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MAX3250 Datasheet(PDF) 7 Page - Maxim Integrated Products |
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MAX3250 Datasheet(HTML) 7 Page - Maxim Integrated Products |
7 / 10 page ±50V Isolated, 3.0V to 5.5V, 250kbps, 2 TX/2 RX, RS-232 Transceiver _______________________________________________________________________________________ 7 AC waveforms are rectified on the isolated side of the dual die to power its internal circuitry (ISOVCC). Capacitor C6 filters the output of the rectifier. See the Typical Operating Circuit. The power controller works in a dual power mode. Power is maximum when the isolated power supply is below its regulation point. Power is reduced when the isolated power supply is above its regulation point. Dual Charge-Pump Voltage Converter The RS-232 drivers are powered from a regulated dual charge pump that provides output voltages of +5.5V (dou- bling charge pump) and -5.5V (inverting charge pump) relative to ISOCOM over the 3.0V to 5.5V VCC range. The charge pumps are powered from ISOVCC and operate in a discontinuous mode. If the output voltages are less than 5.5V, the charge pumps are enabled. If the output voltages exceed 5.5V, the charge pumps are disabled. Each charge pump requires a flying capacitor (C3, C4) and a reservoir capacitor (C7, C8) to generate the V+ and V- supplies. RS-232 Transmitters The transmitters are inverting level translators that convert TTL/CMOS-logic levels to ±5.0V EIA/TIA-232-compliant levels. They guarantee a 250kbps data rate with worst- case loads of 3k Ω in parallel with 1000pF to ISOCOM. In shutdown, the transmitters are disabled and the outputs are forced into a high-impedance state. When powered off or shut down, the outputs can be driven up to ±12V relative to ISOCOM. The transmitter inputs do not have pullup resistors. All unused inputs should be connected to VCC or GND. RS-232 Receivers The receivers convert RS-232 signals to CMOS-logic output levels. The receivers’ outputs are forced into a high-impedance state when the device is in shutdown. This allows a single UART to multiplex between differ- ent protocols. Low-Power Shutdown Mode Shutdown mode is obtained by driving SHDN low. In shutdown, the devices typically draw only 20µA of sup- ply current and no power is transferred across the iso- lation capacitors. The charge pumps are disabled, and the receiver outputs and transmitter outputs are high impedance. When exiting shutdown the charge pumps and transmitter outputs are fully operational in typically 500µs (Figure 3). Connect SHDN to VCC if the shut- down mode is not used. Applications Information Power Isolation Capacitors The values for capacitors C1 and C2 are important for proper operation of the device. These capacitors should be 0.047µF for 4.5V to 5.5V operation, and 0.47µF for 3.0V to 3.6V operation. Smaller values result in insufficient supply voltage on the isolated side. Larger values are not allowed. Capacitor C9 provides an AC feedback path for proper controller operation. Connect C9 from ISOCOM to GND. The values for C1, C2, and C9 determine the maximum frequency and amplitude of the voltage difference between the local and isolated ground. Table 1 shows proper capacitance values. LOAD CURRENT PER TRANSMITTER (mA) 7 6 4 5 2 3 1 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 -6 08 VCC = 3.0V VOUT+ VOUT- T1 T2 VCC VOUT1+ VOUT2- Figure 2. Transmitter Output Voltage vs. Load Current per Transmitter 100 µs/div 2V/div T2OUT VCC = 3.3V SHDN 5V/div T1OUT Figure 3. Transmitter Outputs when Exiting Shutdown or Powering Up |
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