LTC6905

6

6905fa

APPLICATIO S I FOR ATIO

SELECTING THE DIVIDER SETTING AND RESISTOR

The LTC6905’s master oscillator has a frequency range

spanning 68.9MHz to 170MHz. A programmable divider

extends the frequency range from 17.225MHz to 170MHz.

Table 1 describes the recommended frequencies for each

divider setting. Note that the ranges overlap; at some

frequencies there are two divider/resistor combinations

that result in the same frequency. Choosing a higher

divider setting will result in less jitter at the expense of

slightly higher supply current.

Table 1. Frequency Range vs Divider Setting

DIVIDER SETTING

FREQUENCY RANGE

÷1

⇒

68.9MHz to 170MHz

÷2

⇒

DIV (Pin 4) = Floating

34.45MHz to 85MHz

÷4

⇒

DIV (Pin 4) = GND

17.225MHz to 43MHz

After choosing the proper divider setting, determine the

correct frequency-setting resistor. Because of the linear

correspondence between oscillation period and resis-

tance, a simple equation relates resistance with frequency.

R

k

N

MHz

f

MHz

SET

OSC

=

⎛

⎝

⎜

⎞

⎠

⎟

⎧

⎨

⎪

⎩⎪

10

168 5

15

1

2

4

•

.

–.

, N =

START-UP TIME

The start-up time and settling time to within 1% of the final

frequency is typically 100µs.

MAXIMUM OUTPUT LOAD

The LTC6905 output (Pin 5) can drive a capacitive load

to 90% of the rise or fall transition). If the total output rise

time plus fall time is arbitrarily specified to be equal to or

Example: An LTC6905 is operating with a 3V power supply

the Typical Performance Characteristics).

than [45454/(27 • 50)] = 33.6pF.

The lowest resistive load Pin 5 can drive can be calculated

using the minimum high level output voltage in the Elec-

output current, the minimum high level output voltage is

5V and the lowest resistive load Pin 5 can drive is 1.25k

current, the minimum high level output voltage is 1.9V and

the lowest resistive load Pin 5 can drive is 475Ω(1.9V/4mA).

FREQUENCY ACCURACY AND POWER SUPPLY NOISE

The frequency accuracy of the LTC6905 may be affected

when its power supply generates noise with frequency

LTC6905 master oscillator frequency before the divider

quency). If for example, the master oscillator frequency is

set equal to 80MHz and the LTC6905 is powered by a

switching regulator, then the oscillator frequency may

show an additional error if the switching frequency is

1.4MHz (80MHz/64).

JITTER AND POWER SUPPLY NOISE

If the LTC6905 is powered by a supply that has frequency

contents equal to the output frequency then the oscillators

jitter may increase. In addition, power supply ripple in

excess of 20mV at any frequency may increase jitter.

JITTER AND DIVIDE RATIO

At a given output frequency, a higher master oscillator

frequency and a higher divide ratio will result in lower jitter

and higher power supply dissipation. Indeterminate jitter

percentage will decrease by a factor of slightly less than

the square root of the divider ratio, while determinate jitter

will not be similarly attenuated. Please consult the speci-

fication tables and Jitter vs Frequency graph showing jitter

at various divider ratios.