DN293 - Using the LTC6900 Low Power SOT-23 Oscillator as a VCO

Using the LTC6900 Low Power SOT-23 Oscillator as a VCO
Design Note 293
Nello Sevastopoulos
Introduction
The LTC ®6900 is a precision low power oscillator that
is extremely easy to use and occupies very little PC
board space. It is a lower power version of the LTC1799.
controlled by an internal bias, and by the gate to source
voltage of a PMOS transistor. The voltage of the SET
pin (VSET ) is typically 1.1V below V+.
Programming the Output Frequency
The output frequency of the LTC6900 can be programmed by altering the value of RSET as shown in
Figure 1 and the accuracy of the oscillator will be as
specified. The frequency can also be programmed by
steering current in or out of the SET pin, as conceptually shown in Figure 3. This technique can degrade
accuracy as the ratio of (V+ – VSET )/IRES is no longer
uniquely dependent on the value of RSET, as shown in
Figure 2. This loss of accuracy will become noticeable
when the magnitude of IPROG is comparable to IRES. The
frequency variation of the LTC6900 is still monotonic.
The output frequency, fOSC, of the LTC6900 can range
from 1kHz to 20MHz—programmed via an external
resistor, RSET, and a 3-state frequency divider pin, as
shown in Figure 1.
fOSC =
10MHz 20kΩ
•
N
RSET
N = 1, 10, 100
(1)
A proprietary feedback loop linearizes the relationship between RSET and the output frequency so the
frequency accuracy is already included in the expression above. Unlike other discrete RC oscillators, the
LTC6900 does not need correction tables to adjust the
formula for determining the output frequency.
Figure 4 shows how to implement the concept shown in
Figure 3 by connecting a second resistor, RIN, between
the SET pin and a ground referenced voltage source VIN.
Figure 2 shows a simplified block diagram of the
LTC6900. The LTC6900 master oscillator is controlled
by the ratio of the voltage between V+ and the SET pin
and the current, IRES, entering the SET pin. As long as
IRES is precisely the current through resistor RSET, the
ratio of (V+ – VSET )/IRES equals RSET and the frequency
of the LTC6900 depends solely on the value of RSET.
This technique ensures accuracy, typically ±0.5% at
ambient temperature.
For a given power supply voltage in Figure 4, the
output frequency of the LTC6900 is a function of VIN,
RIN, RSET, and (V+ – VSET ) = VRES:
fOSC =
RSET
GAIN = 1
3
5V
1
0.1μF
2
V+
OUT
LTC6900
1kHz ≤ fOSC ≤ 20MHz
5
10k ≤ RSET ≤ 2M
SET
DIV
PROGRAMMABLE
DIVIDER (N)
(÷1, 10 OR 100)
SET
GND
ƒMO.)[tLĀt
+–
VBIAS
4
÷10
IRES
IRES
(V + – VSET)
DIVIDER
SELECT
V
+
–
5
˜"
DIV
THREE-STATE
INPUT DETECT
+
–
4
˜"
OPEN
÷1
GND
%/'
DN293 F01
Figure 1. Basic Connection Diagram
09/02/293_conv
OUT
+
MASTER OSCILLATOR
–
5V
GND
÷100
3
VRES = (V+ – VSET) = 1.1V TYPICAL
V+
+
IRES
(2)
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered
trademarks of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
As shown in Figure 2, the voltage of the SET pin is
1
⎞
10MHz
20kΩ ⎛ VIN – V +
1
•
• 1+
•
N
RSET //RIN ⎜⎝
VRES 1+RIN /RSET ⎟⎠
Figure 2. Simplified Block Diagram
V+
1
0.1μF
2
RSET
V+
OUT
LTC6900
Once RIN/RSET is known, calculate RSET from:
5
÷100
3
SET
DIV
(
4
÷10
OPEN
÷1
IPR
)
⎡
⎛
RIN ⎞ ⎤
+
⎢ VIN(MAX) – V + VRES ⎜ 1+
⎥
⎝ RSET ⎠⎟ ⎥
10MHz 20kΩ ⎢
RSET =
•
⎥
⎛ R ⎞
N
fOSC(MAX) ⎢
VRES ⎜ IN ⎟
⎢
⎥
⎝ RSET ⎠
⎢⎣
⎥⎦
5V
GND
(4)
DN293 F03
+
0.1μF
VRES
RSET
–
VIN
+
–
2
5
V+
OUT
LTC6900
fOSC
5V
GND
÷100
3
SET
4
DIV
RIN
÷10
OPEN
÷1
DN293 F04
Figure 4. Implementation of the Concept Shown in Figure 3
When VIN = V+ the output frequency of the LTC6900
assumes the highest value and it is set by the parallel
combination of RIN and RSET. Also note, the output
frequency, fOSC, is independent of the value of VRES
= (V+ – VSET ) so, the accuracy of fOSC is within the
data sheet limits.
When VIN is less than V+, and especially when VIN approaches the ground potential, the oscillator frequency,
fOSC, assumes its lowest value and its accuracy is
affected by the change of VRES = (V+ – VSET ). At 25°C
VRES varies by ±8%, assuming the variation of V+ is
±5%. The temperature coefficient of VRES is 0.02%/°C.
Note that if VIN is the output of a DAC referenced to V+,
the VRES sensitivity to the power supply is eliminated.
By manipulating the algebraic relation for fOSC above,
a simple algorithm can be derived to set the values of
external resistors RSET and RIN, as shown in Figure 4:
1. Choose the desired value of the maximum oscillator
frequency, fOSC(MAX), occurring at maximum input
voltage VIN(MAX) ≤ V+.
2. Set the desired value of the minimum oscillator
frequency, fOSC(MIN), occurring at minimum input
voltage VIN(MIN) ≥ 0.
3. Choose VRES = 1.1V and calculate the ratio of RIN/
RSET from the following:
RIN
=
RSET
(V
IN(MAX) – V
⎛ fOSC(MAX) ⎞
+
⎟ • VIN(MIN) – V
OSC(MIN) ⎠
–1
⎛ fOSC(MAX) ⎞
VRES ⎜
– 1⎟
⎝ fOSC(MIN) ⎠
+
) – ⎜⎝ f
(
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Linear Technology Corporation
Example 2: Vary the oscillator frequency by one octave
per volt. Assume fOSC(MIN) = 1MHz and fOSC(MAX)
= 2MHz, when the input voltage varies by 1V. The
minimum input voltage is half supply, that is VIN(MIN)
= 1.5V, VIN(MAX) = 2.5V and V+ = 3V.
Equation (3) yields RIN/RSET = 1.273 and equation (4)
yields RSET = 142.8kΩ. RIN = 1.273RSET = 181.8kΩ.
For standard resistor values, use RSET = 143kΩ (1%)
and RIN = 182kΩ (1%).
Figure 6 shows the measured fOSC vs VIN. For VIN
higher than 1.5V the VCO is quite linear; nonlinearities
occur when VIN becomes smaller than 1V, although
the VCO remains monotonic.
The VCO modulation bandwidth is 25kHz that is, the
LTC6900 will respond to changes in the frequency
programming voltage, VIN, ranging from DC to 25kHz. 2.00
3000
RIN = 1.1M
RSET = 110k
V + = 3V
N=1
1.90
1.70
1.50
)
Data Sheet Download
Let V+ = 3V, fOSC(MAX) = 2MHz for VIN(MAX) = 3V and
fOSC(MIN) = 1.5MHz for VIN=0V. Solve for RIN/RSET by
equation (3), yielding RIN/RSET = 9.9/1. RSET = 110.1kΩ
by equation (4). RIN = 9.9RSET = 1.089MΩ. For standard resistor values, use RSET = 110kΩ (1%) and RIN
= 1.1MΩ (1%). Figure 5 shows the measured fOSC vs
VIN. The 1.5MHz to 2MHz frequency excursion is quite
limited, so the curve fOSC vs VIN is linear.
fOSC (kHz)
1
V+
Example 1: In this example, the oscillator output
frequency has small excursions. This is useful where
the frequency of a system should be tuned around
some nominal value.
fOSC (MHz)
Figure 3. Concept for Programming via Current Steering
1000
0
0
2
1
VIN (V)
(3)
RIN = 182k
RSET = 143k
V+ = 3V
2000 N = 1
3
0
DN293 F05
1
2
VIN (V)
3
DN293 F06
Figure 5. Output Frequency Figure 6. Output Frequency
vs Input Voltage
vs Input Voltage
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