ISL59442_07 Datasheet, PDF (9/11 Pages) Intersil Corporation – 1GHz, 4 x 1 Multiplexing Amplifier

ISL59442
Application Information
General
The ISL59442 is a 4:1 mux that is ideal as a matrix element in
high performance switchers and routers. The ISL59442 is
optimized to drive a 2pF in parallel with a 500Ω load. The
capacitance can be split between the PCB capacitance an and
external load capacitance. Their low input capacitance and high
input resistance provide excellent 50Ω or 75Ω terminations.
Capacitance at the Output
The output amplifier is optimized for capacitance to ground
(C_L) 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 (C_T) and an external capacitor
(C_OUT). A good method to maintain control over the output
pin capacitance is to minimize the trace length (C_T) to the
next component, and include a discrete surface mount
capacitor (C_OUT) directly at the output pin.
For large signal applications where overshoot is important
the circuit in Figure 24B should be used. The series resistor
(R_S) and capacitor (C_L) form a low pass network that limits
system bandwidth and reduces overshoot. The component
values shown result in a typical pulse response shown in
Figure 20.
Ground Connections
For the best isolation and crosstalk rejection, the GND pin
and NIC pins must connect to the GND plane. The NIC pins
are placed on both sides of the input pins. These pins are
not internally connected to the die. It is recommended this
pin be tied to ground to minimize crosstalk.
Control Signals
S0, S1, HIZ - These pins are, TTL/CMOS compatible control
inputs. The S0, S1 pins select which one of the inputs
connect to the output. The HIZ pin is used to three-state the
output amplifiers. For control signal rise and fall times less
than 10nsec 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 25) 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 30ns by placing a
logic high (>2V) on the HIZ pin. If the HIZ state is selected, the
output is a high impedance 1.4MΩ. 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.
Limiting the Output Current
No output short circuit current limit exists on these parts. All
applications need to limit the output current to less than 50mA.
Adequate thermal heat sinking of the parts is also required.
V+ SUPPLY
LOGIC
POWER
GND
SIGNAL
DE-COUPLING
CAPS
V- SUPPLY
SCHOTTKY
PROTECTION
V+
S0
GND ^V- V+
IN0
V+
V-
IN1
V-
V+
LOGIC
CONTROL
V-
V+
OUT
V-
FIGURE 25. SCHOTTKY PROTECTION CIRCUIT
9
EXTERNAL
CIRCUITS
FN7452.4
January 5, 2007