LTC6990CS6-PBF(V2) View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LTC6990CS6-PBF
(Rev.:V2)

TimerBlox: Voltage Controlled Silicon Oscillator

Linear Technology 

LTC6990

APPLICATIONS INFORMATION

Equation (2) can be re-written as shown below, where f(0V)

is the output frequency when VCTRL = 0V, and KVCO is the

frequency gain. Note that the gain is negative (the output

frequency decreases as VCTRL increases).

fOUT = f(0V) – K VCO • VCTRL

( ) f(0V)

=

1MHz • 50k

NDIV • RSET PRVCO

K VCO

=

1MHz • 50k

NDIV • VSET • RVCO

The design procedure for a VCO is a simple four step

process. First select the NDIV value. Then calculate the

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

select the RVCO resistor. Finally calculate and select the

RSET resistor.

Step 1: Select the NDIV Frequency Divider Value

For best accuracy, the master oscillator frequency should

fall between 62.5kHz and 1MHz. Since fMASTER = NDIV •

fOUT, choose a value for NDIV that meets the following

conditions

62.5kHz

fOUT(MIN)

≤ NDIV ≤

1MHz

fOUT(MAX )

(3a)

The 16:1 frequency range of the master oscillator and

the 2:1 divider step-size provides several overlapping fre-

quency spans to guarantee that any 8:1 modulation range

can be covered by a single NDIV setting. RVCO allows the

gain to be tailored to the application, mapping the VCTRL

voltage range to the modulation range.

Step 2: Calculate KVCO and f(0V)

KVCO and f(0V) define the VCO’s transfer function and

simplify the calculation of the the RVCO and RSET resis-

tors. Calculate these parameters using the following

equations.

K VCO

=

fOUT(MAX )

VCTRL(MAX )

−

−

fOUT(MIN)

VCTRL(MIN)

(3b)

f(0V) = fOUT(MAX) + KVCO • VCTRL(MIN)

(3c)

16

KVCO and f(0V) are not device settings or resistor values

themselves. However, beyond their utility for the resistor

calculations, these parameters provide a useful and intuitive

way to look at the VCO application. The f(0V) parameter is

the output frequency when VCTRL is at 0V. Viewed another

way, it is the fixed output frequency when the RVCO and

RSET resistors are in parallel. KVCO is actually the frequency

gain of the circuit.

With KVCO and f(0V) determined, the RVCO and RSET values

can now be calculated.

Step 3: Calculate and Select RVCO

The next step is to calculate the correct value for RVCO

using the following equation.

RVCO

=

1MHz • 50k

NDIV • VSET • K VCO

(3d)

Select the standard resistor value closest to the calculated

value.

Step 4: Calculate and Select RSET

The final step is to calculate the correct value for RSET

using the following equation:

( ) RSET = NDIV •

1MHz • 50k

f(0V) − VSET • K VCO

(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. ΔVSET adds no frequency error when

VCTRL = 0V).

6990f

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