### May 2002 How to Use the LTC6900 Low Power SOT-23 Oscillator as a VCO

```DESIGN IDEAS
How to Use the LTC6900 Low Power
SOT-23 Oscillator as a VCO
by Nello Sevastopoulos
Introduction
The LTC6900 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, which was featured in
the February 2001 issue of this magazine.
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.
100
 10MHz 
RSET = 20k • 
 , N = 10
 N • fOSC 
1
(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 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 resis-
1
5V
1
0.1µF
2
3
10k ≤ RSET ≤ 2M
+
V
OUT
LTC6900
1kHz ≤ fOSC ≤ 20MHz
5
5V
GND
SET
DIV
4
÷100
÷10
OPEN
÷1
6900 TA01
Figure 1. Basic connection diagram
tor 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.
As shown in Figure 2, the voltage of
the SET pin is 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+.
RSET
SET
–
+
–
VBIAS
2 GND
)
(2)
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
not be affected. 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
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 datasheet limits.
GAIN = 1
3
(
10MHz
20k
•
•
N
RIN RSET
Programming the Output
Frequency
PROGRAMMABLE
DIVIDER (N)
(÷1, 10 OR 100)
+
IRES
fOSC =

VIN − V + 
1 +
•

VRES


VRES = (V+ – VSET) = 1.1V TYPICAL
V+
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.
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.
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:
1

RIN  

1+
RSET 
OUT
5
V+
MASTER OSCILLATOR
ƒMO = 10MHz • 20kΩ •
IRES
(V + – VSET)
+
–
DIVIDER
SELECT
2µA
DIV
THREE-STATE
INPUT DETECT
IRES
+
–
4
2µA
6900 BD
GND
Figure 2. Simplified block diagram
Linear Technology Magazine • May 2002
23
DESIGN IDEAS
1
0.1µF
RSET
2
3
OUT
V+
LTC6900
5
IPR
DIV
+
5V
GND
SET
÷100
4
÷10
VRES
–
VIN
Figure 3. Concept for programming via current steering
RIN
=
RSET
(3)


(MAX)
+
(VIN(MAX) − V + ) −  ffOSC
 (VIN(MIN) − V )
OSC(MIN) 
(
)

 fOSC(MAX
VRES 
− 1
f
 OSC(MIN)

−1
Once RIN/RSET is known, calculate
RSET from:
RSET =
10MHz
20k
•
•
N
fOSC(MAX)
)
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.
Let V+ = 3V, fOSC(MAX) = 2MHz for
VIN(MAX) = 3V and fOSC(MIN) = 1.5MHz for
24
+
–
OUT
V+
LTC6900
2
RSET
5
fOSC
5V
GND
3
SET
DIV
÷100
4
RIN
÷10
OPEN
÷1
6900 TA01
Figure 4. Implementation of the concept shown in Figure 3
2.00
3000
1.95
2500
1.90
RIN = 1.1M
RSET = 110k
V+ = 3V
N=1
fOSC (MHz)
1.85
1.80
2000
1.75
1.70
1500
RIN = 182k
RSET = 143k
V+ = 3V
N=1
1000
1.65
1.60
500
1.55
1.50
0
0
0.5
1
1.5
VIN (V)
2
2.5
0
3
6900 F09
Figure 5. Output frequency vs input
voltage
0.5
1
1.5
VIN (V)
2
2.5
3
6900 F10
Figure 6. Output frequency vs input
voltage
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.
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 R SET =
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.
Note:
All of the calculations above assume VRES = 1.1V,
although VRES ≈ 1.1V. For completeness, Table 1
shows the variation of VRES against various parallel
combinations of RIN and RSET (VIN = V+). Calculate
first with VRES ≈ 1.1V, then use Table 1 to get a
better approximation of VRES, then recalculate the
resistor values using the new value for VRES.
(4)


RIN  
+

 VIN(MAX) − V + VRES  1 +

R
SET  



 R 
VRES  IN 


 RSET 


(
0.1µF
OPEN
÷1
6900 TA01
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.
By manipulating the algebraic relation for f OSC 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 = 1.1 and calculate
the ratio of RIN/RSET from the
following:
1
V+
fOSC (kHz)
V+
Table 1: Variation of VRES for various values of RIN RSET
RINRSET (VIN = V+)
VRES, V+ = 3V
VRES, V+ = 5V
20k
0.98V
1.03V
40k
1.03V
1.08V
80k
1.07V
1.12V
160k
1.1V
1.15V
320k
1.12V
1.17V
VRES = Voltage across RSET
Linear Technology Magazine • May 2002
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