LTC6908-1/LTC6908-2

9

Rev C

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SELECTING THE FREQUENCY-SETTING RESISTOR

The LTC6908 has an output frequency range spanning

50kHz to 10MHz. However, accuracy may suffer if the

oscillator is operated at a frequency greater than 5MHz

with a supply voltage lower than 4V. With a linear rela-

tionship correspondence between oscillation period and

resistance, a simple equation relates resistance with

frequency.

RSET =10k • 10MHz/fOUT

RSETMIN = 10k (5V supply), 20k (3V supply),

RSETMAX = 2M

Any resistor, RSET, tolerance will shift the output fre-

quency, fOUT.

ALTERNATIVE METHODS OF SETTING THE OUTPUT

FREQUENCY OF THE LTC6908

The oscillator may be programmed by any method that

sources a current into the SET pin. The circuit in Figure 5

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 differ-

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

Figure 5. Current Controlled Oscillator

therefore, the Figure 5 circuit is less accurate than if a

resistor controls the output frequency.

Figure 6 shows the LTC6908 configured as a VCO. A

voltage source is connected in series with an external

10k resistor. The output frequency, fOUT, will vary with

VCONTROL, that is the voltage source connected between

Figure 6. Voltage Controlled Oscillator

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

tionship between the input current and the voltage 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 LTC6908

The modulation rate of the LTC6908 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 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.

DRIVING LOGIC CIRCUITS

The outputs of the LTC6908 are suitable for driving gen-

eral digital logic circuits. However, the form of frequency

spreading used in the LTC6908 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 sys-

tem where the frequency is slowly and linearly modulated

by a triangular waveform, not a pseudorandom waveform.

This type of frequency spreading maintains a minimal dif-

ference in the timing from one clock edge to the next adja-

cent clock edge (cycle-to-cycle jitter). The LTC6908 uses

a pseudorandom modulating signal where the frequency

transitions have been slowed and the corners rounded

by a first order lowpass filter with a corner frequency set

CBYP

ICONTROL

OUT1

OUT2

OUT1

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

OUT2

MOD

V+

V+

V+

FLOAT

GND

SET

690812 F05

CBYP

VCONTROL

OUT1

OUT2

OUT1

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

OUT2

MOD

V+

V+

V+

FLOAT

GND

SET

690812 F06

RSET

+–

APPLICATIONS INFORMATION