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MAX221E データシート(PDF) 5 Page - Maxim Integrated Products |
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MAX221E データシート(HTML) 5 Page - Maxim Integrated Products |
5 / 12 page ±15kV ESD-Protected, +5V, 1µA, Single RS-232 Transceiver with AutoShutdown _______________________________________________________________________________________ 5 _______________Detailed Description Dual Charge-Pump Voltage Converter The MAX221E’s internal power supply consists of a dual charge pump that provides a positive output volt- age (doubling charge pump) and negative output volt- age (inverting charge pump) from a single +5V supply. The charge pumps operate in continuous mode. Each charge pump requires a flying capacitor (C1, C2) and a reservoir capacitor (C3, C4) to generate the V+ and V- supplies. RS-232 Transmitter The transmitter is an inverting level translator that con- verts CMOS-logic levels to 5.0V EIA/TIA-232 levels. It guarantees a 250kbps data rate with worst-case loads of 3k Ω in parallel with 1000pF. When FORCEOFF is driven to ground, or when the AutoShutdown circuitry senses invalid voltage levels on the receiver input, the transmitter is disabled and the output is forced into a high-impedance state. The trans- mitter input does not have a pull-up resistor. RS-232 Receiver The MAX221E’s receiver converts RS-232 signals to CMOS-logic output levels. The receiver has an inverting three-state output and can be active or inactive. In shutdown (FORCEOFF = low) or in AutoShutdown, the receiver is active (Table 1). Drive EN high to place the receiver in a high-impedance state. The receiver is high-impedance when the MAX221E is in shutdown (FORCEOFF = low). The MAX221E’s INVALID output is pulled low when there is no valid RS-232 signal level detected on the receiver input. INVALID is functional in any mode (Figures 2 and 3). AutoShutdown The MAX221E achieves 1µA supply current with Maxim’s AutoShutdown feature, which operates when FORCEON is low and FORCEOFF is high. When the device senses no valid signal levels on the receiver input for 30µs, the on-board charge pump and driver are shut off, reducing supply current to 1µA. This occurs if the RS-232 cable is disconnected or the con- nected peripheral transmitter is turned off. The MAX221E turns on again when a valid level is applied to the RS-232 receiver input. As a result, the system saves power without changes to the existing BIOS or operating system. Table 1 and Figure 2c summarize the MAX221E operat- ing modes. FORCEON and FORCEOFF override AutoShutdown. When neither control is asserted, the device selects between these states automatically, based on the receiver input level. Figures 2a, 2b, and 3a depict valid and invalid RS-232 receiver levels. Figure 3 shows the input levels and timing diagram for AutoShutdown operation. A device or another system with AutoShutdown con- nected to the MAX221E may need time to wake up. Figure 4 shows a circuit that forces the transmitter on for 100ms, allowing enough time for the other system to realize that the MAX221E is awake. If the other system transmits valid RS-232 signals within that time, the RS-232 ports on both systems remain enabled. When shut down, the device’s charge pumps turn off, V+ is pulled to VCC, V- is pulled to ground, and the transmitter output is high impedance. The time required to exit shutdown is typically 100µs (Figure 3b). MAX221E FORCEON EN 12 1 ROUT ISHDN 9 FORCEOFF 16 INVALID 10 GND 14 TIN 11 C2- 6 C2+ 5 C1- 4 C1+ 2 RIN 8 TOUT 13 V- 7 V+ 3 VCC C4 0.1 µF 15 C1 0.1 µF C2 0.1 µF CBYPASS 0.1 µF +5V TO POWER- MANAGEMENT UNIT 5k 3k C3 0.1 µF Figure 1. Shutdown Current Test Circuit |
同様の部品番号 - MAX221E |
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同様の説明 - MAX221E |
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