LTC6990I View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC6990I

TimerBlox: Voltage Controlled Silicon Oscillator

Linear Technology 

LTC6990

APPLICATIONS INFORMATION

Step 2: Calculate KVCO and f(0V)

Next, calculate the intermediate values KVCO and f(0V) using

Equations (3b) and (3c).

K VCO

=

100kHz − 10kHz

4V − 1V

=

30kHz/V

f(0V) = 100kHz + 30kHz/V • 1V = 130kHz

Step 3: Calculate and Select RVCO

The next step is to use Equation (3d) to calculate the cor-

rect value for RVCO.

RVCO

=

8

1MHz • 50k

• 1V • 30kHz/V

=

208.333k

Select RVCO = 210k.

Step 4: Calculate and Select RSET

The final step is to calculate the correct value for RSET

using Equation (3e).

RSET

=

8

•

1MHz • 50k

(130kHz − 1V • 30kHz/V)

=

62.5k

Select RSET = 61.9k

In this design example, with its wide 10:1 frequency

range, the potential output frequency error due to VSET

error alone ranges from less than ±1% when VCTRL is at

its minimum up to ±36% when VCTRL is at its maximum.

This error must be accounted for in the system design.

Depending on the application’s requirements, the non-

inverting VCO circuit in Figure 13 may be preferred for

this wide of a frequency variation as its maximum inac-

curacy due to VSET error is only ±9% and can be reduced

to only ±3% with a small change to the voltage tuning

range specification.

Reducing VSET Error Effects in VCO Applications

Figure 13 shows a VCO that reduces the effect of ΔVSET

by adding an op-amp to make VCTRL dependent on VSET.

This circuit also has a positive transfer function (the out-

put frequency increases as VIN increases). Furthermore,

for positive VIN voltages, this circuit places the greatest

absolute frequency error at the highest output frequency.

Compared to the simple VCO circuit of Figure 11, the

absolute frequency error is unchanged. However, with

the maximum absolute frequency error (in Hertz) now

occurring at the highest output frequency, the relative

frequency error (in percent) is greatly improved.

0.4V TO 4V

VIN

R3

100k

3V

10kHz TO 100kHz

fOUT

OE

OUT

LTC6990

3V

3V

1/2

LTC6078

VSET

RVCO

VCTRL 75k

R4

30.1k

C4

33pF

GND

V+

SET

DIV

6990 F13

C1

0.1µF

R1

1M

DIVCODE = 3

(NDIV = 8, Hi-Z = 0)

R2

280k

RSET

249k

fOUT

=

1MHz

•

50kΩ

•





RVCO

NDIV • RVCO  RSET

+





VIN

VSET

−



1

•

R4

R3







IF R4 = RVCO , THE EQUATION REDUCES TO:

R3 RSET

fOUT

=

1MHz • 50kΩ •

NDIV • RSET

VIN

VSET

= VIN • 25kHz/V

Figure 13. VCO with Reduced ∆VSET Sensitivity

6990fc

18

For more information www.linear.com/LTC6990

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