Datasheet LTC6905 (Analog Devices) - 10

LTC6905
APPLICATIONS INFORMATION ALTERNATIVE METHODS OF SETTING THE OUTPUT
Figure 3 shows a method to control the frequency of the
FREQUENCY OF THE LTC6905
LTC6905 using a current source. RSET , in this case, sets a maximum frequency according to the regular expression The LTC6905 may be programmed by any method that for f sources a current into the SET pin (Pin 3). The accuracy OSC. The current source will subtract current from the SET pin to lower the frequency. of the programming is best with a simple resistor because the LTC6905 takes into account both the voltage at the SET Figure 4 shows a method for controlling the frequency pin and the current into the SET pin when generating the of the LTC6905 using a voltage source. In this case, RSET output frequency. Since the voltage at the SET pin can sets a constant current into the SET pin, and RCNTRL will vary by as much as 5%, setting the frequency using a subtract from this current in order to change the frequency. current rather than a resistor will result in as much as 5% Increasing VCNTRL will increase the output frequency. additional inaccuracy in the output frequency. ⎡ ⎡ V+ – V ⎤ ⎤ ⎢168. MHz 5 • 1 k SET 0 Ω • ⎢ – I ⎥ ⎥ R CNTRL 1 V+ ⎢ ⎢ SET ⎥ ⎥ 1 5 f ⎣ ⎦ OSC = + 1. MHz 5 V+ OUT N ⎢ ⎥ + 0.1μF LTC6905 ⎢ V – VSET ⎥ f 2 OSC GND 69.8MHz TO 170MHz ⎢ ⎥ RSET 10k ⎣ ⎦ 3 4 V+ SET DIV N = 1 I I CNTRL CNTRL Frequency ≤ 100kHz 0μA TO 60μA 6905 F03
Figure 3. Current Controlled Oscillator
Example (Figure 3): VSET = (V+ – 1V), RSET = 10k, N = 1 fOSC = [168.5MHz • (1 – 10kΩ • ICNTRL) + 1.5MHz ⎡ ⎡ V+ – V V – V ⎤ ⎤ V+ = 3V ⎢168. MHz 5 • 1 k SET SET CNTR 0 Ω • 1 5 ⎢ – L ⎥ ⎥ V+ OUT R 1 ⎢ ⎢ SET CN R TRL ⎥ ⎥ 0.1μF LTC6905 f ⎣ ⎦ fOSC OSC = + 1. MHz 5 2 N ⎢ ⎥ 69.8MHz TO 170MHz V+ R GND ⎢ – SE V T ⎥ SET 10k 3 4 ⎢ ⎥ V+ SET DIV N = 1 ⎣ ⎦ RCNTRL 6905 F04 33.2k VCNTRL Frequency ≤ 100kHz V + CNTRL – 0V TO 2V Example (Figure 4): VSET = (V+ – 1V), RSET = 10k, RCNTRL = 33.2k,
Figure 4. Voltage Controlled Oscillator
N = 1, V+ = 3V ⎡ ⎡ 1 2V – V ⎤ ⎤ f = 168. M 5 Hz • 1 k CNTRL OSC 0 ⎢ Ω • ⎢ – 1.5MHz⎥ k 10 ⎥ + Ω 33. k 2 Ω ⎣ ⎣ ⎦ ⎦ 6905fd 10