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ISL4223EIR Scheda tecnica(PDF) 8 Page - Intersil Corporation |
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ISL4223EIR Scheda tecnica(HTML) 8 Page - Intersil Corporation |
8 / 13 page 8 FN6045.2 April 13, 2007 from initiating automatic powerdown for 100ms after powering up. This gives the slow-to-wake peripheral circuit time to reestablish valid RS-232 output levels. The time to recover from automatic powerdown mode is typically 100 μs. INVALID Output The INVALID output always indicates whether or not a valid RS-232 signal (see Figure 6) is present at any of the receiver inputs (see Table 2), 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 by the internal receiver pull down resistors), the INVALID logic detects the invalid levels and drives the output low. The power management logic then uses this indicator to power down the interface block. Reconnecting the cable restores valid levels at the receiver inputs, INVALID switches high, and the power management logic wakes up the interface block. INVALID can also be used to indicate the DTR or RING INDICATOR signal, as long as the other receiver inputs are floating, or driven to GND (as in the case of a powered down driver). INVALID switches low after invalid levels have persisted on all of the receiver inputs for more than 30 μs (see Figure 7). INVALID switches back high 1 μs after detecting a valid RS-232 level on a receiver input. INVALID operates in all modes (forced or automatic powerdown, or forced on), so it is also useful for systems employing manual powerdown circuitry. When automatic powerdown is utilized, INVALID = 0 indicates that the ISL422XE is in powerdown mode. Capacitor Selection The charge pumps require 0.1 μF, or greater, capacitors for proper operation. Increasing the capacitor values (by a factor of 2) reduces ripple on the transmitter outputs and slightly reduces power consumption. When using minimum required capacitor values, make sure that capacitor values do not degrade excessively with temperature. If in doubt, use capacitors with a larger nominal value. The capacitor’s equivalent series resistance (ESR) usually rises at low temperatures and it influences the amount of ripple on V+ and V- . Power Supply Decoupling In most circumstances a 0.1 μF bypass capacitor is adequate. In applications that are particularly sensitive to power supply noise, decouple VCC to ground with a capacitor of the same value as the charge-pump capacitor C1. Connect the bypass capacitor as close as possible to the IC. Transmitter Outputs when Exiting Powerdown Figure 8 shows the response of two transmitter outputs when exiting powerdown mode. As they activate, the two transmitter outputs properly go to opposite RS-232 levels, with no glitching, ringing, nor undesirable transients. Each transmitter is loaded with 3k Ω in parallel with 2500pF. Note that the transmitters enable only when the magnitude of the supplies exceed approximately 3V. FIGURE 6. DEFINITION OF VALID RS-232 RECEIVER LEVELS 0.3V -0.3V -2.7V 2.7V INVALID LEVEL - POWERDOWN OCCURS AFTER 30 μs VALID RS-232 LEVEL - ISL422XE IS ACTIVE VALID RS-232 LEVEL - ISL422XE IS ACTIVE INDETERMINATE - POWERDOWN MAY OR INDETERMINATE - POWERDOWN MAY OR MAY NOT OCCUR MAY NOT OCCUR RECEIVER INPUTS TRANSMITTER OUTPUTS INVALID OUTPUT V+ VCC 0 V- VCC 0 tINVL tINVH INVALID REGION } FIGURE 7. AUTOMATIC POWERDOWN AND INVALID TIMING DIAGRAMS AUTOPWDN PWR UP ISL4221E, ISL4223E |
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