Quickswitch Basics - Integrated Device Technology, Inc. (IDT)

QUICKSWITCH GENERAL INFORMATION
QUICKSWITCH® GENERAL INFORMATION
QUICKSWITCH BASICS AND APPLICATIONS
16
INTRODUCTION
14
12
The QuickSwitch family of FET switches was pioneered in 1990 to offer
designers products for high-speed bus connection and logic replacement
with sub-nano-second propagation delay, zero added noise and timing
skew, and no additional static power dissipation. Since their introduction,
QuickSwitch products have gained acceptance as ideal devices for hotdocking, voltage translation, capacitance isolation, logic replacement, clock
gating, and several other applications.
RON
5Ω
10
8
6
4
2
0
0
WHAT IS A QUICKSWITCH?
3
4
VCC - 1
Figure 2. QuickSwitch ON Resistance vs. VIN
two switch terminals can be regarded as the input, the other being the output.
A plot of the ON resistance of a QuickSwitch is shown in Figure 2.
A typical QuickSwitch device has an ON resistance of less than 5Ω for
input voltages near Ground. The resistance rises as the switch input
voltage rises. At the TTL High voltage of 2.4V at the switch inputs, the ON
resistance is typically 10Ω. As the switch input voltage rises towards VCC,
the switch exhibits higher ON resistance. At input voltages approaching
approximately VCC –1, the switch enters cut-off region.
5Ω when on
OFF / ON
2
VIN
In its basic form, a QuickSwitch is an N-channel FET switch
controlled by either combinatorial or sequential control logic using
CMOS technology. The low ON resistance (typically 5Ω), low
capacitance, high current capacity, and very high OFF resistance,
make the FET switch an ideal element for bus connection. Figure 1
shows the basic QuickSwitch configuration.
A
1
B
0 / VCC
The input range also extends to 0.5V below ground. Below this voltage,
the clamp diode connected to the switch terminal begins to draw current.
Figure 1. Basic QuickSwitch
QUICKSWITCH WITH SERIES RESISTOR OPTION
When the switch is enabled, the gate of the N-channel switch transistor
driven by a CMOS logic gate is at VCC and the switch exhibits a typical ON
resistance of 5Ω. When disabled, the gate of the switch is at Ground potential
and the switch offers very high resistance between the A and B terminals.
In the OFF state, the leakage current at the switch terminals is typically 10nA,
and the capacitance between the terminals is low. Typical capacitance at
the switch terminals is 5pF in the OFF state. These properties make the
QuickSwitch an ideal device for unbuffered bus connection.
IDToffers most QuickSwitch functions with a series resistor option. These
switches have an integrated resistor of typically 23Ω value in series with the
switch. To obtain the ON resistance vs. VIN characteristics for a switch with
the resistor option, the reader should add 23Ω to the graph in Figure 2. The
series resistor serves as a source-termination resistor or a damping resistor
to reduce noise caused by signals with fast transitions.
QUICKSWITCH ON RESISTANCE
The QuickSwitch provides a low resistance connection between the input
and output over a wide input voltage range. Starting from VIN of approximately –0.5V, the output voltage equals the input voltage. This relationship
is maintained until the input voltage reaches approximately VCC –1V. For
input voltages between VCC –1 and VCC , the output voltage gets clipped to
approximately VCC –1 due to the ‘source-follower’ configuration of the Nchannel switch. The VIN vs. VOUT characteristics for three different output
loads is shown in Figure 3.
VOUT vs. VIN CHARACTERISTICS
The ON resistance of the QuickSwitch is determined by the size of the
switch FET and the voltage applied to the switch terminals. The ON
resistance decreases as the voltage between the switch gate and the switch
terminals increases. Therefore, the switch exhibits a somewhat nonlinear
ON resistance characteristics with respect to the voltage at its input terminal.
Note that the FET switch is essentially bidirectional. Therefore, either of the
The IDT logo is a registered trademark of Integrated Device Technology, Inc.
1
c /-
1999
Integrated Device Technology, Inc.
DSC-5241/1
QUICKSWITCH GENERAL INFORMATION
VOUT
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0.5
1KΩ
10MΩ
10KΩ
10KΩ
4.0
10MEGΩ
3.5
ΔV@10Ω
3.0
2.5
VOUT
2.0
1.5
1.0
0.0
1.0
2.0
3.0
4.0
1.0
5.0
0.5
VIN, Volts
0.0
3.0
Figure 3. Typical VOUT vs. VIN for Various Loads
4.0
4.5
5.0
Figure 5. VOUT and Voltage Drop vs.VCC at VIN = VCC
The voltage drop across the switch when VIN = VCC is substantially
constant for a wide range of supply voltage. The "clipping" action at high
input voltages and the negligible voltage drop across the switch at lower
input voltages makes the QuickSwitch an ideal device for voltage level
translation in mixed 5V/ 3.3V systems. By using the QuickSwitch at a
supply voltage of approximately 4.3V between a 5V bus and a 3.3 Volt
bus, 3.3V signals can be passed to the 5V side without attenuation, and
5V signals will be clipped to 3.3V to interface safely with 3.3V circuits
connected to the bus.
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
3.5
VCC
The switch develops a voltage drop across its terminals as a function of
the load resistance. Because of the non-linear nature of switch resistance,
the voltage drop is also non-linear as shown in Figure 4.
ΔV
ΔV@10MΩ
1KΩ
10KΩ
10MEG Ω
QUICKSWITCH OPERATION WITH TTL SIGNALS
0.5
0.0
0.5
1.0
1.5
2.0
VIN, Volts
2.5
3.0
3.5
One common application of the QuickSwitch is its use as a bus switch.
Some popular devices such as QS3245 are used for replacing FCT245
or ABT245 logic functions which interface between two TTL buses. In this
application the QuickSwitch performs bus connection and bus isolation with
sub-nanosecond propagation delay and zero added skew and ground
bounce.
4.0
Figure 4. Typical Voltage Drop vs. VIN
QUICKSWITCH VOUT vs. VCC
Figure 6 below shows the QuickSwitch as a bus switch driven by a TTL
driver at its input. The switch output is connected to CMOS inputs which offer
a capacitive load to the driver through the switch.
When the input voltage to the switch is at or near VCC, the output voltage
is approximately 1V below VCC due to the "source-follower" configuration
of the N-channel switch as described earlier. This voltage drop is process
dependent and may vary from 1V to 1.3V. Increasing or decreasing the VCC
will increase or decrease the output voltage by the same amount as shown
in Figure 5.
3.5V
3.5V
0
0
Input
TTL Driver
74FCT245
Figure 6. Bus Switch Operation
2
5
+5V
Output
QUICKSWITCH GENERAL INFORMATION
When the switch is closed, connecting the driver to its load, the gate of the
N-channel FET is at VCC and the switch transistor is fully ON. During the lowto-high TTL transitions at the switch input, the load capacitance will charge
via the low ON resistance of the switch. Typically, a 50pF load will charge
with a time-constant of 250ps through the 5Ω switch. During the high-to-low
transitions, the load capacitance will discharge with the same time-constant.
Since the time-constant is much less than the rise or fall time of the driver,
the signal transition at the load is determined by the driver, and not by the
switch. To a first approximation, the switch adds zero propagation delay. If
the signal transitions at the switch input are TTL compatible, i.e. from 0 to
3.5V, these transitions are faithfully reproduced at the output with no
additional noise.
When the switch is open, disconnecting the driver from the load, the
gate of the N-channel FET is at 0V, and the switch transistor is OFF. The
impedance between the switch terminals is extremely high. The parasitic
capacitance at the switch terminals offers low AC impedance, thus
minimizing signal feed-through between the switch terminals in the OFF
state.
When the QuickSwitch is either powered down or is in the disabled
state, there is no DC path from either switch terminal to either ground or
VCC for voltages above –0.5V. This feature makes the QuickSwitch ideal
as an interface device for isolating system components during hot
docking (live insertion).
The bus switch can replace drivers and transceivers in systems if bus
repowering is not required. It provides no drive of its own, but relies on the
driver connected to its input. For moderately capacitive loading, the
QuickSwitch provides a net gain in speed because of the sub-nanosecond
propagation delay through the switch, which is much smaller than the
propagation delay through a buffer or a transceiver and the additional
derating for the load capacitance.
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QUICKSWITCH GENERAL INFORMATION
QUICKSWITCH RATINGS AND SPECIFICATIONS
INTRODUCTION
The DC Switch Current per pin defines the maximum current that
can be drawn through the switch in ON state to pull down an external
load to ground.
In this section, the ratings, AC and DC specifications, and test method for
various parameters are discussed in order to provide information and
clarification to the user.
The Power Dissipation defines the maximum total power dissipation
capability of the device. It represents the package power dissipation limit
based on the thermal time constant for the package/die combination and the
maximum permissible chip junction temperature. The total power dissipation
is the sum of static and dynamic dissipation components.
ABSOLUTE MAXIMUM RATINGS
The absolute maximum continuous ratings are those values beyond
which permanent and irreversible damage to the device may occur.
Exposure to these conditions or conditions beyond those indicated may
adversely affect device reliability. Functional operation under absolute
maximum conditions is not implied.
The Storage Temperature rating defines the extremes of temperature
range that a package may be subjected to for long term storage, even the
power being applied to the device.
The Supply Voltage rating defines the absolute maximum VCC that can
be safely applied to the device. This rating is consistent with that of other TTL
logic devices. The Supply Voltage rating is determined by the breakdown
limits of internal components. If the VCC is taken beyond the rated limit,
excessive current may be drawn from the power supply due to device
breakdown and permanent damage may occur due to excessive heat
generated. As device dimensions are reduced to improve circuit performance, the breakdown voltage also decreases. As a rule of thumb, the
absolute maximum Supply Voltage rating is normally 40% above the
nominal operating supply voltage. For 5V circuits, the absolute maximum
supply voltage is 7V. It is 4.6V for circuits operating at a nominal 3.3V supply.
It is a prudent practice to limit even the transient supply voltage rise to the
absolute maximum rating.
INPUT AND SWITCH CAPACITANCE
Each control input pin and switch pin has a capacitance associated with
it. The capacitance at a control input is due to the package and the input
circuitry connected to the pin. It is package dependent and typically ranges
from 3pF to 5pF.
The switch pins have two capacitance values—one for the OFF state and
one for the ON state, as shown in Figure 7. In the OFF state, each switch
pin is isolated and has a capacitance to ground. The capacitance in this state
is the sum of pin capacitance and the total capacitance associated with the
switches connected to the pin. For example, an individual switch in the
QS3384 device will have a lower capacitance than the Mux pin of QS3253
Dual 4:1 Mux/Demux.
The DC Input Voltage rating is the maximum voltage that can be
applied to the TTL control inputs of a QuickSwitch device. Since IDT’s
QuickSwitch devices are designed without a clamp diode to VCC to enable
hot insertion, the DC input voltage may exceed VCC provided it is less than
the rated voltage. This rating is also determined by the breakdown
characteristics of internal components.
C1
The DC Switch Voltage is maximum voltage that can be applied to the
switch terminals. For all devices which use an N-channel FET switch, the
rated switch voltage is the same as the rated input voltage. For devices
that use N-FET and P-FET combination, the absolute maximum switch
voltage is limited to 0.5V above VCC to prevent damage to the circuit due
to excessive current flow through the parasitic diode associated with the
P-channel FET.
C2
Figure 7. Switch Capacitance
In the ON state, the low impedance switch connects both ends of the switch,
and the capacitance seen at one pin is the sum of the capacitances at the
two pins. If multiple switches connected to a pin are ON at the same time (i.e.
in the case of Crossbar products), the capacitance seen at the pin is the sum
of all pin capacitances connected by the ON switches.
The AC Input Voltage applies to the transient voltage (for example,
voltage undershoot) at the TTL control input or switch input. It states that
pulses of up to –3V can be tolerated for a duration less than 20ns without
damaging the device. However, large undershoots can cause significant
clamp current and local heating. If the transient pulses have a high duty
cycle, the average power dissipation must be taken into account to ensure
that the average DC current and power dissipation do not exceed the
rated values.
DC ELECTRICAL CHARACTERISTICS
The DC electrical characteristics define the input operating conditions
for proper operation, responses to applied DC signals, and Switch
characteristics over specified voltage and temperature range. Limits of all
DC electrical characteristics are guaranteed over the recommended
operating temperature and power supply range as stated in the datasheets for individual products.
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QUICKSWITCH GENERAL INFORMATION
VIH and VIL define the limits of guaranteed logic HIGH and logic LOW
recognition levels at the control inputs. For example, V IH limit of 2.0V
implies that any voltage greater than 2.0V shall be recognized by the
input circuit as a logic HIGH. Similarly, VIL limit of 0.8V implies that any
voltage less than 0.8V shall be recognized as logic LOW. Typical applied
input voltages are between 3.5V and V CC for logic HIGH and between
0V and 0.5V for logic LOW, thus assuring adequate noise margin. Note
that the AC characteristics are guaranteed for input signal swing of 0V
to 3.0V.
ICCQ is the Quiescent (static) power supply current when all control inputs
are at logic LOW or HIGH levels and the switches are OFF. This current
represents the leakage current between the VCC and Ground pins of a
QuickSwitch device. It will also include DC current through the device due
to a resistive path from VCC to Ground, if applicable.
IIN defines the input leakage current at the control inputs of a QuickSwitch.
The typical input leakage current is of the order of 1 to 5nA at room
temperature.
QCCD is the dynamic power supply current component expressed in terms
of mA or mA per MHz. This current is measured with switch pins open and
switching the control inputs with 50% duty cycle, so that it measures only the
current required to switch the internal nodes of the circuit. Note that measured
value of dynamic supply current at a given frequency includes the ICCQ
component which needs to be subtracted to obtain the QCCD value.
ΔICC is the power supply current component per each TTL control input
when the input is in logic HIGH state. It represents the current through the
input stage.
IOZ defines the maximum leakage current at the switch pin in OFF state.
The typical switch leakage current is also of the order of 1 to 5nA at room
temperature, indicating the excellent Off-isolation characteristics of the
QuickSwitch devices.
SWITCHING CHARACTERISTICS
The switching characteristics discussed below relate specifically to the
switches. Other parameters such as set-up and hold times, release times,
clock pulse width etc. have the same meaning and interpretation as those
in logic circuits, and are excluded from the following discussion.
RON defines the resistance offered by a switch in the ON state. Since the
resistance is a function of the input voltage applied to the switch, it is normally
specified at two different values of input voltage. RON is measured by forcing
the specified amount of current through the switch as shown in Figure 8. It
is calculated by taking the ratio of voltage drop across the switch and the
current forced through the switch.
VCC
Switch
ON
Data Propagation Delay parameters, tPLH and tPHL, refer to the delay
through the switch in ON state. Since the switch behaves like a low value
resistor, the propagation delay is related to the RC time-constant RON x CL,
where CL is the load capacitance. The time-constant is of the order of 250ps
for RON = 5Ω and CL= 5pF. Note that the time-constant is of the order of 1.4ns
for QuickSwitch devices with resistor options with typical RON = 28Ω.
IIN
Switch Turn-on Delay specifications, tPZL and tPZH, define the time taken
to cross nominal TTL threshold of 1.5V at the switch output when the switch
turns on in response to the control signal.
Switch Turn-off Delay specifications, tPLZ and tPHZ, define the time taken
to place the switch in high-impedance OFF state in response to the control
signal. Since the output is undriven in the OFF state, external load must be
used to move the output away from the previous logic state in order to create
a measurable voltage change at the output. The turn-off delay is the time
taken for the output voltage to change by 300mV from the original quiescent
level, with reference to logic level transition at the control input.
VIN
Figure 8. QuickSwitch RON Measurement
RON increases with the applied input voltage. The relationship between
RON and VIN is nonlinear and the ON resistance begins to increase
significantly as VIN approaches VCC.
AC Test Conditions
The Pass Voltage, VP, relates to the output voltage "clipping" feature of
the N-channel QuickSwitch when the switch input voltage is equal to or
greater than the supply voltage. This feature is used in the 5V/3.3V
translation applications. VP is measured at the switch output at IOUT = –5μA
when the switch input is held at VCC. Typical voltage drop across the switch
under these conditions is 1V, thus giving a typical VP of 4V.
The AC test conditions used for the verification and guarantee of switching
characteristics follow industry-accepted practices for high-performance
standard logic products.
Control signals for changing the state of the switches transition between
ground and 3.0V with a rise or fall time of 2.5ns measured between 10%
and 90% points.
The AC test circuit for 5-volt products is shown in Figure 9. The
inputs under test are driven by a pulse generator with a source impedance
of 50Ω. The output load consists of a 50pF capacitance (including jig and
probe capacitance) and a resistor network. The 500Ω resistance to ground
normally consists of a 450Ω discrete resistor in series with the 50Ω
impedance of the co-ax probe which offers very low capacitance for
accurate measurements.
POWER SUPPLY CHARACTERISTICS
The Power Supply characteristics define the components of power
supply current under normal operating conditions.
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QUICKSWITCH GENERAL INFORMATION
7V
500 Ω
IN
DUT
OUT
50pF
500 Ω
Figure 9. AC Test Circuit for 5V Products
For all tests except tPLZ and tPZL, the switch connected to the 7-volt supply
is open. The AC load is the parallel combination of 50pF and 500Ω.
To perform the tPHZ and tPZH tests, the switch shown in Figure 9 is
connected to the 7V supply. This creates an "artificial" logic HIGH level when
the QuickSwitch is in the OFF state, so that output voltage transitions can
occur during logic LOW to high-impedance state when the switch is
undriven.
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QUICKSWITCH GENERAL INFORMATION
TEST CIRCUIT AND WAVEFORMS
LOAD SWITCH POSITION
500Ω
VCC
7.0V
VIN
S1
VOUT
DUT
Pulse
Generator
50Ω
Parameter
S1
Position
tPLH, tPHL
Open
tPZH, tPHZ
Open
tPZL, tPLZ
Closed
450Ω
50pF
50Ω Coax to
Oscilloscope
INPUT CONDITIONS: Input Voltage = 0V to 3V, tR, tF = 2.5ns (10% to 90%), RL = 500Ω, CL= 50pF
PROPAGATION DELAY
3V
1.5V
Switch Input
0V
tPLH
tPHL
VOH
1.5V
Switch Output
VOL
ENABLE AND DISABLE TIMES
3V
1.5V
Control Input
0V
tPHZ
tPZH
Switch Output
VOH
(Switch Input = 3V)
0.3V
1.5V
tPLZ
tPZL
0V
~3.5V
1.5V
Switch Output
0.3V
VOL
(Switch Input = 0V)
NOTES:
1. tPLH and tPHL: Data propagation delays through the switch when the Switch is ON.
2. tPZH: The output goes from Hi-Z (Switch OFF) to a High State (Switch ON).
3. tPZL: The output goes from Hi-Z (Switch OFF) to a Low State (Switch ON).
4. tPLZ: The output goes from Low State (Switch ON) to a Hi-Z (Switch OFF).
5. tPHZ: The output goes from High State (Switch ON) to a Hi-Z (Switch OFF).
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for Tech Support:
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