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MAX3250 Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX3250 Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 10 page ±50V Isolated, 3.0V to 5.5V, 250kbps, 2 TX/2 RX, RS-232 Transceiver 8 _______________________________________________________________________________________ To achieve full isolation capability, C1, C2, and C9 should be rated for 100V or higher operation and be X7R or X5R type or metalized film dielectric. Y5V and Z5U dielectrics should be avoided as their voltage and tem- perature coefficients make their power-transfer capabili- ties insufficient. Charge-Pump and Bypass Capacitors Capacitors C3–C8 should be X7R or X5R type dielec- tric. Their voltage rating needs to be 10V or higher. Layout Information Because the MAX3250 is intended for systems requir- ing ±50V isolation, some consideration in component placement is necessary. A 20mil air gap should isolate the logic side and the isolated RS-232 side, across the N.C. pins (pin num- bers 6, 7, 22, and 23) of the MAX3250. The only com- ponents that cross this air gap should be C1, C2, and C9, which should all have a minimum 100V rating. All capacitors should be located as close to the MAX3250 as possible. Maximum Voltage Between ISOCOM and Logic GND High values of applied isolation voltage and frequency can cause ripple on ISOVCC, V+, V-, and in extreme cases on VCC. Therefore, it is recommended that the isolation-voltage and frequency be limited to the values shown in the Typical Operating Characteristics. Insert a 1k Ω 1/4W resistor in series with any isolation test voltage when testing for maximum values of applied isolation voltage. Exceeding the maximum lim- its of voltage and frequency (see the Typical Operating Characteristics) could trigger a holding current in the internal ESD-protection device if the ±80V isolation limit is exceeded. This resistor should not be used in normal application. Transmitter Outputs when Exiting Shutdown Figure 3 shows two transmitter outputs when exiting shutdown mode. As they become active, the two trans- mitter outputs are shown going to opposite RS-232 lev- els (one transmitter input is high, the other is low). Each transmitter is loaded with 3k Ω in parallel with 2500pF. The transmitter outputs display no ringing or undesir- able transients as they come out of shutdown. Note that the transmitters are enabled only when the magnitude of V- exceeds approximately -3V. VCC (V) C1, C2 (µF) C3 (µF) C4, C7, C8 (µF) C5, C6 (µF) C9 (nF) 3.0 to 3.6 0.47 0.1 0.47 1 10 4.5 to 5.5 0.047 0.1 0.47 1 10 Table 1. Required Capacitor Values |
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