Classic application of MOS transistor circuit design (1) -iic bidirectional level shift

MOS tube , As one of the most important components in hardware design , It will be used in many application scenarios .

In circuit design , Level conversion circuit is an indispensable part . In terms of power supply , In addition to system power supply , In many other cases, level conversion circuits are needed , To get the voltage we need . At the same time, there are various circuits in level conversion , One of them is useful to MOS Case of management . So today, , Xiaobian will explain to you MOS Application of transistor in level conversion circuit .

two-way level shift

This circuit belongs to IIC Level conversion circuit . The main use is NMOS tube . Connected in series with clock and data signal lines respectively . For convenience , Let me introduce SCL Level conversion on this line .

As a clock line , Nothing more than low level , Or high level .

So , This line is divided into four situations .

1. When the left end is pulled low, it is the low level ,Vgs=3.3V.
   here MOS The pipe is open , The signal on the right is also in a low-level state .

2. When the left end is pulled high, it is the high level .Vgs=0V.
   here MOS Pipe cut off ,MOS The pipe is not connected . The left end is still high , be in 5V.

On the line , The left end is 3.3V, On the right is 5V. That is to form 3.3V turn 5V Level transition state of .

3. When the right end is pulled low, it is the low level
   here MOS The diode of the tube is on , At this time, the left end is also in a low-level state .

4. When the right end is pulled high, it is the high level .
   here MOS Pipe cut off , The left end is high 3.3V.

On the line , The left end is 3.3V, On the right is 5V., That is to form 5V turn 3.3V Level transition state of .

Circuit design considerations

1. MOS Tube selection enhanced depletion NMOS tube .
2. NMOS tube , The left and right ends are adjustable , but S The pole voltage is always less than G Pole voltage .
3. MOS Tube on voltage Vgs(th) Slightly less than level shift Left end level value .
4. IIC Different selection rates , Yes MOS The switching time of the tube is required . The data has a time difference from left to right . On the one hand, this time is affected by MOS The closing time parameter of the tube affects , On the other hand, it is affected by the capacitive reactance of the bus + Effect of pull-up resistance （RC Time constant ）. The most common is I2C The transmission rate is 400kbit/s. namely 1bit The transmission time of is 2.5us,MOS Tube when selecting , Its switching time should also be less than this value . Otherwise , Here 2.5us within ,1 The transmission of bit data may not meet the requirements of timing , Cause communication failure .

In addition ,MOS Tube is doing 3.3V turn 5V On the level conversion circuit , And here's more , It is also one of the most common methods .