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MAX339 Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX339 Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 11 page 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. All pins require this protection during manufacturing, not just RS-232 inputs and outputs. Therefore, after PC board assembly, the Machine Model is less relevant to I/O ports. __________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 capacitors for 2.5V operation (Table 2). Do not use values smaller than those listed in Table 2. 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 capacitors). When using the minimum required capacitor values, make sure the capacitor value does not degrade excessively with temperature. If in doubt, use capacitors with a larger nominal value. The capacitor’s equivalent series resistance (ESR), which usually rises at low tem- peratures, influences the amount of ripple on V+ and V-. Power-Supply Decoupling In most circumstances, a 0.1µF bypass capacitor is adequate. In applications that are sensitive to power- supply noise, decouple VCC to ground with a capacitor of the same value as charge-pump capacitor C1. Connect bypass capacitors as close to the IC as possible. Transmitter Outputs when Exiting Shutdown Figure 2 shows two transmitter outputs when exiting shutdown mode. As they become active, the two transmitter outputs are shown going to opposite RS-232 levels (one transmitter input is high; the other is low). Each transmitter is loaded with 3k Ω in parallel with 1000pF. The transmitter outputs display no ringing or undesirable transients as they come out of shutdown. Note that the transmitters are enabled only when the magnitude of V- exceeds approximately 2.5V. 2.5V, ±15kV ESD-Protected RS-232 Transceivers for PDAs and Cell Phones 8 _______________________________________________________________________________________ CHARGE-CURRENT LIMIT RESISTOR DISCHARGE RESISTANCE STORAGE CAPACITOR Cs 150pF RC 50M Ω to 100MΩ RD 330 Ω HIGH- VOLTAGE DC SOURCE DEVICE UNDER TEST Figure 4a. IEC 1000-4-2 ESD Test Model tR = 0.7ns to 1ns 30ns 60ns t 100% 90% 10% I Figure 4b. IEC 1000-4-2 ESD Generator Current Waveform Table 2. Minimum Required Capacitor Values VCC (V) C1–C4 (µF) 2.5 to 3.0 0.1 |
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