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ISL59420IUZ-T7 Datasheet(PDF) 10 Page - Intersil Corporation |
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ISL59420IUZ-T7 Datasheet(HTML) 10 Page - Intersil Corporation |
10 / 13 page ISL59420 10 FN7459.2 July 3, 2012 Application Information General The ISL59420 is a 2:1 mux that is ideal as a matrix element in high performance switchers and routers. The ISL59420 is optimized to drive 5pF in parallel with a 500 Ω load. The capacitance can be split between the PCB capacitance and an external load capacitance. Its low input capacitance and high input resistance provide excellent 50 Ω or 75Ω terminations. Parasitic Effects on Frequency Performance Capacitance at the Inverting Input The AC performance of current-feedback amplifiers in the non-inverting gain configuration is strongly affected by stray capacitance at the inverting input. Stray capacitance from the inverting input pin to the output (CF), and to ground (CG), increase gain peaking and bandwidth. Large values of either capacitance can cause oscillation. The ISL59420 has been optimized for a 0.4pF to 0.7pF capacitance (CG). Capacitance (CF) to the output should be minimized. To achieve optimum performance the feedback network resistor(s) must be placed as close to the device as possible. Trace lengths greater than 1/4 inch combined with resistor pad capacitance can result in inverting input to ground capacitance approaching 1pF. Inverting input and output traces should not run parallel to each other. Small size surface mount resistors (604 or smaller) are recommended. Capacitance at the Output The output amplifier is optimized for capacitance to ground (CL) directly on the output pin. Increased capacitance causes higher peaking with an increase in bandwidth. The optimum range for most applications is ~1.0pF to ~6pF. The optimum value can be achieved through a combination of PC board trace capacitance (CT) and an external capacitor (COUT). A good method to maintain control over the output pin capacitance is to minimize the trace length (CT) to the next component, and include a discrete surface mount capacitor (COUT) directly at the output pin. Feedback Resistor Values The AC performance of the output amplifier is optimized with the feedback resistor network (RF, RG) values recommended in the application circuits. The amplifier bandwidth and gain peaking are directly affected by the value(s) of the feedback resistor(s) in unity gain and gain >1 configurations. Transient response performance can be tailored simply by changing these resistor values. Generally, lower values of RF and RG increase bandwidth and gain peaking. This has the effect of decreasing rise/fall times and increasing overshoot. Ground Connections For the best isolation and crosstalk rejection, the GND pin and NIC pins must connect to the GND plane. Control Signals S0, ENABLE, HIZ - These pins are TTL/CMOS compatible control inputs. The S0 pin selects which one of the inputs connect to the output. The ENABLE, HIZ pins are used to disable the part to save power and three-state the output amplifiers, respectively. For control signal rise and fall times less than 10ns the use of termination resistors close to the part will minimize transients coupled to the output. Power-Up Considerations The ESD protection circuits use internal diodes from all pins the V+ and V- supplies. In addition, a dV/dT- triggered clamp is connected between the V+ and V- pins, as shown in the Equivalent Circuits 1 through 4 section of the Pin Description table. The dV/dT triggered clamp imposes a maximum supply turn-on slew rate of 1V/µs. Damaging currents can flow for power supply rates-of-rise in excess of 1V/µs, such as during hot plugging. Under these conditions, additional methods should be employed to ensure the rate of rise is not exceeded. Consideration must be given to the order in which power is applied to the V+ and V- pins, as well as analog and logic input pins. Schottky diodes (Motorola MBR0550T or equivalent) connected from V+ to ground and V- to ground (Figure 24) will shunt damaging currents away from the internal V+ and V- ESD diodes in the event that the V+ supply is applied to the device before the V- supply. If positive voltages are applied to the logic or analog video input pins before V+ is applied, current will flow through the internal ESD diodes to the V+ pin. The presence of large decoupling capacitors and the loading effect of other circuits connected to V+, can result in damaging currents through the ESD diodes and other active circuits within the device. Therefore, adequate current limiting on the digital and analog inputs is needed to prevent damage during the time the voltages on these inputs are more positive than V+. HIZ State An internal pull-down resistor connected to the HIZ pin ensures the device will be active with no connection to the HIZ pin. The HIZ state is established within approximately 25ns (Figure 19) by placing a logic high (>2V) on the HIZ pin. If the HIZ state is selected, the output is a high impedance 1.4M Ω. Use this state to control the logic when more than one mux shares a common output. In the HIZ state the output is three-stated, and maintains its high Z even in the presence of high slew rates. The supply current during this state is basically the same as the active state. ENABLE & Power Down States The enable pin is active low. An internal pull-down resistor ensures the device will be active with no connection to the ENABLE pin. The Power Down state is established when a logic high (>2V) is placed on the ENABLE pin. In the Power Down state, the output has no leakage but has a large capacitance (on the order of 15pF), and is capable of being back-driven. Under this condition, large incoming slew rates can cause fault currents of tens of mA. Do not use this state as a high Z state for applications driving more than one mux on a common output. |
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