Datasheet

LTC6990
16
6990fc
For more information www.linear.com/LTC6990
APPLICATIONS INFORMATION
Frequency Modulated Operation (Voltage-Controlled
Oscillator)
Operating the LTC6990 as a voltage-controlled oscillator in
its simplest form is achieved with one additional resistor.
As shown in Figure 11, voltage V
CTRL
sources/sinks a cur-
rent through
R
VCO
to vary the I
SET
current, which in turn
modulates the output frequency as described in Equation (2).
f
OUT
=
1MHz • 50k
N
DIV
•R
VCO
• 1+
R
VCO
R
SET
−
V
CTRL
V
SET






(2)
f
OUT
, choose a value for N
DIV
that meets the following
conditions
62.5kHz
f
OUT(MIN)
 ≤ N
DIV
≤
1MHz
f
OUT(MAX)
(3a)
The 16:1 frequency range of the master oscillator and
the 2:1 divider step-size provides several overlapping fre
-
quency spans to guarantee that any 8:1 modulation range
can be covered by a single N
DIV
setting. R
VCO
allows the
gain to be tailored to the application, mapping the V
CTRL
voltage range to the modulation range.
Step 2: Calculate K
VCO
and f
(0V)
K
VCO
and f
(0V)
define the VCO’s transfer function and sim-
plify the calculation of the the R
VCO
and R
SET
resistors.
Calculate these parameters using the following equations.
K
VCO
=
f
OUT(MAX)
− f
OUT(MIN)
V
CTRL(MAX)
− V
CTRL(MIN)
(3b)
f
(0V)
= f
OUT(MAX
) + K
VCO
• V
CTRL(MIN)
(3c)
K
VCO
and f
(0V)
are not device settings or resistor values
themselves. However, beyond their utility for the resistor
calculations, these parameters provide a useful and intuitive
way to look at the VCO application. The f
(0V)
parameter is
the output frequency when V
CTRL
is at 0V. Viewed another
way, it is the fixed output frequency when the R
VCO
and
R
SET
resistors are in parallel. K
VCO
is actually the frequency
gain of the circuit.
With K
VCO
and f
(0V)
determined, the R
VCO
and R
SET
values
can now be calculated.
Step 3: Calculate and Select R
VCO
The next step is to calculate the correct value for R
VCO
using the following equation.
R
VCO
=
1MHz • 50k
N
DIV
• V
SET
•K
VCO
(3d)
Select the standard resistor value closest to the calculated
value.
Figure 11. Voltage Controlled Oscillator
6990 F08
LTC6990
OE
GND
SET
OUT
V
+
DIV
C1
0.1µF
R1
R2
R
SET
V
+
V
+
R
VCO
V
CTRL
Equation (2) can be re-written as shown below, where
f
(0V)
is the output frequency when V
CTRL
= 0V, and K
VCO
is the frequency gain. Note that the gain is negative (the
output frequency decreases as V
CTRL
increases).

f
OUT
= f
(0V)
– K
VCO
• V
CTRL
f
(0V)
=
1MHz • 50k
N
DIV
• R
SET
R
VCO
(
)
K
VCO
=
1MHz • 50k
N
DIV
• V
SET
•R
VCO
The design procedure for a VCO is a simple four step
process. First select the N
DIV
value. Then calculate the
intermediate values K
VCO
and f
(0V)
. Next, calculate and
select the R
VCO
resistor. Finally calculate and select the
R
SET
resistor.
Step 1: Select the N
DIV
Frequency Divider Value
For best accuracy, the master oscillator frequency should
fall between 62.5kHz and 1MHz. Since f
MASTER
= N
DIV
•