ON Semiconductor AND8133 Technical data

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AND8133/D

Analog Switches Solve
Many Problems within a
Cell Phone

Prepared by: Fred Zlotnick

ON Semiconductor

BACKGROUND

Analog switches have been available since the 60’s as a
component for systems designers. With the advent of ASICs
and ASSPs, many designers have not experienced the real
value of these devices. Today’s designers may be familiar
with the venerable MC14066, a workhorse that has been
around for 30 years or more. The device consists of 4
independent switches that provide bilateral capabilities
when “On” and nearly infinite impedance when “OFF”.

The 4066 was done in 9.0 m metal gate CMOS. It has a
voltage rating of 3.0 to 18 V, and when switched “ON” a
resistance of 500 . The smallest package is a 14 pin
TSSOP with approximately 32 mm
occupied. If today’s designer is not aware of the strides made
in analog switches, many of the issues associated with the
old metal gate part have been solved with today’s
sub−micron silicon gate CMOS in tiny transistor−like
packages.

Enter the single gate solution: On Semiconductor now
offers a low voltage, single version of the 4066 in a one gate
package. The MC74VHC1G66DFT2 is a single switch
(SPST) occupying less than 4.5 mm
package. The device is specified from 2.0−5.5 V and offers
<25  resistance when turned “ON” and almost infinite
impedance when turned off. Interestingly, the device can
pass/stop either a digital or analog signal. Digital signals get
passed with <1.0 nsec delay, and very nearly no change in
the signal. Analog signals get passed with less than 0.1%
distortion and the device has a –3.0 dB point of >100 MHz.
Because of its utility, ON Semiconductor has elected to add
several more devices to the portfolio including SPST, SPDT,
dual SPST, DPDT, 2:1 Mux, dual DPDT functions. These
are all available in tiny packages from 2.1 x 2.0 mm to 3.0
x 3.0 mm.

The following problems are presented to illustrate the use
of analog switches in cell phone applications.

2

of board space

2

in a tiny SC70/SC88A

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APPLICATION NOTE

Problem: To design an oscillator with two
frequencies. The purpose of this is to illustrate how

an analog switch may be used to switch in a
different crystal and have a single input to a device.
The purpose might be to slow a microprocessor or
DSP down to conserve power, yet still function. In
this application it is assumed that the MCU has two
inputs for a crystal. The NLAS4599 SPDT switch
selects one of two crystals electronically under
control of the MCU/DSP. Only one tiny SC70 type
package is needed. The diagram doesn’t show
power and ground to the part for simplicity.

32.768 kHz

CTRL

XSTAL1

XSTAL2

MCU

or

DSP

Problem: To cut power consumption on a system
using a PLL. A PLL operating at 1.0 GHz or more,

can often draw more than 50 mA continuously. If
the design is such that the frequency is constant for
a period of time, say, for several minutes at a time
(or longer), it is possible to use a sample and hold
output where the PLL charges a capacitor, hold th e
charge for a period of time (e.g. 50 ms) and go back
and reclose the loop. The amount of droop from
nominal voltage will depend upon the capacitor
used and the time the circuit is held open loop.

NLAS4599

20 MHz

Figure 1.

 Semiconductor Components Industries, LLC, 2003

September, 2003 − Rev. 0

1 Publication Order Number:

AND8133/D

MCU

Power Enable

PLL

Figure 2.

NLAS4501

Problem: Improving lock time of a PLL by
changing the time constant, having a fast “attack”
and long hold time constant. An analog switch can

be used to change either the resistor or capacitor
that makes up the time constant for a PLL.
Changing the time constant allows the loop to get
close to lock very quickly then go to a long time
constant for maximum PLL noise rejection.

To MCU

NLAS4501

PLL

AND8133/D

tester. I will make the assumption that the designer
cannot afford to give up 2 I/O pins. Analog
switches then make the perfect solution. They
introduce nearly zero delay time, cause almost no
distortion to the signal, and all the configuration to
be switched by an external pin. The NLAS4599 is
a Single Pole Double Throw (SPDT) switch, that
will allow the normal configuration to be wired
through. If the control pin is grounded through a
resistor, the when an external plug is presented, it
needs to take the (assumed) 3.0 V supply and route
it out to the control pin, and then it will have the
2 I/O pins available to use. The result is only

8.0 mm
and no degradation to the signal. When the external
connector is plugged in, the enable changes the
circuit over and the 2 I/O pins are routed from their
normal position within the circuit, to be used as
external pins for test.

2

of board space used, with near zero delay

3.0 V

CTRL

NLAS4599

NLAS4599

GND

10 K

I/O 1

MCU

Figure 3.

Design for testability. Cell phones are complete
communication systems built into tiny housings.
They are comprised of I/O, memory, a CPU and
firmware. With firmware becoming such a large
part of the design, it is crucial that the designer be
able to enable software debugging, and create a
system that can be analyzed for field problems. The
analog switch permits the designer to re−route
some of the I/Os to create a new path for analysis.
For purposes of illustration, I will show two I/Os
that need re−routing to be tested by an external

CTRL

Connect wherever it needs to be in circuit

Figure 4.

Conclusion

ON Semiconductor offers more than a dozen analog
switches in tiny packages varying from SC70 and TSOP−5
to a 3.0 x 3.0 mm QFN (Quad flat no−lead) 16 pin device.
The complete list is shown following. The ideas shown here
and many more can be implemented by one or more of these
devices.

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