Most popular use charge pump to power high-speed c

2022-08-26
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Using charge pump to power high-speed can transceiver (1)

using charge pump to power high-speed can transceiver (1)

in the past decades, from the development trend of the automotive industry, the requirements of automotive manufacturing for comfort, efficiency and environmental friendliness have been continuously improved, and the expectations for performance and automotive safety have also been continuously improved. Driven by this trend, the number of electronic subsystems in cars and the wiring connecting these subsystems has increased significantly. The increase of cables leads to the increase of vehicle weight and, of course, the cost. However, in the early 1980s, Bosch introduced the CAN bus network, which effectively reduced the complexity of line connection, reduced the weight of cables and saved costs, so it is widely used in the automotive industry

the transformation of automobile manufacturing from centralized control system to distributed control system helps automobile manufacturers achieve the goal of reducing automobile weight and cost. The centralized control system connects all execution devices, sensors and switches to the control system through a large number of cables, while the distributed management system places the electronic control unit (ECU) at the position that needs to be controlled and communicates with each other through the bus system (for example, two-wire can main network) (Figure 1)

can network is composed of multiple transceiver modules, which are linked through a pair of buses. Each module is a can transceiver, which is used to support the physical layer interconnection between the protocol controller (microcontroller, state machine or other processing engine in the module) and the physical medium (cable). This new can bus design needs rapid standardization to ensure correct communication between ECUs from different manufacturers. ISO (International Organization for Standardization) first defined it in 1993 and made further amendments in 2003 and 2007. The current iso11898 standard has been adopted by original equipment manufacturers (OEMs) as the current standard for can communication in all vehicles

in order to meet ISO standards and provide correct bus level, most can transceiver bus drivers need 5V power supply. However, the main power supply of the electronic system usually cannot meet the power requirements of the subsystem. In this case, the system power supply provided usually cannot directly supply power to the can transceiver. For example, the system may only provide a 3.3V power supply. Sometimes, due to space constraints, the most appropriate number of power supplies cannot be accommodated; Sometimes, due to the heating problem, 5V voltage cannot be directly generated from the battery, especially in the can communication system with high battery voltage (such as the case of dual batteries in cars, or 24V truck system)

voltage converters can be used to generate the required power supply voltage. For low-cost designs with low power consumption and simple structure, charge pumps are usually the best choice. It does not need expensive inductors or extra half. In view of the fact that the domestic rubber and plastic, packaging and other light industry quality testing equipment has not yet had the corresponding national standards and industry standards conductor devices, and it is easy to use

selection of charge pump

1 Transceiver power supply

at present, there are simple and mature can transceivers on the market. Some transceivers need single power supply, while some transceivers need multi-channel power supply. In order to make modules from different ECU suppliers interoperate correctly and realize high-speed can communication complying with iso11898 standard, most modules need a 5V power supply that meets the maximum tolerance requirements

some transceivers also have built-in i/o level adapters. Using the power supply of the protocol controller (acting on the separate power pin of the transceiver), the level adapter adjusts the i/o level of the transceiver in proportion to make it reach the controller level. Thus, the transceiver can be directly connected to the controller working below 5V without any gluing logic

the low-power management transceiver supports local and remote wakeup, so it has another power pin. This pin must be continuously powered by the car battery and consumes very little current

therefore, ECU requires that the high-speed can bus must work effectively even when the ignition key is "off"

for the function description of other pins of the can transceiver, please refer to the data of the selected device

2. The power supply current

can bus is usually in one of two logic states: implicit or explicit (Figure 2). In the normal communication mode, the transceiver needs the maximum input current in the dominant state and the minimum input current in the recessive state. At this time, the current consumed by i/o level adapter and remote wake-up function can be ignored, because they usually consume the current of microcontroller power supply and car battery, and the value is very small

when the bus fails, the power supply current will increase significantly, especially in can_ When the H line is short circuited to the ground. Most transceivers limit the short-circuit current to a specific maximum. In order to prevent power supply voltage drop, it is best to define the output current specification of the charge pump according to the current requirements in this case

based on the above considerations, in order to provide an appropriate power supply for the can transceiver, the charge pump must maintain a 5V output voltage and meet the nominal voltage tolerance in the transceiver data. The minimum output current must support can_ H short circuit to ground

max1759 charge pump is used to supply power to max13041 transceiver.

a variety of traditional can transceivers and charge pump devices can be found in the market. This paper mainly focuses on the design of max13041hscan transceivers and max1759buck/boost regulated charge pump to solve the power supply problem of transceivers. The transceiver is powered by the VCC pin. In order to support the standard iso11898can communication, the VCC must be maintained between 4.75V and 5.25V (nominal working voltage range). This voltage establishes the correct communication signal between the buses (can-h, can-l) and supplies power to the receiving circuit when the IC is in the normal working mode

the vi/o input of the transceiver enables the interface circuit of the 3.3vi/o microcontroller to establish the correct level at the receive/transmit (rxd/txd) level of the controller and the transceiver. Of course, when communicating with the 5V controller, the vi/o pin can also be powered by the 5V power supply

The Vbat pin (usually connected to the vehicle 12V battery) supplies power to the wake-up detection circuit with extremely low quiescent current. According to the information of CAN bus, this pin can control max13041 to wake up from sleep mode. For detailed description of other pins, please refer to max13041 data

in the normal communication mode, the maximum input current (VCC pin) required by max13041 in the dominant state is 80mA and 10mA in the recessive state (Figure 2). The current flowing into vi/o and Vbat is negligible. When the bus fails, the VCC power supply current will increase significantly, especially when the can_ When the H signal line is short circuited to the ground. Max13041 limits the short-circuit current to IO (SC) =95ma

based on the above considerations, in order to meet the power supply requirements of can transceiver, the charge pump must have a stable 5V output voltage to ensure that it meets the requirements of voltage tolerance, and the minimum output current is 95ma

1. Charge pump requires that the max1759 architecture allows the input voltage to be higher or lower than the regulated output value. In this application, the charge pump only works as a boost converter. When Vin is lower than Vout, the charge pump works as a voltage stabilizing booster. Under light load, the charge pump only performs switching operation when it needs to maintain the power supply energy of the load, consuming a small amount of quiescent current. Under light load, the output voltage ripple will not increase

for a detailed description of other characteristics of the charge pump, please refer to the max1759 data

2. Realizing the 3.3V scheme

it can be seen from the circuit in Figure 3 that it is very simple to use a charge pump to power the max13041. Simply connect the max1759 to the VCC input of the can transceiver (shown by the blue dotted line from the government layer), and a 5V output voltage that meets the tolerance and output current requirements can be generated. This configuration allows other circuits to use low voltage power supply. In this example, the external 3.3V power supply (green) supplies power to the charge pump (in), the microcontroller, and the vi/o level converter of the transceiver. Pull up the/shdn of the charge pump and put the device in the on state. Max1759 data details the input/output, so the heat treatment process has an important impact on the mechanical properties of the alloy (CIN, cout) capacitor and flying capacitor (Cx) selection

3. Electromagnetic compatibility

electromagnetic compatibility (EMC) is a design challenge of CAN communication network, especially when switching regulator is used for power supply. The network wiring of can system is a key problem, because the can transceiver can_ H and can_ L pin is the interface connecting the whole automobile bus network. If you are not careful in design, there may be large interference from the can power supply, and the interference signal will eventually enter other ECUs through the transceiver, bus, or adjacent cables. Such interference will cause communication failure or transmission failure between other control units of the system

using charge pump to power high-speed can transceiver (2)

based on this consideration, we tested the EMC characteristics of max13041 powered by max1759 charge pump and compared it with the EMC characteristics of transceiver powered by filtered 5V power supply. From this, we can see the EMC interference of the charge pump and the ability of the charge pump to suppress the interference of the CAN bus transmission power supply. In this test, we mainly consider two aspects: electromagnetic immunity (EMI) and electromagnetic radiation (EME)

4. Electromagnetic immunity test (EMI)

iso11452 specification specifies several test methods for RF interference resistance, including high current injection (BCI), transverse electromagnetic cell (TEM cell), stripline and direct power injection (DPI)

because the DPI method has good repeatability (using a well-designed test board) and the test workload is small, we chose this method. The DPI test principle is very simple: inject a specific modulated or unmodulated AC voltage into the bus cable, and detect the signal integrity of the transmitted data through the RXD pin of the transceiver. This method is also convenient for comparing the designs of different manufacturers, and can test can transceivers (such as ibee) in an independent laboratory

5. The test device

the test device (Figure 4) includes three identical transceivers welded on the PCB, one of which is powered by the max1759 charge pump, and node 1 is used as the transmitter to simulate the bitstream template of can data. The data is received by all transceivers and monitored at the RXD port. For RF decoupling of rx1 to RX3 outputs and txd1 inputs, 1K? Resistance. The VCC and Vbat power ports of each transceiver IC are buffered with ceramic capacitors (c=100nf). The resistance of the wake-up pin is 33K?. Set the EN pin and/stb pin to high level to make the device in normal working mode. The VCC voltage of node 1 is generated by max1759 charge pump, which is powered by 3.3V. The 3.3V power supply is also used as the vi/o voltage of node 1 transceiver

charge pump output capacitance C1 is 10 μ F. The flying capacitor C2 is 330nf, and the in input pin is 10 μ F capacitor decoupling

in the test circuit, the bus termination adopts 60? R4 resistor for central termination. Symmetrical RF coupling/decoupling adopts RC and connection, and r5/r6=120 C3/c4=4.7nf composition. The external 3.3V, 5V and 12V power supply is provided by the standard power supply and filtered through the filter network

6. Test steps

test the can transceiver max13041 under normal working mode. In the first round of test, all transceivers use standard vcc=5v power supply; In the second round of tests, one of the transceivers was powered by a charge pump (Figure 4). The template generator generates a 500kbps square wave with a duty cycle of 50%, and simulates the can signal (alternating data) of the TXD pin of node 1. RF input HF generator (hf1) injects a fixed frequency amplitude modulated (AM) AC voltage with a power of 36dbm into the CAN bus to simulate interference signals

evaluation inhibition

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