip's cost performance.

figure 2

In order to solve the output swing problem, an N-well floating structure is proposed. Its principle is shown in Figure 3. This structure uses an N-well floating circuit to provide reasonable bias for the substrate of the output stage PMOS tube and suppress the conduction of the parasitic diode. when

0, output

When a certain positive voltage bias is obtained from the outside, M3 is turned on, and the external voltage reaches the gate of M2 through M3. Because the gate and drain potentials are equal, M2 is turned off. At this time, the gate of M1 is 0, so M1 is turned off, which isolates the influence of external voltage on the internal of the device. However, when the circuit is working normally and the input voltage of V in is high, the gate and source potentials of M1 are both the power supply voltage and M1 is in the off state. At this time, M1's on-resistance is extremely large, which seriously affects the time characteristics of the circuit.

image 3

Working principle of reverse voltage reverse voltage protection circuit

The reverse voltage protection circuit consists of a substrate voltage protection circuit and a gate voltage protection circuit. Its basic principle is shown in Figure 4.

Figure 4

MP1 and MN1 form the output stage of the device.

Is the output. MP2 and MP3 form a substrate voltage protection circuit, and the unit has a positive feedback structure. For output port voltage

And device supply voltage

For comparison, choose a relatively high voltage for the substrate potential of MP1. In addition, the positive feedback circuit has a fast response speed and can provide reasonable substrate potentials for MP1 to MP3 and MP6 in time to ensure that the circuit has good anti-latch capability. In FIG. 4, MN2 to MN4 and MP4 to MP7 constitute a gate voltage protection circuit. When the power supply voltage is 0, if

For a certain positive voltage, MP4 / MN2 is an inverter structure, its output is high, and MP6 is turned off; MN3 is turned on so that the gate potential of MN4 is 0, thereby turning off MN4 and turning on MP7. MP7 is turned on to make the source and drain voltages equal, and the gate potential of MP1 is equal to the substrate potential.

, MP1 is completely turned off to ensure the safety of the device. When the device supply voltage

Under normal conditions, if the device

Above

, Then the MP1 substrate potential is selected as

; Reasonably adjusting the width-to-length ratio of MN2 and MP4 can make the output voltage of MN2 / MP4 and

Follow, turn off the switch pair MN4 and MP6, MP7 is turned on, set the gate potential and substrate potential of MP1 to

Therefore, MP1 is turned off, and reverse voltage protection at the output terminal is implemented.

When the device is operating normally, the power supply voltage V dd is normal,

0

Between a certain voltage, at this time the substrate voltage protection circuit will select the power supply voltage

Is the substrate potential of MP1; the output of the MP4 / MN2 inverter is low, and MP6 is turned on; the output of the MP5 / MN3 inverter is high, MN4 is turned on, and MP7 is turned off. input signal

After the switch pair MN4 / MP6 reaches the gate potential of MP1, the normal function of the circuit is realized. Because the on-resistance of the switch to the tube is much smaller than the PMOS switch. In addition, when the circuit is working normally, the inverters MP4 / MN2 and MP5 / MN3 are in the off state, which will not increase the static power consumption of the circuit.

Verification and test results of reverse voltage circuit

This reverse voltage protection circuit is used for an interface circuit, and it is designed with a 0.6m standard CMOS process. In the Cadence environment, the working state of the device is simulated when the power supply is powered down and the power supply voltage is normal. Figure 5 (a) shows

Gate potential of MP1 at = 0 (

), Substrate potential (

) And output voltage (

) Follow the curve. As can be seen,

with

Can ideally follow

The voltage guarantees the safety of the output CMOS power switch. Figure 5 (b) shows when the power supply voltage is normal (

= 3.3V) input (

), Output (

) Transient waveform.

Figure 5a

Figure 5b

The output high level is equal to the power supply voltage, which indicates that the reverse voltage protection circuit designed in this article will not affect the output amplitude of the device; the input and output signals follow well, indicating that the circuit has good transient response characteristics. This protection circuit has a total of 9 MOS transistors, the circuit structure is simple, and the layout area is small. The layout design was carried out using the 0.6 m standard CMOS process design specification, as shown in Figure 6.

Test the chip obtained from the streamer, and the device power supply voltage

When = 0, 0 ~ 12V voltage is applied to the output terminal, and the current drawn into the device port by the test output terminal is less than 1 A. Test results show that the protection circuit proposed in this paper can provide good protection when the CMOS power switch withstands a maximum 12V reverse voltage.

Reverse voltage conclusion

A reverse voltage protection circuit is proposed. This circuit provides reasonable gate and substrate bias potentials for CMOS power switches. The circuit structure is simple and the layout area is small. It does not affect the circuit output driving ability and circuit static power consumption. It is suitable for various interface circuits and can achieve reverse Voltage protection function. This circuit uses a 0.6 m standard CMOS process tapeout to achieve + 12V reverse voltage protection and achieve good reverse voltage protection. ^[2]