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ISL54047 Datasheet(PDF) 8 Page - Intersil Corporation |
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ISL54047 Datasheet(HTML) 8 Page - Intersil Corporation |
8 / 12 page 8 FN6503.0 May 31, 2007 the input current below the threshold that produces permanent damage, and the sub-microamp input current produces an insignificant voltage drop during normal operation. This method is not acceptable for the signal path inputs. Adding a series resistor to the switch input defeats the purpose of using a low RON switch. Connecting schottky diodes to the signal pins as shown in Figure 8 will shunt the fault current to the supply or to ground thereby protecting the switch. These schottky diodes must be sized to handle the expected fault current. Power-Supply Considerations The ISL54047 construction is typical of most single supply CMOS analog switches, in that they have two supply pins: V+ and GND. V+ and GND drive the internal CMOS switches and set their analog voltage limits. Unlike switches with a 4V maximum supply voltage, the ISL54047 5.5V maximum supply voltage provides plenty of room for the 10% tolerance of 4.3V supplies, as well as room for overshoot and noise spikes. The minimum recommended supply voltage is 1.65V. It is important to note that the input signal range, switching times, and on-resistance degrade at lower supply voltages. Refer to the Electrical Specification Tables on page 5 and Typical Performance Curves on page 9 for details. V+ and GND also power the internal logic and level shifters. The level shifters convert the input logic levels to switched V+ and GND signals to drive the analog switch gate terminals. This family of switches cannot be operated with bipolar supplies, because the input switching point becomes negative in this configuration. Logic-Level Thresholds This switch family are 1.8V CMOS compatible (0.5V and 1.4V) over a supply range of 2.7V to 4.5V (see Figure 18). At 2.7V the VIL level is about 0.53V. This is still above the 1.8V CMOS guaranteed low output maximum level of 0.5V, but noise margin is reduced. The digital input stages draw supply current whenever the digital input voltage is not at one of the supply rails. Driving the digital input signals from GND to V+ with a fast transition time minimizes power dissipation. The ISL54047 has been designed to minimize the supply current whenever the digital input voltage is not driven to the supply rails (0V to V+). For example driving the device with 2.85V logic (0V to 2.85V) while operating with a 4.2V supply the device draws only 12μA of current (see Figure 16 for VIN = 2.85V). Frequency Performance In 50 Ω systems, the ISL54047 has a -3dB bandwidth of 27MHz (see Figure 21). The frequency response is very consistent over a wide V+ range, and for varying analog signal levels. An OFF switch acts like a capacitor and passes higher frequencies with less attenuation, resulting in signal feed through from a switch’s input to its output. Off Isolation is the resistance to this feed through, while Crosstalk indicates the amount of feed through from one switch to another. Figure 22 details the high Off Isolation and Crosstalk rejection provided by this part. At 100kHz, Off Isolation is about 102dB in 50 Ω systems, 118dB into 8Ω, and 124dB into 4Ω, decreasing approximately 20dB per decade as frequency increases. Higher load impedances decrease Off Isolation and Crosstalk rejection due to the voltage divider action of the switch OFF impedance and the load impedance. Leakage Considerations Reverse ESD protection diodes are internally connected between each analog-signal pin and both V+ and GND. One of these diodes conducts if any analog signal exceeds V+ or GND. Virtually all the analog leakage current comes from the ESD diodes to V+ or GND. Although the ESD diodes on a given signal pin are identical and therefore fairly well balanced, they are reverse biased differently. Each is biased by either V+ or GND and the analog signal. This means their leakages will vary as the signal varies. The difference in the two diode leakages to the V+ and GND pins constitutes the analog- signal-path leakage current. All analog leakage current flows between each pin and one of the supply terminals, not to the other switch terminal. This is why both sides of a given switch can show leakage currents of the same or opposite polarity. There is no connection between the analog signal paths and V+ or GND. FIGURE 8. OVERVOLTAGE PROTECTION GND VCOM VNX V+ INX OPTIONAL PROTECTION RESISTOR OPTIONAL SCHOTTKY DIODE OPTIONAL SCHOTTKY DIODE ISL54047 |
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