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MAX3229 Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX3229 Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 14 page MAX3228/MAX3229 feature a separate logic supply input (VL) that sets VOH for the receiver and INVALID outputs. The transmitter inputs (T_IN), FORCEON and FORCEOFF, are also referred to VL. This feature allows maximum flexibility in interfacing to different systems and logic levels. Connect VL to the system’s logic sup- ply voltage (+1.65V to +5.5V), and bypass it with a 0.1µF capacitor to GND. If the logic supply is the same as VCC, connect VL to VCC. Always enable VCC before enabling the VL supply. VCC must be greater than or equal to the VL supply. Software-Controlled Shutdown If direct software control is desired, connect FORCEOFF and FORCEON together to disable AutoShutdown. The microcontroller then drives FORCEOFF and FORCEON like a SHDN input, INVALID can be used to alert the microcontroller to indicate serial data activity. Applications Information Capacitor Selection The capacitor type used for C1–C4 is not critical for proper operation; either polarized or nonpolarized capacitors may be used. However, ceramic chip capacitors with an X7R or X5R dielectric work best. The charge pump requires 0.1µF capacitors for 3.3V opera- tion. For other supply voltages, refer to Table 4 for required capacitor values. Do not use values smaller than those listed in Table 4. Increasing the capacitor values (e.g., by a factor of 2) reduces ripple on the transmitter outputs and slightly reduces power con- sumption. 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 capac- itors). When using the minimum required capacitor values, make sure the capacitor value does not degrade excessively with temperature. If in doubt, use capaci- tors with a larger nominal value. The capacitor’s equiv- alent series resistance (ESR) usually rises at low temperatures and influences the amount of ripple on V+ and V-. Power-Supply Decoupling In most circumstances, a 0.1µF VCC bypass capacitor is adequate. In applications that are sensitive to power- supply noise, use a capacitor of the same value as the charge-pump capacitor C1. Connect bypass capaci- tors as close to the IC as possible. +2.5V to +5.5V RS-232 Transceivers in UCSP _______________________________________________________________________________________ 9 Table 3. Output Control Truth Table TRANSCEIVER STATUS FORCEON FORCEOFF RECEIVER STATUS INVALID Shutdown (AutoShutdown) Low High High-Z L Shutdown (Forced Off) X Low High-Z † Normal Operation (Forced On) High High Active † Normal Operation (AutoShutdown) Low High Active H X = Don’t care. † = INVALID output state is determined by R_IN input levels. VCC (V) C1, CBYPASS ( µF) C2, C3, C4 ( µF) 2.5 to 3.0 0.22 0.22 3.0 to 3.6 0.1 0.1 4.5 to 5.5 0.047 0.33 3.0 to 5.5 0.22 1 Table 4. Required Capacitor Values Figure 5. Transmitter Outputs Exiting Shutdown or Powering Up 5V/div FORCEON = FORCEOFF TOUT 0 2V/div 4 µs/div 0 |
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