Datasheet

LTC6990
17
6990fc
For more information www.linear.com/LTC6990
Figure 12. VCO Transfer Function
V
CTRL
(V)
1
f
OUT
(kHz)
100
80
40
20
60
0
2
6990 F12
43
APPLICATIONS INFORMATION
Step 4: Calculate and Select R
SET
The final step is to calculate the correct value for R
SET
using the following equation:
R
SET
=
1MHz • 50k
N
DIV
• f
(0V)
− V
SET
•K
VCO
( )
(3e)
Select the standard resistor value closest to the calculated
value.
Some applications require combinations of f
OUT(MIN)
,
f
OUT(MAX)
, V
CTRL(MIN)
and V
CTRL(MAX)
that are not achiev-
able. These applications result in unrealistic or unrealiz-
able (e.g. negative value) resistors. These applications
will require preconditioning of the V
CTRL
signal via range
scaling and/or level shifting to place the V
CTRL
into a range
that yields realistic resistor values.
Frequency Error in VCO Applications Due to V
SET
Error
As stated earlier, f
(0V)
represents the frequency for V
CTRL
= 0V, which is the same value as would be generated by
a single resistor between SET and GND with a value of
R
SET
|| R
VCO
. Therefore, f
(0V)
is not affected by error or
drift in V
SET
(i.e. ∆V
SET
adds no frequency error when
V
CTRL
= 0V).
The accuracy of K
VCO
does depend on V
SET
because the
output frequency is controlled by the ratio of V
CTRL
to
V
SET
. The frequency error (in Hertz) due to ∆V
SET
is ap-
proximated by:
∆f
OUT
≅ K
VCO
• V
CTRL
•
∆V
SET
V
SET
As the equation indicates, the potential for error in output
frequency due to V
SET
error increases with K
VCO
and is
at its largest when V
CTRL
is at its maximum. Recall that
when V
CTRL
is at its maximum, the output frequency is
at its minimum. With the maximum absolute frequency
error (in Hertz) occurring at the lowest output frequency,
the relative frequency error (in percent) can be significant.
V
SET
is nominally 1.0V with a maximum error of ±30mV
for at most a ±3% error term. However, this ±3% po-
tential error
term is multiplied by both V
CTRL
and K
VCO
.
Wide frequency range applications (high K
VCO
) can have
frequency errors greater than ±50% at the highest V
CTRL
voltage (lowest f
OUT
). For this reason the simple, two
resistor VCO circuit must be used with caution for ap-
plications where
the frequency range is greater than 4:1.
Restricting the range to 4:1 typically keeps the frequency
error due to V
SET
variation below 10%.
For wide frequency range applications, the non-inverting
VCO circuit shown in Figure 13 is preferred because the
maximum frequency error occurs when the frequency
is highest, keeping the relative error (in percent) much
smaller.
Example: Design a VCO with the Following Parameters
f
OUT(MAX)
= 100kHz at V
CTRL(MIN)
= 1V
f
OUT(MIN)
= 10kHz at V
CTRL(MAX)
= 4V
Step 1: Select the N
DIV
Value
First, choose an N
DIV
that meets the requirements of
Equation (3a).
6.25 ≤ N
DIV
≤ 10
The application’s desired frequency range is 10:1, which
isn’t always possible. However, in this case N
DIV
= 8 meets
both requirements of Equation (3).