LTC6906 datasheet(17/30 Pages) AD

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LTC6990

Rev. D

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Figure 12. VCO Transfer Function

VCTRL (V)

100

6990 F12

APPLICATIONS INFORMATION

Step 4: Calculate and Select RSET

The final step is to calculate the correct value for RSET

using the following equation:

RSET =

1MHz • 50k

NDIV • f(0V) – VSET •KVCO

(

)

(3e)

Select the standard resistor value closest to the calculated

value.

Some applications require combinations of fOUT(MIN),

fOUT(MAX), VCTRL(MIN) and VCTRL(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 VCTRL signal via range

scaling and/or level shifting to place the VCTRL into a range

that yields realistic resistor values.

Frequency Error in VCO Applications Due to VSET Error

As stated earlier, f(0V) represents the frequency for VCTRL

= 0V, which is the same value as would be generated by

a single resistor between SET and GND with a value of

RSET || RVCO. Therefore, f(0V) is not affected by error or

drift in VSET (i.e. ∆VSET adds no frequency error when

VCTRL = 0V).

The accuracy of KVCO does depend on VSET because

the output frequency is controlled by the ratio of VCTRL

to VSET. The frequency error (in Hertz) due to ∆VSET is

approximated by:

∆fOUT ≅KVCO • VCTRL •

∆VSET

VSET

As the equation indicates, the potential for error in output

frequency due to VSET error increases with KVCO and is

at its largest when VCTRL is at its maximum. Recall that

when VCTRL 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.

VSET is nominally 1.0V with a maximum error of ±30mV

for at most a ±3% error term. However, this ±3% potential

error term is multiplied by both VCTRL and KVCO. Wide

frequency range applications (high KVCO) can have fre-

quency errors greater than ±50% at the highest VCTRL

voltage (lowest fOUT). For this reason the simple, two

resistor VCO circuit must be used with caution for appli-

cations where the frequency range is greater than 4:1.

Restricting the range to 4:1 typically keeps the frequency

error due to VSET variation below 10%.

For wide frequency range applications, the non-invert-

ing VCO circuit shown in Figure 13 is preferred because

the maximum frequency error occurs when the fre-

quency is highest, keeping the relative error (in percent)

much smaller.

Example: Design a VCO with the Following Parameters

fOUT(MAX) = 100kHz at VCTRL(MIN) = 1V

fOUT(MIN) = 10kHz at VCTRL(MAX) = 4V

Step 1: Select the NDIV Value

First, choose an NDIV that meets the requirements of

Equation (3a).

6.25 ≤ NDIV ≤ 10

The application’s desired frequency range is 10:1, which

isn’t always possible. However, in this case NDIV = 8 meets

both requirements of Equation (3).

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