LTC6908CS6-2 datasheet(9/16 Pages) LINER

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LTC6908-1/LTC6908-2

690812fa

APPLICATIO S I FOR ATIO

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 relationship

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 frequency,

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 current

source and in the expression for fOUT, the resistor RSET is

replaced by the ratio of 1.1V/ICONTROL.Asalreadyexplained

in the Operation section, the voltage difference between

V+ and SET is approximately 1.1V ±5%, therefore, the

Figure 5 circuit is less accurate than if a resistor controls

the output frequency.

Figure 5. Current Controlled Oscillator

Figure 6 shows the LTC6908 conﬁgured as a VCO. A voltage

source is connected in series with an external 10k resis-

tor. The output frequency, fOUT, will vary with VCONTROL,

that is the voltage source connected between V+ and the

SET pin. Again, this circuit decouples the relationship

between the input current and the voltage between V+

Figure 6. Voltage Controlled Oscillator

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 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 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

to the modulation rate (fOUT/N), where N is the modula-

tion rate divider setting, which is determined by the state

CBYP

ICONTROL

OUT1

OUT2

OUT1

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

OUT2

MOD

V+

FLOAT

GND

SET

690812 F05

CBYP

VCONTROL

OUT1

OUT2

OUT1

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

OUT2

MOD

V+

FLOAT

GND

SET

690812 F06

RSET

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