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ISL4244EIRZ Datasheet(PDF) 10 Page - Intersil Corporation |
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ISL4244EIRZ Datasheet(HTML) 10 Page - Intersil Corporation |
10 / 17 page 10 Powerdown Functionality The already low current requirement drops significantly when the device enters powerdown mode. In powerdown, supply current drops to 10nA, because the on-chip charge pump turns off (V+ collapses to VCC, V- collapses to GND), and the transmitter outputs tristate. Inverting receiver outputs disable only in manual 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. ISL4238E/4XE inverting (standard) receiver outputs also disable when the device is in manual 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 ISL4238E/4XE 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 ISL4238E/4XE powers up as soon as FORCEOFF switches high. The rising edge of the Master Powerdown signal forces the device to power up, and the ISL4238E/4XE returns to enhanced automatic powerdown mode an RC time constant after this rising edge. The time constant isn’t critical, because the ISL4238E/4XE 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. INVALID Output The INVALID output always indicates (see Table 2) whether or not 30 µs have elapsed with invalid RS-232 signals (see Figures 6 and 9) persisting on all of the receiver inputs, giving the user an easy way to determine when the interface block should power down. Invalid receiver levels occur whenever the driving peripheral’s outputs are shut off (powered down) or when the RS-232 interface cable is disconnected. In the case of a disconnected interface cable where all the receiver inputs are floating (but pulled to GND FIGURE 3. DISABLED RECEIVERS PREVENT POWER DRAIN ISL4238E/4XE TRANSITION RX TX ROUTB ROUT T1IN FORCEOFF = GND VCC VCC TO RIN 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 ISL4238E/4XE MGT LOGIC UART FIGURE 5. CIRCUIT TO ENSURE IMMEDIATE POWER UP WHEN EXITING FORCED POWERDOWN FORCEOFF FORCEON POWER MASTER POWERDOWN LINE 1M Ω 0.1 µF MANAGEMENT UNIT ISL4238E/4XE ISL4238E, ISL4244E, ISL4245E |
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