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MAX3222EWN Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX3222EWN Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 16 page 3.0V to 5.5V, Low-Power, up to 1Mbps, True RS-232 Transceivers Using Four 0.1µF External Capacitors 8 _______________________________________________________________________________________ RS-232 Receivers The receivers convert RS-232 signals to CMOS-logic out- put levels. The MAX3222/MAX3237/MAX3241 receivers have inverting three-state outputs. In shutdown, the receivers can be active or inactive (Table 1). The complementary outputs on the MAX3237 (R1OUTB) and the MAX3241 (R1OUTB, R2OUTB) are always active, regardless of the state of EN or SHDN. This allows for Ring Indicator applications without forward biasing other devices connected to the receiver outputs. This is ideal for systems where VCC is set to 0V in shutdown to accommodate peripherals, such as UARTs (Figure 2). MAX3222/MAX3237/MAX3241 Shutdown Mode Supply current falls to less than 1µA in shutdown mode (SHDN = low). When shut down, the device’s charge pumps are turned off, V+ is pulled down to VCC, V- is pulled to ground, and the transmitter outputs are dis- abled (high impedance). The time required to exit shut- down is typically 100µs, as shown in Figure 3. Connect SHDN to VCC if the shutdown mode is not used. SHDN has no effect on R_OUT or R_OUTB. MAX3222/MAX3237/MAX3241 Enable Control The inverting receiver outputs (R_OUT) are put into a high-impedance state when EN is high. The complemen- tary outputs R1OUTB and R2OUTB are always active, regardless of the state of EN and SHDN (Table 1). EN has no effect on T_OUT. __________Applications Information Capacitor Selection The capacitor type used for C1–C4 is not critical for proper operation; polarized or nonpolarized capacitors can be used. The charge pump requires 0.1µF capaci- tors for 3.3V operation. For other supply voltages, refer to Table 2 for required capacitor values. Do not use values lower than those listed in Table 2. Increasing the capaci- tor values (e.g., by a factor of 2) reduces ripple on the transmitter outputs and slightly reduces power consump- tion. C2, C3, and C4 can be increased without changing C1’s value. However, do not increase C1 without also increasing the values of C2, C3, and C4, to maintain the proper ratios (C1 to the other capacitors). When using the minimum required capacitor values, make sure the capacitor value does not degrade exces- sively with temperature. If in doubt, use capacitors with a higher nominal value. The capacitor’s equivalent series resistance (ESR), which usually rises at low tempera- tures, influences the amount of ripple on V+ and V-. MAX3237 MAX3241 T1OUT R1OUTB Tx 5k UART VCC T1IN LOGIC TRANSITION DETECTOR R1IN R1OUT EN = VCC SHDN = GND VCC TO µP Rx PREVIOUS RS-232 Tx UART PROTECTION DIODE PROTECTION DIODE SHDN = GND VCC VCC GND Rx 5k a) OLDER RS-232: POWERED-DOWN UART DRAWS CURRENT FROM ACTIVE RECEIVER OUTPUT IN SHUTDOWN. b) NEW MAX3237/MAX3241: EN SHUTS DOWN RECEIVER OUTPUTS (EXCEPT FOR B OUTPUTS), SO NO CURRENT FLOWS TO UART IN SHUTDOWN. B OUTPUTS INDICATE RECEIVER ACTIVITY DURING SHUTDOWN WITH EN HIGH. GND Figure 2. Detection of RS-232 Activity when the UART and Interface are Shut Down; Comparison of MAX3237/MAX3241 (b) with Previous Transceivers (a). |
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