The working principle, main features and structure of MC33399 applied to automobile body control system

1. Overview

LIN (Local Interconnect Network) is a low-cost bus network. Its original development purpose was to make up for the shortcomings of CAN bus. It is mainly used in some occasions in automobiles that do not require high communication speed. LIN bus can be used as an auxiliary network or sub-network of CAN bus to solve many problems caused by too many wires in the car.

In addition to the microcontroller, a simple LIN node requires two chips, namely the LIN interface chip and the 5V voltage regulator. Freescale's MC33399 chip is a single-wire physical interface device dedicated to LIN. The device has very low power consumption, can control an external voltage regulator, safely complies with LIN specifications, has strong anti-interference ability, and is a high-performance analog device suitable for automobiles with complex working environments. The main features of MC33399 are as follows:

Communication rate range is 1~20kb/s;

Rated working voltage: 8V～18V, normal voltage: 7V～27V;

Reactive nodes do not affect the bus status;

There are two working modes: normal and sleep. The quiescent current in sleep mode is only 20μA.

LIN bus wake-up, MCU command wake-up and interface external high-voltage switch input wake-up;

With external voltage regulator control function;

Built-in pull-up resistor;

The ESD voltage of LIN pin can reach 4kV;

·Has good electromagnetic compatibility;

·Working temperature range is -40℃～125℃.

2. Pin Function MC33399 adopts SO8 type SMD package. Figure 1 shows the pin arrangement, and the function description of each pin is listed in Table 1.

3. Internal structure and working principle

The internal structure block diagram of MC33399 is shown in Figure 2. Its main working principles are introduced as follows:

3.1 Main functions

The working mode of MC33399 is controlled by the enable terminal EN. When EN is 1, the circuit is in normal working state, and the two transmission paths from TX pin to LIN bus and from LIN bus to RX pin are activated. When EN is 0, the chip enters sleep or low power mode, and the two transmission paths are disabled. In sleep mode, the LIN pin can be kept at a high level Vsup through a pull-up resistor and an upper current source, and a static current (typical value is 20μA) is provided to the chip by the power supply through the Vsup pin.

The LIN pin of MC33399 is used to complete the single bus transceiver function. The TX pin is used to control the output state of the LIN pin. When working, the states of these two pins are always consistent. The RX pin can be used to display the state of the LIN bus. When the LIN bus is at a high level (recessive), RX is high; conversely, when the LIN bus is at a low level (dominant), RX is low. The RX output is a typical CMOS push-pull output structure.

Due to its specific internal structure, when the ground drifts or the power connection fails, there will be no reverse current from the LIN bus into the chip. MC33399 allows the Wake pin to input high voltage wake-up, and can also be awakened by two jumps from high to low or from low to high. When the chip enters the sleep state, the chip will record the current state of the Wake pin. When the circuit detects the opposite state of the Wake terminal, it is considered that a wake-up event has occurred. In addition, the chip also has a built-in noise filter, which can suppress high-frequency noise interference on the bus and prevent false wake-ups.

3.2 Sleep wake-up

MC33399 can wake up from sleep mode in the following three ways:

(1) LIN bus wake-up

When any node on the bus sends a "wake-up frame", the state of the LIN pin can be changed from recessive to dominant to wake up the chip. When awakened, the MC33399 is in "wait mode" and the INH pin is set high to activate the external regulator. After that, the MCU is powered on and the EN pin is set high, so that the chip enters "normal mode" from "wait mode".

(2) Internal node activation wake-up

When the MCU in the local node issues an instruction to set the EN pin high, the chip will also enter "normal mode".

(3) Wake up through the Wake pin

By setting a switch outside the MC33399 chip, the Wake pin can input high voltage to wake up.

4. Typical application circuit

Figure 3 is a typical application circuit of MC33399. This circuit adopts the Wake pin input switch wake-up method, in which the 5V external voltage regulator is controllable. In the figure, the EN pin is directly connected to the I/O port of the MCM to select the working state of the MC33399. The communication receiving pin RX and the transmitting pin TX are respectively connected to the serial communication interface (SCI) of the MCU to complete the communication function. The INH pin is directly connected to the control end of the external voltage regulator. When the circuit is working normally, the INH pin outputs a high level, the external voltage regulator is activated, and the normal output 5V voltage is used to power the MCU; when the chip enters the sleep mode, the INH pin outputs a low level to disable the external voltage regulator and the MCU is powered off. In addition, the INH pin can also be connected to the IRQ or XIRQ pin of the MCU through a transistor to generate an interrupt. At this time, the external voltage regulator should be selected to be uncontrollable. The application method of the chip is similar to Figure 3, except that the connection method of the INH pin is inconsistent.

Since the circuit integrates pull-up resistors and series diodes between the LIN pin and the Vsup pin, the bus slave node does not need an external pull-up component. However, for the master node, a 1kΩ pull-up resistor must be added externally, and a diode must be connected in series to prevent the MC33399 from being powered on through the bus when the battery is off.

By connecting an external switch to the Wake pin, a high voltage can be input to the Wake pin to wake up the MC33399 from sleep mode. When the input voltage of the Wake pin exceeds 14V, a series resistor must be connected to limit the instantaneous pulse current (the resistance is generally 33kΩ) to prevent excessive input current. At the same time, it must be noted that the Wake pin must not be left floating. If this pin is not used, it must be grounded to prevent false wake-up.

5. Conclusion

The MC33399 circuit is widely used in automobile body control systems for its perfect performance. The external voltage regulator used with this circuit can be the 8-pin circuit LT1121. In fact, these two chips will be replaced by Freescale's new system basis chip (SBC).