introduction
FPGA, microprocessor, ds2482-100 and ds2480b are common 1-wire host devices. 1-Wire/iButton The slave device is produced by Maxim, and the typical working voltage of this series of devices is 2.8V to 5.25V. In the past, the traditional 1-wire master and slave devices used 5V drain open circuit logic. Now, designers need 1-wire host IO to provide 1.8V drain open circuit logic. Most 1-wire slave devices can work safely at 5V, and most of them cannot work at 1.8V. A bidirectional level converter is needed to overcome this limitation. This reference design (RD) uses Maxim Max3394e bidirectional level converter is used to solve the problems in such applications.
Level converter
The max3394e bidirectional level converter is packaged in 8-pin, 3mm x 3mm tdfn. With the help of its internal slew rate enhancement circuit, it can be ideal for driving large capacitive loads. The capacitive load of 1-wire slave devices is usually greater than 500pf. The VCC I / O pin of max3394e has 15kV HBM (body mode) electrostatic protection to protect the 1-wire host. 1-wire bus is usually used to connect the external world, and HBM protection is the basic requirement. It is recommended to use ds9503p at pull-up resistor (R3), optional strong pull-up circuit and 1-wire slave device to enhance ESD protection.
Application circuit
The circuit shown in Figure 1 uses max3394e to realize bidirectional level conversion from 1.8V to 5V, and the system adopts open drain port. Figure 1. Circuit schematic diagram of 1-wire bidirectional level (1.8V to 5V) converter. Note that pin I / O VL and I / O VCC have 10K internal pull-up. The BOM (bill of materials) of the reference design is shown in Table 1. Table 1. Bill of materials
Waveform measurement / test results
Figs. 2 to 5 are the results of testing the evaluation circuit board. The test conditions are:
VL = 1.8V
VCC = 5.0V
Ch1: 1-wire host (ow_master)
OW_ Slave line length: 2.4m, max.
The optional strong pull-up circuit in Figure 1 is not used in the test.
Measure at room temperature only.
Figure 2. The performance of max3394e can be seen from the 1-wire reset waveform. The online response pulse amplitude does not exceed 250Mv, which is lower than the 0.4V maximum voltage of the typical 1-wire host VIL. Figure 3. 1-wire write operation waveform, write "1" time slot, tlow1 conclusion
This reference design is used to realize 1.8V to 5V 1-wire bidirectional level conversion and drive typical 1-wire slave devices. This paper introduces the construction and test of the designed circuit, and gives the circuit schematic diagram, BOM and typical test waveform.
Bao’an District Shenzhen City, China
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