Datasheet LTC6905 (Analog Devices) - 8

LTC6905
APPLICATIONS INFORMATION SELECTING THE DIVIDER SETTING AND RESISTOR
to 90% of the rise or fall transition). If the total output rise time plus fall time is arbitrarily specifi ed to be equal to The LTC6905’s master oscillator has a frequency range or less than 20% of the oscillator’s period (1/fOSC), then spanning 68.9MHz to 170MHz. A programmable divider the maximum output CLOAD in picofarads (pF) should be extends the frequency range from 17.225MHz to 170MHz. equal to or less than [45454/(ROUT • fOSC)] (ROUT in ohms Table 1 describes the recommended frequencies for each and fOSC in MHz). divider setting. Note that the ranges overlap; at some fre- quencies there are two divider/resistor combinations that Example: An LTC6905 is operating with a 3V power supply result in the same frequency. Choosing a higher divider and is set for a fOSC = 50MHz. setting will result in less jitter at the expense of slightly ROUT with V+ = 3V is 27Ω (using the ROUT vs V+ graph in higher supply current. the Typical Performance Characteristics).
Table 1. Frequency Range vs Divider Setting
The maximum output CLOAD should be equal to or less
DIVIDER SETTING FREQUENCY RANGE
than [45454/(27 • 50)] = 33.6pF. ÷1 ⇒ DIV (Pin 4) = V+ 68.9MHz to 170MHz The lowest resistive load Pin 5 can drive can be calculated ÷2 ⇒ DIV (Pin 4) = Floating 34.45MHz to 85MHz using the minimum high level output voltage in the Electrical ÷4 ⇒ DIV (Pin 4) = GND 17.225MHz to 43MHz Characteristics. With a V+ equal to 5.5V and 4mA output After choosing the proper divider setting, determine the current, the minimum high level output voltage is 5V and correct frequency-setting resistor. Because of the linear the lowest resistive load Pin 5 can drive is 1.25k (5V/4mA). correspondence between oscillation period and resistance, With a V+ equal to 2.7V and 4mA output current, the a simple equation relates resistance with frequency. minimum high level output voltage is 1.9V and the lowest resistive load Pin 5 can drive is 475Ω (1.9V/4mA). ⎧1 ⎛ 168. MHz 5 ⎞ ⎪ R = k 10 • , N SET ⎜ ⎟ = 2 ⎨
FREQUENCY ACCURACY AND POWER SUPPLY NOISE
⎝ f • N – 1. M 5 Hz⎠ OSC ⎪4 ⎩ The frequency accuracy of the LTC6905 may be affected (R when its power supply generates noise with frequency SETMIN = 10k, RSETMAX = 25k) contents equal to fMO/64 or its multiples (fMO is the Any resistor, RSET , tolerance adds to the inaccuracy of internal LTC6905 master oscillator frequency before the the oscillator. fOSC. divider and fMO/64 is the master oscillator control loop frequency). If for example, the master oscillator frequency
START-UP TIME
is set equal to 80MHz and the LTC6905 is powered by a The start-up time and settling time to within 1% of the switching regulator, then the oscillator frequency may fi nal frequency is typically 100μs. show an additional error if the switching frequency is 1.4MHz (80MHz/64).
MAXIMUM OUTPUT LOAD JITTER AND POWER SUPPLY NOISE
The LTC6905 output (Pin 5) can drive a capacitive load (C If the LTC6905 is powered by a supply that has frequency LOAD) of 5pF or more. Driving a CLOAD greater than 5pF depends on the oscillator’s frequency (f contents equal to the output frequency then the oscillators OSC) and output resistance (R jitter may increase. In addition, power supply ripple in OUT). The output rise time or fall time due to R excess of 20mV at any frequency may increase jitter. OUT and CLOAD is equal to 2.2 • ROUT • CLOAD (from 10% 6905fd 8