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ISL83220E Datasheet(PDF) 5 Page - Intersil Corporation |
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ISL83220E Datasheet(HTML) 5 Page - Intersil Corporation |
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5 / 11 page 
5 Receivers The ISL83220E device contains a standard inverting receiver that three-states via the EN control line. Receivers convert RS-232 signals to CMOS output levels and accept inputs up to ±25V while presenting the required 3kΩ to 7kΩ input impedance (see Figure 1) even if the power is off (VCC = 0V). The receiver’s Schmitt trigger input stage uses hysteresis to increase noise immunity and decrease errors due to slow input signal transitions. The ISL83220E receiver disables only when EN is driven high. (see Table 2). This allows the receiver to monitor external devices, like a modem, even when the ISL83220E is in its 1 µA powerdown state. Standard receivers driving powered down peripherals must be disabled to prevent current flow through the peripheral’s protection diodes (see Figures 2). This renders them useless for wake up functions. Operation down to 2.7V ISL83220E transmitter outputs meet RS-562 levels ( ±3.7V) with VCC as low as 2.7V. RS-562 levels typically ensure inter operability with RS-232 devices. 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), which 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. This micro-power mode makes these devices ideal for battery powered and portable applications. Software Controlled (Manual) Powerdown On the ISL83220E, the powerdown control is via a simple shutdown (SHDN) pin. Driving this pin high enables normal operation, while driving it low forces the IC into it’s powerdown state. Connect SHDN to VCC if the powerdown function isn’t needed. Note that the receiver output remains enabled during shutdown (see Table 2). For the lowest power consumption during powerdown, the receiver should also be disabled by driving the EN input high (see next section). The time to recover from manual powerdown mode is typically 100 µs. Receiver ENABLE Control The ISL83220E also features an EN input to control the receiver output. Driving EN high disables the receiver output placing it in a high impedance state. This is useful to eliminate supply current, due to a receiver output forward biasing the protection diode, when driving the input of a powered down (VCC = GND) peripheral (see Figure 2). Capacitor Selection The charge pumps require 0.1 µF capacitors for 3.3V operation. Do not use values smaller than 0.1 µF. 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 -. R1OUT GND ≤ VROUT ≤ VCC 5k Ω R1IN -25V ≤ VRIN ≤ +25V GND VCC FIGURE 1. INVERTING RECEIVER CONNECTIONS TABLE 2. POWERDOWN AND ENABLE LOGIC TRUTH TABLE SHDN INPUT EN INPUT TRANSMITTER OUTPUT RECEIVER OUTPUT MODE OF OPERATION L L High-Z Active Manual Powerdown L H High-Z High-Z Manual Powerdown w/Rcvr. Disabled H L Active Active Normal Operation H H Active High-Z Normal Operation w/Rcvr. Disabled 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 ISL83220E |
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