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MAX3158CPI Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX3158CPI Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 14 page High CMRR RS-485 Transceiver with ±60V Isolation 10 ______________________________________________________________________________________ Detailed Description The MAX3158 is a high CMRR RS-485/RS-422 data- communications interface providing ±60V isolation in a hybrid microcircuit. A single +5V supply on the logic side powers both sides of the interface, with external 100V capacitors transferring power from the logic side to the isolated side (see Block Diagram). The MAX3158 typically draws 25mA of supply current when unloaded or when fully loaded with the driver disabled. Supply current drops to 25µA when the device is placed in shutdown mode (see Low-Power Shutdown Mode sec- tion). The MAX3158 transceiver for RS-485/RS-422 communication contains one driver and one receiver. This device features fail-safe circuitry, which guaran- tees a logic-high receiver output when the receiver inputs are open or shorted, or when they are connected to a terminated transmission line with all drivers dis- abled (see Fail-Safe section). The MAX3158 is selec- table between half- and full-duplex communication by connecting a selector pin to ISOVCC or ISOCOM, respectively. Drivers are output short-circuit current lim- ited. Thermal shutdown circuitry protects drivers against excessive power dissipation. When activated, the thermal shutdown circuitry places the driver outputs into a high-impedance state. The device also features independently programmable receiver and transmitter output phase through separate pins. The MAX3158 is a low-cost replacement for opto-isolated transceivers. Fail-Safe The MAX3158 guarantees a logic-high receiver output when the receiver inputs are shorted or open, or when they are connected to a terminated transmission line with all drivers disabled. The receiver threshold is fixed between -50mV and -200mV. If the differential receiver input voltage (A - B) is greater than or equal to -50mV, RO is logic high. If A - B is less than or equal to -200mV, RO is logic low. In the case of a terminated bus with all transmitters disabled, the receiver’s differential input volt- age is pulled to 0V by the termination. With the receiver thresholds of the MAX3158, this results in a logic high with a 50mV minimum noise margin. Unlike competitor’s fail-safe devices, the -50mV to -200mV threshold com- plies with the ±200mV EIA/TIA-485 standard. MAX3158 Programming The MAX3158 has several programmable operating modes. Occasionally, twisted-pair lines are reversed. The MAX3158 has two pins that invert the phase of the driver and the receiver to correct for this problem. For normal operation, drive TXP and RXP low, connect them to ground, or leave them unconnected (internal pulldown). To invert the driver phase, drive TXP high or connect it to VCC. To invert the receiver phase, drive RXP high or connect it to VCC. Note that the receiver threshold is positive when RXP is high. The MAX3158 can operate in full- or half-duplex mode. Connect H/F to ISOCOM for full-duplex mode, and con- nect it to ISOVCC for half-duplex operation. In half- duplex mode, the receiver inputs are switched to the driver outputs, connecting outputs Y and Z to inputs A and B, respectively. In half-duplex mode, the internal full-duplex receiver input resistors are still connected to pins A and B. Low-Power Shutdown Mode The low-power shutdown mode is initiated by bringing both RE high and DE low. In shutdown, this device typi- cally draws only 25µA of supply current, and no power is transferred across the isolation capacitors in this mode. 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 700ns, the parts are guaranteed to enter shutdown. Enable times t_ZH and t_ZL in the Switching Characteristics table assume the part was not in a low-power shutdown state. Enable times t_ZH(SHDN) and t_ZL(SHDN) assume the parts were shut down. It takes drivers and receivers longer to become enabled from low-power shutdown mode t_ZH(SHDN), t_ZL(SHDN) than from driver/receiver-disable mode (t_ZH, t_ZL). Driver Output Protection Two mechanisms prevent excessive output current and power dissipation caused by faults or by bus con- tention. The first, a foldback current limit on the output stage, provides immediate protection against short cir- cuits over the whole common-mode voltage range (see Typical Operating Characteristics). The second, a ther- mal shutdown circuit, forces the driver outputs into a high-impedance state if the die temperature becomes excessive—typically around +150°C. Applications Information Capacitor and Grounding Resistor Selection The value for the charge-pump capacitors C1 and C2 should be between 47nF and 100nF. Smaller values will result in insufficient supply voltage on the isolated side. Larger values are allowed but will not result in better charge-pump capacity. The values for C1 and C2, as well as that of C4, determine the maximum frequency and amplitude of the voltage difference (under operat- ing conditions) between the local and isolated ground. Besides the capacitor values, this maximum frequency |
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