Interfacing to Analog Switches

AND8304/D
Interfacing to Analog
Switches
Driving the Control Input of an Analog
Switch with 1.8 V or Lower − Is it Safe?
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Introduction
Analog switches are everywhere today. Due to their small
size and low current consumption, they are popular in
portable devices where they are effective in a variety of
subsystems including audio and data communications, port
connections, and even test. They can be used to facilitate
signal routing, allow multiple data types to share an interface
connector, or permit temporary access to internal processors
during manufacturing. Analog switches are often used to
give portable system designers a convenient method of
increasing their features or accessibility without duplicating
any circuitry. Understanding the key specifications and
tradeoffs can make the difference between a temporary fix
and a truly optimized solution.
NO
COM
NC
Control Input
Figure 1. Control Input of Switch
For most analog switches out on the market today, the
input buffer that lies directly at the interface of the control
input is a standard CMOS or TTL inverter. These structures
are modeled as seen in Figure 2. The input signal that drives
these inverters, is ideally a digital signal that toggles
between ground and the VCC applied to the inverter. For an
analog switch, the VCC of the inverter is usually the same as
the VCC of the switch. Many systems designers using analog
switches today, however, want the flexibility to operate the
control input voltage, VIN, at a lower voltage than that
applied to the switch VCC.
The Control Input
One aspect of the analog switch that has come under
recent attention is the control input voltage range which
defines how low the voltage can be that drives the control
input pin. The control input, sometimes called select,
determines the state of the switch − open/closed or NO/NC
− as illustrated in Figure 1. This input is typically driven by
a digital signal that toggles between ground and a set DC
voltage and often comes from a much larger, integrated
chipset, such as a CODEC or baseband processor. In general,
these chipsets are operating off of ever−decreasing supply
voltages which in turn sets limits on the voltages they are
capable of sourcing. At the same time analog switches are
not necessarily seeing the same drop in supply voltages. For
example, one popular technique is to power the switch
directly off the battery, causing it to not only see a potentially
high voltage relative to the control input voltage, but also
one that varies constantly over time. This can pose a problem
for some system designs that employ switches optimized to
accept control input voltages that must toggle strictly
between ground and the switch VCC.
© Semiconductor Components Industries, LLC, 2011
November, 2011 − Rev. 1
VCC
Output to Switch
Control Input
Figure 2. Static CMOS Inverter
The first place a designer might look to see whether he or
she can safely lower the high level of VIN is the VIH
specification of the analog switch datasheet. VIH and VIL
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Publication Order Number:
AND8304/D
AND8304/D
levels define the minimum high voltage and maximum low
voltage the input to the inverter must maintain in order to be
considered valid, as illustrated in Figure 3.
TTL, VIH = 2.0 V. These specifications gave ample room for
compliance when the driving signal toggled between ground
and something relatively high, such as 3 V or 5 V. Even if the
VCC were lowered the signals driving the control inputs
were still coming from generally the same sources and
therefore easily able to meet whatever the switch VCC was
at the time. The main difference today is that analog switches
are now often simultaneously interfacing with very different
circuitry blocks, analog and digital. It is not unusual that the
signals that drive VCC, VIN, and the data paths are coming
from three distinct sources, such as in the example
application of Figure 4. This can create a mishmash of signal
swings and voltage requirements for the switch. If the VIH
level is specified using just the traditional standards and
without elaboration, the system designer may find that the
switch does not seem to have the range or flexibility needed
for the application.
VIN toggles between Low and High States and must swing
Above VIH and Below VIL to be considered valid
VCC
VIH
VTH
VIL
GND
Figure 3. VIH and VIL Levels with
Respect to VCC and Ground
Traditionally, VIH was specified fairly simply, often citing
standards for either 5 V CMOS, VIH = 0.7 * VCC, or 5 V
+
Audio
Amp
Battery of
− Portable Device
VCC
CODEC
Figure 4. Example of Various Sources for Voltages and Signal Swings Surrounding an Analog Switch
Pushing the Limits
2.0
It may be possible to safely operate a switch with a much
lower VIH value than VCC, but to understand what the
tradeoffs are, it helps to understand the source of the VIH
specification. Both the VIH and the VIL specifications are
actually derived values based on what’s known as the
threshold voltage, VTH, of the inverter. When ramping up
the voltage to the inverter, VTH is defined as the voltage at
which the inverter, and subsequently the switch, will toggle.
This voltage varies across VCC voltage and temperature and
falls in between VIH and VIL. There are two reasons this
threshold voltage is not simply specified directly as a
minimum voltage switch point. The first is that this
threshold is not exact, it varies a little, even for a given VCC
and temperature. Figure 5 shows an example of the threshold
voltage as it varies across VCC. The voltage falls at slightly
different points depending on the direction of approach
when ramping VIN: ground to VCC or VCC to ground. When
specifying a safe VIH level, the analog switch designer must
take into consideration all of these variations and guarantee
a value with some safety margin.
1.8
1.6
VIN (V)
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
2.7
3.1
3.5
3.9
4.3
4.7
5.1
5.5
VCC (V)
Figure 5. VCC vs. VTH
The second reason VTH is not used directly as a VIH or VIL
value is that if the system designer were to drive the control
input exactly at the threshold voltage the switch would pull
a lot of current from the VCC line causing the device to be
very leaky − sometimes in the mA range. The farther away
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AND8304/D
from the threshold voltage the high and low values of VIN
are set, the less leaky the switch will be. This is demonstrated
in Figure 6, which shows the leakage current at different VIN
values for a handful of different VCC’s. It is evident from this
graph that if VIN switches rail−to−rail, then the static leakage
current will be very low, regardless of the VCC. But, as the
high state voltage of VIN is lowered, the leakage creeps up.
For example, for a VCC of 3.3 V, if the high state voltage of
VIN is 2.5 V the leakage current will easily be less than 5 mA.
But if you lower it down to 1.8 V, the leakage current jumps
up to 100 mA. This problem is exacerbated when the
application attempts to pair a fixed VIN of, say, 1.8 V with
a variable VCC voltage of 2.7 V to 4.2 V, as in the case when
a switch is operating directly off a portable battery. Here, the
leakage current would vary between < 1 mA and 450 mA
depending on the VCC at the moment.
Many system designers may think that 450 mA of leakage
is too much for any amount of time. But it must be
understood that even when swinging rail−to−rail, this
leakage current is seen for at least a brief amount of time as
the control input voltage is ramped up or down. Most control
input signals are digital−type signals, but they are not perfect
and there is some amount of rise and fall time associated
with each transition. When swinging from ground to VCC,
the input signal still has to pass through each VIN level,
momentarily exhibiting the ICC leakage associated with
each level. When the high state of VIN is lowered with
respect to VCC, the leakage will remain at its associated
value for as long as VIN is in its high state. This is true for
all CMOS input structures.
ON Semiconductor’s Solutions
ON Semiconductor has developed a new control input
buffer designed specifically to interface with low voltage
chipsets. The new structure achieves two important goals −
it lowers the minimum allowable VIH value that guarantees
switching and maintains low leakage for the new expanded
range of VIN values. Figure 7 shows leakage curves from
ON Semiconductor’s NLAS5223BL. The graph shows the
typical leakage current across VIN for three different switch
VCC voltages − 2.7 V, 3.3 V, 4.2 V. It is clear that with the
new structure the leakage current is significantly reduced for
a wider range of VIH values. For example comparing the
graphs in Figures 6 and 7, the leakage current for a VCC of
4.2 V and a VIH of 1.8 V is 450 mA for the original structure
and 100 mA for the new structure, yielding a significant
improvement.
500
2000
1900
450
1800
1700
VCC = 3.3 V
400
1600
1500
1400
350
VCC = 4.2 V
VCC = 4.2 V
1300
300
1100
ICC (mA)
ICC (mA)
1200
1000
900
250
200
800
VCC = 3.3 V
700
150
600
500
100
400
VCC =
2.7 V
300
50
200
100
0
VCC =
2.7 V
0
0
0.5
1.0
1.5
2.0
1.8
VIN (V)
2.5
3.0
3.5
4.0
0
0.5
1
1.5
VIN (V)
2
2.5
3
3.5
Figure 7. VIN vs. ICC of ON Semiconductor’s
NLAS5223BL with Optimized Control Input
Figure 6. VIN vs. ICC, Standard Control Input
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AND8304/D
ON Semiconductor’s portfolio of data and audio switches
includes new devices designed to allow low baseband
voltages, such as 1.8 V or lower, to drive the control input.
The datasheet for each newly released device will include
ICC leakage graphs for varying levels of VIN− giving the
system designer a more complete picture of the tradeoffs and
options available for interfacing to the switch. With this
additional information, the designer is one step closer to an
analog switch solution that is truly optimized for the
application.
It is important to remember that even with the lower
leakage values, there is still a limit to how low the VIH can
safely operate. This goes back to the arguments made
previously that the threshold voltage which defines the
switch point varies with a number or parameters and a safety
margin must be maintained in order to guarantee an effective
switch. The VIH values specified in the datasheet take both
factors into account. ON Semiconductor’s NLAS5223BL is
designed to safely operate with VIH levels down to 1.6 V for
VCC levels up to 4.3 V. In this scenario, a battery voltage that
varies between 2.7 V and 4.2 V will never induce a leakage
above 200 mA, typically much lower, when operated by a
control input that toggles between ground and 1.6 V.
ON Semiconductor and
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