LTC6990CDCB#TRMPBF Linear Technology, LTC6990CDCB#TRMPBF Datasheet - Page 17

• Manufacturer Part Number: LTC6990CDCB#TRMPBF
• Description: IC MPU SUPERVISORY 6DFN
• Manufacturer: Linear Technology

Specifications of LTC6990CDCB#TRMPBF

• Frequency: *
• Voltage - Supply: *
• Current - Supply: 235µA
• Operating Temperature: *
• Package / Case: 6-WFDFN Exposed Pad
• Count: *
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LTC6990CDCB#TRMPBFTR

APPLICATIONS INFORMATION
The accuracy of K
output frequency is controlled by the ratio of V
V
proximated by:
As the equation indicates, the potential for error in output
frequency due to V
at its largest when V
when V
its minimum. With the maximum absolute frequency error
(in Hertz) occurring at the lowest output frequency, the
relative frequency error (in percent) can be signifi cant.
V
for at most a ±3% error term. However, this ±3% potential
error term is multiplied by both V
quency range applications (high K
errors greater than ±50% at the highest V
(lowest f
circuit must be used with caution for applications where
the frequency range is greater than 4:1. Restricting the
range to 4:1 typically keeps the frequency error due to
V
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.
SET
SET
SET
Δf
. The frequency error (in Hertz) due to ΔV
is nominally 1.0V with a maximum error of ±30mV
variation below 10%.
OUT
CTRL
OUT
≅ K
100
80
60
40
20
0
). For this reason the simple, two resistor VCO
is at its maximum, the output frequency is at
1
VCO
Figure 12. VCO Transfer Function
• V
VCO
SET
CTRL
CTRL
does depend on V
2
error increases with K
V
•
CTRL
is at its maximum. Recall that
ΔV
V
SET
(V)
SET
CTRL
3
VCO
) can have frequency
and K
6990 F12
SET
4
VCO
CTRL
because the
VCO
. Wide fre-
SET
CTRL
voltage
and is
is ap-
to
Example: Design a VCO with the Following Parameters
Step 1: Select the N
First, choose an N
Equation (3a).
The application’s desired frequency range is 10:1, which
isn’t always possible. However, in this case N
both requirements of Equation (3).
Step 2: Calculate K
Next, calculate the intermediate values K
Equations (3b) and (3c).
Step 3: Calculate and Select R
The next step is to use Equation (3d) to calculate the cor-
rect value for R
Select R
Step 4: Calculate and Select R
The fi nal step is to calculate the correct value for R
using Equation (3e).
Select R
In this design example, with its wide 10:1 frequency range,
the potential output frequency error due to V
ranges from less than ±1% when V
up to ±36% when V
must be accounted for in the system design.
f
f
6.25 ≤ N
K
R
R
f
OUT(MAX)
OUT(MIN)
(0V)
VCO
SET
VCO
= 100kHz + 30kHz/V • 1V = 130kHz
VCO
SET
=
=
=
8 • 130kHz − 1V • 30kHz/V
100kHz − 10kHz
8 • 1V • 30kHz/V
DIV
= 61.9k
= 10kHz at V
= 210k.
= 100kHz at V
1MHz • 50k
(
≤ 10
VCO
4V − 1V
.
1MHz • 50k
DIV
VCO
DIV
CTRL
that meets the requirements of
and f
Value
CTRL(MAX)
is at its maximum. This error
CTRL(MIN)
= 208.333k
= 30kHz/V
(0V)
VCO
SET
CTRL
= 4V
= 1V
)
VCO
LTC6990
is at its minimum
= 62.5k
SET
and f
DIV
error alone
= 8 meets
(0V)
17
using
6990f
SET