Dec 2003 Digital Programmable Oscillator Is Smaller, Sturdier and More Versatile than Crystal Oscillators

DESIGN FEATURES
Digital Programmable Oscillator Is
Smaller, Sturdier and More Versatile
by Albert Huntington
than Crystal Oscillators
Introduction
Open just about any electronic gadget
these days, and you will find a crystal oscillator driving a microcontroller,
providing a timebase or clocking any
number of discrete time circuits. Crystal oscillators provide a reasonably
priced, and highly stable time base.
They are relatively easy to use, and
are available in increasingly smaller
packages. Thus, the venerable crystal oscillator has become the defacto
timebase solution. Designers often do
not even consider asking if it is the
best solution to a problem, when, in
fact, crystal oscillators are not without
their drawbacks. They can be power
hungry, inflexible, board space hogging, and above all, shock sensitive
components.
Enter the LTC6903 and LTC6904.
These programmable oscillators
provide a smaller, more reliable and
vastly more versatile clocking solution.
In a small MS8 package, the LTC6903
and LTC6904 use less board space
than almost all crystal oscillators.
Whereas crystal oscillators contain
a quartz crystal and are sensitive to
mechanical shock, the LTC6903 and
LTC6904 are a fully electronic devices,
and relatively insensitive to vibration
and mechanical shock. While crystal
oscillators output a set frequency, the
LTC6903 and LTC6904 are fully programmable between 1kHz and 68MHz.
The frequency is set by a 16-bit control
word via a serial port and is typically
D15
D14
D13
D12
D11
OCT3 OCT2 OCT1 OCT0 DAC9
4-Bit Control
Divider 2N
V+
OUT
R
RESISTOR
CONTROLLED
OSCILLATOR
R
DIVIDE
2N
OUT
RESISTOR
DAC
f=
68MHz • R
, WHERE R ≤ RDAC ≤ 2R
R DAC
LTC6903/LTC6904
Figure 1. LTC6903 and LTC6904 conceptual block diagram. The LTC6903 and LTC6904 consist of
a resistor controlled oscillator coupled to a serial port controlled resistor DAC and output divider.
accurate to within 1.1%, with a resolution of 0.1% or better.
to give the oscillator a frequency range
of 34MHz to 68MHz:
Device Description
f=
The LTC6903 and LTC6904 are resistor controlled oscillators, similar to the
popular LTC1799. These new oscillators offer an integrated serial resistor
DAC and a set of digital frequency dividers, as shown in Figure 1.
The LTC6903 takes commands via
an SPI-compatible 3-wire serial port,
and the LTC6904 communicates
through an I2C-compatible 2-wire
serial port. The serial port bit maps
are shown in Figure 2. Ten DAC bits
control the resistor DAC, four OCT
bits control the output dividers, and
2 MODE bits control the outputs. The
LTC6904 can respond to one of two
different serial port addresses (set by
the state its ADR pin).
The resistor DAC ranges linearly in
value from R to 2R, where R is trimmed
The oscillator frequency is inversely
proportional to the resistance of the
DAC. At frequencies just above 34MHz,
the step size is 16.6kHz. At frequencies immediately below 68MHz, the
step size is 66.4kHz. The step size
ranges between 0.05% and 0.1% of
the frequency. The output frequency
divider divides the internal oscillator
frequency by 2N, where N ranges from
0 to 15. N is calculated from the OCT
bits of the control word, and is simply
the complement of those bits. Higher
values of N (lower values of OCT) yield
lower output frequencies. The combination of the OCT and DAC bits into a
single 14-bit control word provides a
simple and consistent interface where
higher control codes always result in
68MHz • R
, where R ≤ RDAC ≤ 2R
R DAC
D10
D9
D8
D7
D6
D5
D4
D3
DAC8
DAC7
DAC6
DAC5
DAC4
DAC3
DAC2
DAC1
10-Bit Control
DAC
D2
D1
D0
DAC0 MODE1 MODE0
2-Bit Control
OUT and OUT
Figure 2. LTC6903 and LTC6904 serial port bitmap. OCT[3:0] controls the octave, DAC[9:0]
controls the frequency setting within an octave, and MODE[1:0] sets the active output.
Linear Technology Magazine • December 2003
7
DESIGN FEATURES
Minimum
Frequency
Maximum
Frequency
OCT
Code
34.05MHz
68.03MHz
15
17.02MHz
34.01MHz
14
8.511MHz
17.01MHz
13
4.256MHz
8.503MHz
12
2.128MHz
4.252MHz
11
1.064MHz
2.126MHz
10
532kHz
1063kHz
9
266kHz
531.4kHz
8
133kHz
265.7kHz
7
66.5kHz
132.9kHz
6
33.25kHz
66.43kHz
5
16.62kHz
33.22kHz
4
8.312kHz
16.61kHz
3
4.156kHz
8.304kHz
2
2.078kHz
4.152kHz
1
1.039kHz
2.076kHz
0
higher frequencies. Across all control
codes, the LTC6903/LTC6904 is guaranteed to be completely monotonic.
The output pins are controlled by
the output control bits MODE1 and
MODE0. Either of the outputs can
be disabled through these bits. When
both outputs are disabled through the
mode control bits, the internal oscillator is also disabled. The OE pin can
also be used to asynchronously disable
either output without shutting down
the oscillator entirely.
V+
V–
V+
SDI
OE
SCK
LTC6903
0.1µF
CLK
OUTPUTS
SEN
CLK
Figure 3. LTC6903 minimal circuit. The
LTC6903 and LTC6904 have a simple
external interface—the only required
external component is a bypass capacitor.
8
A Minimal Circuit
The LTC6903 and LTC6904 require
no external components other than a
small power supply bypass capacitor.
For best performance, this capacitor
should have low series resistance
and be mounted directly adjacent to
the power supply pins. The minimal
circuit shown in Figure 3 results in an
oscillator frequency of 1.039kHz upon
power-up. The LTC6903/LTC6904
incorporates power on reset circuitry
which sets the control code to all zeros when power is first applied. Other
frequencies may be set through the
serial port.
Calculating the Frequency Code
In order to set a frequency, an OCT code
and a DAC code must be calculated.
The OCT code may be chosen from
Table 1 or it may be calculated as:
OCT = 3.322 log
f ,
1039
(1)
where f is the desired frequency in
Hertz.
When using the equation, it is necessary to round the OCT code down
(truncate) to the nearest integer.
The DAC code is:
DAC = 2048 –
2078(Hz) • 210 + OCT ,
f
where f is the desired frequency in
Hertz and OCT is the previously determined OCT code.
Round the DAC code to the nearest integer value, up or down. The
frequency may be calculated from
the OCT and DAC settings through
the formula:
2078
f = 2OCT •

DAC 
2 –

1024 

(2)
For instance, to set a frequency of
1.00MHz, first chose the OCT code
from Table 1 or calculate OCT from
equation [1] above.
OCT = 3.322 log
1 • 106
= 9.91
1039
Round down (truncate) for an
OCT code of 9. Next, calculate the
DAC code:
DAC = 2048 –
2078(Hz) • 210 + 9
1 • 106
= 958.53
Rounding to the nearest integer,
the DAC code is 959.
Verify the calculations by plugging
the result back into the formula [2]
for frequency:
f = 29 •
2078
= 1.00 × 106

959 
2 –

1024 

In order to determine the 16-bit control word for the LTC6903/LTC6904,
values for the mode control bits,
MODE0 and MODE1, must be chosen. To enable both outputs, set both
MODE0 and MODE1 to “0”.
The control word is composed of the
OCT, DAC and MODE control bits:
OCT • 212 + DAC • 22 + MODE,
or,
9 • 212 + 959 • 22 + 0 = 40,700,
or in binary,
1001111011111100.
Figure 4 shows that frequency
resolution over the entire range of the
control word is roughly proportional
to the set frequency.
Writing the Control Code
The LTC6903 can be configured
through its SPI-compatible serial
port. Similarly, the LTC6904 can be
100M
10M
FREQUENCY (Hz)
Applications Example
Table 1. Choosing the OCT code
1M
100k
10k
1k
0
10k
20k
30k 40k
CODE
50k
60k
70k
Figure 4. Frequency vs control code. The
LTC6903 and LTC6904 achieve 0.1%
resolution across all specified frequencies
with a smooth, monotonic transfer function.
Linear Technology Magazine • December 2003
DESIGN FEATURES
OCT = 9
DAC = 959
MODE=0
D15
D14
D13
D12
D11
D10
D9
D8
D7
D6
D5
D4
D3
D2
D1
D0
1
0
0
1
1
1
1
0
1
1
1
1
1
1
0
0
Figure 5. Setting the frequency to 1MHz and choosing a mode after calculating OCT and DIV codes. The codes
must be translated into a binary number and written to the LTC6903 or LTC6904 through its serial port.
addressed through its I2C-compatible
2-wire serial port. Both serial ports
are set up so that the serial transfer
is accomplished in 8-bit chunks, the
MSB being transferred first. Therefore,
writing just a single byte to the serial
port will result in the most significant
byte being changed. Additionally, the
bytes are written to the registers as
they are received, so a pause between
writing the first and second bytes may
temporarily result in an unintended
frequency output.
Driving Loads
The LTC6903 and LTC6904 output
drivers present a low output impedance of 45Ω, and are capable of driving
substantial resistive and capacitive
loads of as much as 1kΩ and 100pF
at frequencies up to 1MHz. At higher
frequencies, two effects must be taken
into account. First, the impedance presented by the capacitive load becomes
a substantial factor in the shape of
the output waveform. At the maximum
operating frequency of 68MHz, in order
to achieve full swing, an output load of
5pF or less is recommended. Second,
the current drawn through the output
drivers at high frequencies becomes
excessive with capacitive loads. This
results in greatly increased power
C3
1µF
SDI
SDI
SERIAL SCK
CONTROL
SCK
CLK
SEN
SEN
CLK
LTC6903
IS = CLOAD • VSUPPLY • FCLK
The recommended 5pF load is
equivalent to two HC CMOS logic inputs, and is substantially less than the
12pF –15pF of a standard oscilloscope
probe. It is also recommended that
the connection to the output of the
LTC6903/LTC6904 be kept shorter
than 5cm in order to reduce ringing
and reflections from transmission line
effects.
Jitter
Crystal oscillators traditionally excel in
frequency accuracy with low jitter. The
LTC6903 and LTC6904 do not reach
the level of a crystal oscillator by those
measures, but it is comparable enough
to make it a good choice in most applications, especially when size, cost
and durability are important.
Frequency accuracy is trimmed in at
<0.75% at 1kHz under nominal power
IN
VCC
V+
GND
dissipation, and will contribute to
frequency inaccuracy at frequencies
above about 1MHz. Under a 5V power
supply, the output drivers each draw
1.7mA at 68MHz for every 5pF of load.
This is simply a calculation of the energy necessary to charge and discharge
the output load capacitance to 5V at
68MHz, following the formula:
R1
3.3k
OE
R2
2k
IN+
OUT
IN–
V+
LTC1569-7
GND
RX
CLK
V–
C1
1µF
Figure 6. This simple circuit can generate a lowpass frequency
response anywhere from 100Hz to 200kHz with a 0.1% resolution.
Linear Technology Magazine • December 2003
OUT
C2
1µF
supply and temperature conditions.
DAC variation over frequency settings
adds an additional 0.35%, while temperature variation across 0°C–70°C
adds 0.9%, for a total variation of 2%
over temperature and setting. Power
supply variations, mostly at the upper
end of the supply range, account for an
additional 0.25% inaccuracy, leading
to 2.25% over all conditions.
Due to the large number of dividers
used when operating at low frequencies, the LTC6903 is able to provide
typical peak-to-peak jitter of less than
0.1% at frequencies up to 500kHz, and
less than 0.4% at frequencies up to
8.5MHz. At 68MHz, jitter increases to
just under 3% because the averaging effects of the dividers are absent.
These specs are acceptable in all but
the most demanding precision timing
applications.
A Tunable Lowpass Filter
The LTC6903 and LTC6904 are
uniquely well suited to interface with
switched capacitor devices such as
filters and data converters. The tunable lowpass filter of Figure 6 is a
typical example. Using the LTC6903
in combination with an LTC1569-7
tunable filter, it is possible to generate a lowpass frequency response
anywhere from 94Hz to 300kHz with
a 0.1% resolution, using a circuit consisting of only two small integrated
circuits and no external components
other than two 10% resistors and
power supply bypass capacitors.
By tuning the LTC6903 over a frequency range of 3kHz to 9.5MHz at
5V power supply using the equations
presented earlier, a corner frequency
of between 94Hz and 300kHz may be
set. The current draw of the combined
circuits is typically 10mA, the majority
of which is in the LTC1569-7 tunable
lowpass filter.
continued on page 30
9
DESIGN FEATURES
such that the desired output voltages
ramp characteristic is achieved. The
gate pullup currents are controlled via
the FB+ and FB– pins.
Figure 4 shows coincident tracking for a system operating with +12V
and –12V supplies as per the circuit
in Figure 2. The circuit in Figure 2
is easily converted to work with –5V
and +12V supplies by simply changing
R3, R9 and R11 to 12.4kΩ. The new
coincident tracking behavior is shown
in Figure 5. Ratiometric tracking is
sometimes preferable, especially in
signal processing applications. Figure 6 shows this mode of operation,
obtained by changing only R3 and R11
to 12.4kΩ. Note that in this case the
supply ramps are made to start and
finish at the same time.
Short-Circuit Protection
Current limiting provides protection
for the output MOSFET devices. The
current limit for either supply is set by
sense resistors RS+ and RS– (Figure 2).
The voltage across the sense resistor is
regulated by the current limit circuitry
to 50mV for conditions where foldback
current limiting is not enabled. The
LTC6903/LTC6904, continued from page 9
Conclusion
Though crystal based oscillators have
dominated the timing and clocking
market for many years, the LTC6903
(I2C) and LTC6904 (SPI) offer solutions
that are smaller, more flexible, more
LTC3205, continued from page 23
Both of these features are required to
keep the LTC3205 in direct-connect
mode as long as possible.
Conclusion
The LTC3205, designed specifically
for portable backlighting applications,
provides all of the necessary current
TIMER pin provides a means for setting the maximum time the LT4220
is allowed to operate in current limit.
Whenever the current limit circuitry
becomes active, by either the positive
or negative sense amplifier operating in
current limit, a pull-up current source
of 60uA is connected to the TIMER pin
and the voltage rises with a slope of
dV/dt = 60µA/CTIMER. If the overload
is removed, a small 3µA pulldown
current slowly discharges the timer
pin. If the timer succeeds in charging to a 1.24V threshold, an internal
fault latch is set and the FAULT pin is
pulled low. Both MOSFETs are quickly
turned off while the TIMER pin is slowly
discharged to ground.
The power dissipation will be high
in the output MOSFET devices when
the output is shorted with zero ohms.
To prevent excessive power dissipation
in these pass transistors the current
limit on each supply is reduced as the
output voltage falls. This characteristic, commonly referred to as “current
foldback”, reduces the fault current as
the output voltage drops and reaches
the lowest level into the short. The
foldback current limiting reduces
short circuit MOSFET dissipation by
a factor of 2.5. The FB± pins effectively
measure the MOSFET VDS voltage and
control the appropriate current limit
sense amplifier input offset to provide
the foldback current limit.
Automatic Restart
Normally the LT4220 latches off in
the presence of a fault. Nevertheless,
by removing R15 in Figure 2, you can
connect the FAULT and ON+ together to
enable automatic restart. FAULT pulls
the ON+ pin low allowing an automatic
restart to be initiated once the TIMER
pin ramps below 0.5V.
Conclusion
The LT4220 combines all of the functions necessary for split supply Hot
Swap control in one small 16-lead
SSOP plastic package. This device is
adaptable to applications covering a
wide range of positive and negative
supply voltages, ramping profiles,
capacitance and load currents, including optical/laser, audio and ECL
systems.
robust and lower power. Selecting
a frequency from the 1kHz–68MHz
frequency range is simple through
the serial ports, and both devices
operate over a wide range of supply
voltages.
regulation, power circuitry and control
logic to deliver efficient and accurate
power to a large number of LEDs in
a portable product. To further reduce
board level complexity, it uses only four
0603 sized ceramic capacitors keeping
the total solution height under 1mm.
A straightforward serial interface re-
for
the latest information
on LTC products,
visit
www.linear.com
duces the number of wires needed to
control all of the LEDs. Given its feature
set, the LTC3205 packs an amazing
amount of backlighting horsepower,
flexibility and performance into a very
small 4mm × 4mm footprint.
For more information on parts featured in this issue, see
http://www.linear.com/go/ltmag
30
Linear Technology Magazine • December 2003
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