LTC6905IS5 Linear Technology, LTC6905IS5 Datasheet - Page 8

• Manufacturer Part Number: LTC6905IS5
• Manufacturer: Linear Technology
• Type: Silicon Oscillatorr

Specifications of LTC6905IS5

• Mounting Style: Surface Mount
• Screening Level: Industrial
• Product Length (mm): 2.9mm
• Product Depth (mm): 1.75mm
• Product Height (mm): 0.9mm
• Package / Case: TSOT-23
• Lead Free Status / RoHS Status: Not Compliant

APPLICATIONS INFORMATION
LTC6905
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 fre-
quencies 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
÷1
÷2
÷4
After choosing the proper divider setting, determine the
correct frequency-setting resistor. Because of the linear
correspondence between oscillation period and resistance,
a simple equation relates resistance with frequency.
Any resistor, R
the oscillator. f
START-UP TIME
The start-up time and settling time to within 1% of the
fi nal frequency is typically 100μs.
MAXIMUM OUTPUT LOAD
The LTC6905 output (Pin 5) can drive a capacitive load
(C
depends on the oscillator’s frequency (f
resistance (R
R
8
OUT
LOAD
(R
R
SET
SETMIN
⇒
⇒
⇒
and C
) of 5pF or more. Driving a C
=
10
LOAD
DIV (Pin 4) = V
DIV (Pin 4) = Floating
DIV (Pin 4) = GND
= 10k, R
OUT
k
OSC
•
SET
is equal to 2.2 • R
⎛
⎝ ⎜
). The output rise time or fall time due to
f
, tolerance adds to the inaccuracy of
.
OSC
SETMAX
168 5
• – .
+
N
.
MHz
1 5
= 25k)
MHz
OUT
LOAD
FREQUENCY RANGE
68.9MHz to 170MHz
34.45MHz to 85MHz
17.225MHz to 43MHz
⎞
⎠ ⎟
• C
, = ⎨ ⎨
N
greater than 5pF
OSC
LOAD
⎧
⎪
⎪
⎩
) and output
1
2
4
(from 10%
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
or less than 20% of the oscillator’s period (1/f
the maximum output C
equal to or less than [45454/(R
and f
Example: An LTC6905 is operating with a 3V power supply
and is set for a f
R
the Typical Performance Characteristics).
The maximum output C
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 Electrical
Characteristics. With a V
current, the minimum high level output voltage is 5V and
the lowest resistive load Pin 5 can drive is 1.25k (5V/4mA).
With a V
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
contents equal to f
internal LTC6905 master oscillator frequency before the
divider and f
frequency). 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.
OUT
OSC
with V
in MHz).
+
equal to 2.7V and 4mA output current, the
+
MO
= 3V is 27Ω (using the R
/64 is the master oscillator control loop
OSC
= 50MHz.
MO
LOAD
/64 or its multiples (f
LOAD
+
equal to 5.5V and 4mA output
in picofarads (pF) should be
should be equal to or less
OUT
• f
OSC
OUT
)] (R
vs V
OUT
OSC
MO
+
graph in
in ohms
), then
is the
6905fd