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MAX485EESA Datasheet(PDF) 12 Page - Maxim Integrated Products |
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MAX485EESA Datasheet(HTML) 12 Page - Maxim Integrated Products |
12 / 16 page ±15kV ESD-Protected, Slew-Rate-Limited, Low-Power, RS-485/RS-422 Transceivers 12 ______________________________________________________________________________________ The major difference between tests done using the Human Body Model and IEC1000-4-2 is higher peak current in IEC1000-4-2, because series resistance is lower in the IEC1000-4-2 model. Hence, the ESD with- stand voltage measured to IEC1000-4-2 is generally lower than that measured using the Human Body Model. Figure 7 shows the current waveform for the 8kV IEC1000-4-2 ESD contact-discharge test. The air-gap test involves approaching the device with a charged probe. The contact-discharge method connects the probe to the device before the probe is energized. Machine Model The Machine Model for ESD tests all pins using a 200pF storage capacitor and zero discharge resis- tance. Its objective is to emulate the stress caused by contact that occurs with handling and assembly during manufacturing. Of course, all pins require this protec- tion during manufacturing—not just inputs and outputs. Therefore, after PC board assembly, the Machine Model is less relevant to I/O ports. MAX487E/MAX1487E: 128 Transceivers on the Bus The 48k Ω, 1/4-unit-load receiver input impedance of the MAX487E and MAX1487E allows up to 128 transceivers on a bus, compared to the 1-unit load (12k Ω input impedance) of standard RS-485 drivers (32 transceivers maximum). Any combination of MAX487E/MAX1487E and other RS-485 transceivers with a total of 32 unit loads or less can be put on the bus. The MAX481E, MAX483E, MAX485E, and MAX488E–MAX491E have standard 12k Ω receiver input impedance. MAX483E/MAX487E/MAX488E/MAX489E: Reduced EMI and Reflections The MAX483E and MAX487E–MAX489E are slew-rate limited, minimizing EMI and reducing reflections caused by improperly terminated cables. Figure 16 shows the driver output waveform and its Fourier analy- sis of a 150kHz signal transmitted by a MAX481E, MAX485E, MAX490E, MAX491E, or MAX1487E. High- frequency harmonics with large amplitudes are evident. Figure 17 shows the same information displayed for a MAX483E, MAX487E, MAX488E, or MAX489E transmit- ting under the same conditions. Figure 17’s high-fre- quency harmonics have much lower amplitudes, and the potential for EMI is significantly reduced. Low-Power Shutdown Mode (MAX481E/MAX483E/MAX487E) A low-power shutdown mode is initiated by bringing both RE high and DE low. The devices will not shut down unless both the driver and receiver are disabled. In shutdown, the devices typically draw only 0.5µA of supply current. RE and DE may be driven simultaneously; the parts are guaranteed not to enter shutdown if RE is high and DE is low for less than 50ns. If the inputs are in this state for at least 600ns, the parts are guaranteed to enter shutdown. For the MAX481E, MAX483E, and MAX487E, the tZH and tZL enable times assume the part was not in the low-power shutdown state (the MAX485E, MAX488E– MAX491E, and MAX1487E can not be shut down). The tZH(SHDN) and tZL(SHDN) enable times assume the parts were shut down (see Electrical Characteristics). 500kHz/div 0Hz 5MHz 10dB/div Figure 16. Driver Output Waveform and FFT Plot of MAX485E/MAX490E/MAX491E/MAX1487E Transmitting a 150kHz Signal 500kHz/div 0Hz 5MHz 10dB/div Figure 17. Driver Output Waveform and FFT Plot of MAX483E/MAX487E–MAX489E Transmitting a 150kHz Signal |
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