LTC6906 datasheet(18/30 Pages) AD

18 / 30 page

LTC6990

Rev. D

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APPLICATIONS INFORMATION

Step 2: Calculate KVCO and f(0V)

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

using Equations (3b) and (3c).

KVCO =

100kHz

−10kHz

−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 =

1MHz • 50k

8 • 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 =

1MHz •50k

8 • 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 ∆VSET

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 occur-

ring at the highest output frequency, the relative frequency

error (in percent) is greatly improved.

Figure 13. VCO with Reduced ∆VSET Sensitivity

6990 F13

LTC6990

GND

SET

OUT

V+

DIV

0.1µF

280k

DIVCODE = 3

(NDIV = 8, Hi-Z = 0)

RSET

249k

RVCO

75k

–

10kHz TO 100kHz

fOUT

VCTRL

VSET

1/2

LTC6078

30.1k

33pF

100k

0.4V TO 4V

VIN

fOUT =

1MHz •50k

NDIV •RVCO

•

RVCO

RSET

VIN

VSET

•

)

RVCO

RSET

, THE EQUATION REDUCES TO:

fOUT =

1MHz • 50k

NDIV •RSET

•

VIN

VSET

= VIN •25kHz/V

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