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ICL3244EIA Datasheet(PDF) 10 Page - Intersil Corporation |
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ICL3244EIA Datasheet(HTML) 10 Page - Intersil Corporation |
10 / 20 page 10 Powerdown Functionality This 3V family of RS-232 interface devices requires a nominal supply current of 0.3mA during normal operation (not in powerdown mode). This is considerably less than the 5mA to 11mA current required of 5V RS-232 devices. The already low current requirement drops significantly when the device enters powerdown mode. In powerdown, supply current drops to 1 µA, because the on-chip charge pump turns off (V+ collapses to VCC, V- collapses to GND), and the transmitter outputs three-state. Inverting receiver outputs may or may not disable in powerdown; refer to Table 2 for details. This micro-power mode makes these devices ideal for battery powered and portable applications. Software Controlled (Manual) Powerdown These devices allow the user to force the IC into the low power, standby state, and utilize a two pin approach where the FORCEON and FORCEOFF inputs determine the IC’s mode. For always enabled operation, FORCEON and FORCEOFF are both strapped high. To switch between active and powerdown modes, under logic or software control, only the FORCEOFF input need be driven. The FORCEON state isn’t critical, as FORCEOFF dominates over FORCEON. Nevertheless, if strictly manual control over powerdown is desired, the user must strap FORCEON high to disable the enhanced automatic powerdown circuitry. ICL3244E inverting (standard) receiver outputs also disable when the device is in powerdown, thereby eliminating the possible current path through a shutdown peripheral’s input protection diode (see Figures 2 and 3). Connecting FORCEOFF and FORCEON together disables the enhanced automatic powerdown feature, enabling them to function as a manual SHUTDOWN input (see Figure 4). With any of the above control schemes, the time required to exit powerdown, and resume transmission is only 100 µs. When using both manual and enhanced automatic powerdown (FORCEON = 0), the ICL32XXE won’t power up from manual powerdown until both FORCEOFF and FORCEON are driven high, or until a transition occurs on a receiver or transmitter input. Figure 5 illustrates a circuit for ensuring that the ICL32XXE powers up as soon as FORCEOFF switches high. The rising edge of the Master Powerdown signal forces the device to power up, and the ICL32XXE returns to enhanced automatic powerdown mode an RC time constant after this rising edge. The time constant isn’t critical, because the ICL32XXE remains powered up for 30 seconds after the FORCEON falling edge, even if there are no signal transitions. This gives slow-to-wake systems (e.g., a mouse) plenty of time to start transmitting, and as long as it starts transmitting within 30 seconds both systems remain enabled. FIGURE 2. POWER DRAIN THROUGH POWERED DOWN PERIPHERAL OLD VCC POWERED GND SHDN = GND VCC Rx Tx VCC CURRENT VOUT = VCC FLOW RS-232 CHIP DOWN UART FIGURE 3. DISABLED RECEIVERS PREVENT POWER DRAIN ICL3244E TRANSITION RX TX R2OUTB R2OUT T1IN FORCEOFF = GND VCC VCC TO R2IN T1OUT VOUT = HI-Z POWERED DETECTOR DOWN UART WAKE-UP LOGIC FIGURE 4. CONNECTIONS FOR MANUAL POWERDOWN WHEN NO VALID RECEIVER SIGNALS ARE PRESENT PWR FORCEOFF INVALID CPU I/O FORCEON ICL32XXE MGT LOGIC UART ICL3224E, ICL3226E, ICL3244E |
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