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LTC6909

6909f

SELECTING THE FREQUENCY-SETTING RESISTOR

The LTC6909 has a master oscillator frequency range

spanning 100kHz to 20MHz depending on the RSET resis-

tor value. However, accuracy may suffer if the oscillator

is operated at a master oscillator frequency greater than

10MHz with a supply voltage lower than 4V. With a linear

correspondence between the master oscillator period and

the RSET resistance, a simple equation relates resistance

with frequency.

RSET =10k • 20MHz/fMASTER

RSETMIN = 10k (5V supply), 20k (2.7V supply),

RSETMAX = 2M

Any RSET resistor tolerance will shift the output frequency

by the same amount.

ALTERNATIVE METHODS OF SETTING THE OUTPUT

FREQUENCY OF THE LTC6909

The oscillator may be programmed by any method that

sources a current into the SET pin. The circuit in Figure 4

sets the oscillator frequency using a programmable cur-

rent source and in the expression for fOUT, the resistor

RSET is replaced by the ratio of 1.1V/ICONTROL. As already

explained in the Operation section, the voltage difference

between V+ and SET is approximately 1.1V ±5%, there-

fore, the Figure 4 circuit is less accurate than if a resistor

controls the output frequency.

Figure 4. Current Controlled Oscillator

Figure 5 shows the LTC6909 conﬁgured as a VCO. A volt-

age source is connected in series with an external 10k

resistor. The master oscillator frequency, fMASTER, will

vary with VCONTROL, that is the voltage source connected

between V+ and the SET pin. Again, this circuit decouples

Figure 5. Voltage Controlled Oscillator

the relationship between the input current and the volt-

age between V+ and SET; the frequency accuracy will be

degraded. The oscillator frequency, however, will increase

monotonically with decreasing VCONTROL.

SETTING THE MODULATION RATE OF THE LTC6909

The modulation rate of the LTC6909 is equal to fOUT/N,

where N is the modulation rate divider setting, which is

determined by the state of the MOD pin. The MOD pin

should be tied to ground for the N = 16 setting. Floating

the MOD pin selects N = 32. The MOD pin should be tied

to V+ for the N = 64 setting. To disable the SSFM, connect

one of the active outputs to the MOD pin. An AC detector

circuit shuts down the modulation circuitry if a frequency

that is close to the output frequency is detected at the

MOD pin.

When the MOD pin is ﬂoated, for the fOUT/32 modulation

rate, it must be bypassed by at least a 1nF capacitor to

GND. Any AC signal coupling to the MOD pin could poten-

tially be detected and stop the frequency modulation.

DRIVING LOGIC CIRCUITS

The outputs of the LTC6909 are suitable for driving gen-

eral digital logic circuits. However, the form of frequency

spreading used in the LTC6909 may not be suitable for

many logic designs. Many logic designs have fairly tight

timing and cycle-to-cycle jitter requirements. These sys-

tems often beneﬁt from a spread spectrum clocking system

where the frequency is slowly and linearly modulated by a

triangular waveform, not a pseudorandom waveform. This

type of frequency spreading maintains a minimal difference

in the timing from one clock edge to the next adjacent

clock edge (cycle-to-cycle jitter). The LTC6909 uses a

CBYP

ICONTROL

fMASTER = 10k • (20MHz/1.13V) • ICONTROL(A)

V+A

V+

GND

SET

6909 F04

CBYP

VCONTROL

fOUT = 10k • 20MHz/RSET(1 – VCONTROL/1.13V)

V+A

V+

GND

SET

6909 F05

RSET

–

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