LTC6906 Linear Technology, LTC6906 Datasheet - Page 11

• Manufacturer Part Number: LTC6906
• Description: Micropower
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
unpredictable oscillator behavior. Accuracy under higher
supplies may be estimated from the typical Frequency vs
Supply Voltage curves in the Typical Performance Charac-
teristics section of this data sheet.
Alternative Methods for Setting the Output Frequency
Any means of sinking current from the SET pin will control
the output frequency of the LTC6906. Equation 2 (re-
peated below) gives the fundamental relationship between
frequency and the SET pin voltage and current:
This equation shows that the LTC6906 converts conduc-
tance (I
resistance (R
V
it is given approximately by:
where
V
response of V
SET
SET
V
I
(I
t
V
S
Figure 16. Using the LTC6906 at Higher Supply Voltages
OSC
T
S
SET
varies with temperature and the SET pin current. The
is the voltage across an internal diode, and as such
≅ 82 • 10
= kT/q = 25.9mV at T = 300°K (27°C)
is also temperature dependent)
SET
=
≅
≅
25 9
ƒ
/V
V
T
OSC
3.6V TO 5.5V DC
1
SET
.
SET
•
SET
–18
Log
mV Log
) to frequency or, equivalently, converts
=
= V
V
to temperature is shown in the Typical
Amps
+
100Ω
e
V
•
U
I
1µF
SET
SET
SET
I
SET
I
S
/I
e
•
SET
U
⎛
⎜
⎝
10
82 10
) to period.
V
GND
DIV
pF
+
LTC6906
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
•
I
SET
GRD
OUT
SET
–
W
18
A
⎞
⎟
⎠
6906 F16
R
SET
– .
2 3
U
mV C
/
°
(2)
Performance graphs. V
°C. At room temperature V
60mV/decade of increase in I
If the SET pin is driven with a current source generating
I
Figure 17 and Figure 18 show a current controlled oscilla-
tor and a voltage controlled oscillator. These circuits are
not highly accurate if used alone, but can be very useful if
they are enclosed in an overall feedback circuit such as a
phase-locked loop.
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 for typical jitter at various divider ratios.
SET
ƒ
, the oscillator output frequency will be:
OSC
≅
Figure 18. Voltage Controlled Oscillator
25 9
Figure 17. Current Controlled Oscillator
V
V
.
+
+
mV
V
GND
DIV
V
GND
DIV
•
+
LTC6906
+
LTC6906
SET
n
GRD
⎛
⎜
⎝
6906 F18
OUT
GRD
SET
OUT
SET
changes approximately –2.3mV/
82 10
SET
•
SET
10
I
I
increases 18mV/octave or
SET
SET
R
100k
100kHz TO 1MHz
6906 F17
SET
pF
.
I
0.65µA TO 6.5µA
CTRL
1MHz TO 100kHz
–
18
V
0V TO 0.585V
A
CTRL
⎞
⎟
⎠
LTC6906
– .
2 3
mV C
/
11
°
6906fa