Datasheet LTC6908-1, LTC6908-2 (Analog Devices) - 9

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APPLICATIONS INFORMATION SELECTING THE FREQUENCY-SETTING RESISTOR
CBYP OUT1 V+ V+ OUT1 The LTC6908 has an output frequency range spanning 50kHz to 10MHz. However, accuracy may suffer if the V + CONTROL GND OUT2 – OUT2 oscillator is operated at a frequency greater than 5MHz V+ with a supply voltage lower than 4V. With a linear rela- SET MOD FLOAT tionship correspondence between oscillation period and RSET 690812 F06 resistance, a simple equation relates resistance with fOUT = 10k • 10MHz/RSET(1 – VCONTROL/1.13V) frequency.
Figure 6. Voltage Controlled Oscillator
RSET =10k • 10MHz/fOUT V+ and the SET pin. Again, this circuit decouples the rela- RSETMIN = 10k (5V supply), 20k (3V supply), tionship between the input current and the voltage between RSETMAX = 2M V+ and SET; the frequency accuracy will be degraded. The Any resistor, R oscillator frequency, however, will increase monotonically SET, tolerance will shift the output fre- quency, f with decreasing V OUT. CONTROL.
ALTERNATIVE METHODS OF SETTING THE OUTPUT SETTING THE MODULATION RATE OF THE LTC6908 FREQUENCY OF THE LTC6908
The modulation rate of the LTC6908 is equal to fOUT/N, The oscillator may be programmed by any method that where N is the modulation rate divider setting, which is sources a current into the SET pin. The circuit in Figure 5 determined by the state of the MOD pin. The MOD pin sets the oscillator frequency using a programmable cur- should be tied to ground for the N = 16 setting. Floating rent source and in the expression for f the MOD pin selects N = 32. The MOD pin should be tied OUT, the resistor R to V+ for the N = 64 setting. To disable the SSFM, connect SET is replaced by the ratio of 1.1V/ICONTROL. As already explained in the Operation section, the voltage differ- one of the outputs to the MOD pin. An AC detector circuit ence between V+ and SET is approximately 1.1V ±5%, shuts down the modulation circuitry if a frequency that is close to the output frequency is detected at the MOD pin. CBYP OUT1
DRIVING LOGIC CIRCUITS
V+ V+ OUT1 The outputs of the LTC6908 are suitable for driving gen- ICONTROL GND OUT2 eral digital logic circuits. However, the form of frequency OUT2 V+ spreading used in the LTC6908 may not be suitable for SET MOD FLOAT many logic designs. Many logic designs have fairly tight 690812 F05 timing and cycle-to-cycle jitter requirements. These sys- fOUT = 10k • (10MHz/1.13V) • ICONTROL(A) tems often benefit from a spread spectrum clocking sys-
Figure 5. Current Controlled Oscillator
tem where the frequency is slowly and linearly modulated by a triangular waveform, not a pseudorandom waveform. therefore, the Figure 5 circuit is less accurate than if a This type of frequency spreading maintains a minimal dif- resistor controls the output frequency. ference in the timing from one clock edge to the next adja- Figure 6 shows the LTC6908 configured as a VCO. A cent clock edge (cycle-to-cycle jitter). The LTC6908 uses voltage source is connected in series with an external a pseudorandom modulating signal where the frequency 10k resistor. The output frequency, f transitions have been slowed and the corners rounded OUT, will vary with V by a first order lowpass filter with a corner frequency set CONTROL, that is the voltage source connected between Rev B For more information www.analog.com 9 Document Outline Features Applications Description Absolute Maximum Ratings Electrical Characteristics Pin Functions Operation Applications Information Typical Applications Package Description Typical Application