LTC6900 Linear Technology, LTC6900 Datasheet - Page 10

• Manufacturer Part Number: LTC6900
• Description: 1kHz to 20MHz Resistor Set SOT-23 Oscillator
• Manufacturer: Linear Technology

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LTC6900
APPLICATIO S I FOR ATIO
Once R
Example 1:
In this example, the oscillator output frequency has small
excursions. This is useful where the frequency of a system
should be tuned around some nominal value.
Let V
f
Equation (2), yielding R
Equation (4). R
resistor values, use R
(1%). Figure 10 shows the measured f
1.5MHz to 2MHz frequency excursion is quite limited, so
the curve of f
10
OSC(MIN)
R
SET
V
+
IN MAX
IN
= 3V, f
2.00
1.95
1.90
1.85
1.80
1.75
1.70
1.65
1.60
1.55
1.50
(
Figure 10. Output Frequency vs Input Voltage
/R
= 1.5MHz for V
10
SET
0
)
OSC
MHz
N
OSC(MAX)
is known, calculate R
V
0.5
V
IN
RES
vs V
•
U
= 9.9R
f
OSC MAX
1
IN
R
SET
R
V
R
R
V
N = 1
SET
IN
20
is linear.
IN
SET
+
RES
IN
V
= 2MHz for V
(
= 3V
IN
/R
IN
= 1.1M
1.5
U
k
= 110k (1%) and R
= 110k
SET
(V)
SET
= 0V. Solve for R
1
)
•
= 1.089M. For standard
2
= 9.9/1. R
R
R
SET
W
IN
2.5
SET
6900 F10
IN(MAX)
from:
OSC
3
SET
= 110.1k by
vs V
IN
U
IN
= 3V and
/R
= 1.1M
IN
SET
. The
(3)
by
Example 2:
Vary the oscillator frequency by one octave per volt.
Assume f
input voltage varies by 1V. The minimum input voltage is
half supply, that is V
V
Equation (2) yields R
yields R
standard resistor values, use R
182k (1%). Figure 11 shows the measured f
V
occur when V
VCO remains monotonic.
Maximum VCO Modulation Bandwidth
The maximum VCO modulation bandwidth is 25kHz; that
is, the LTC6900 will respond to changes in V
to 25kHz. In lower frequency applications however, the
modulation frequency may need to be limited to a lower
rate to prevent an increase in output jitter. This lower limit
is the master oscillator frequency divided by 20, (f
In general, for minimum output jitter the modulation
frequency should be limited to f
ever is less. For best performance at all frequencies, the
value for f
(N = 1) when V
+
IN
= 3V.
higher than 1.5V, the VCO is quite linear; nonlinearities
SET
Figure 11. Output Frequency vs Input Voltage
OSC(MIN)
OSC
3000
2500
2000
1500
1000
500
= 142.8k. R
0
IN
should be the master oscillator frequency
0
IN
becomes smaller than 1V, although the
= 1MHz and f
is at the lowest level.
0.5
IN(MIN)
IN
/R
1
IN
SET
V
= 1.5V, V
= 1.273R
IN
1.5
OSC(MAX)
= 1.273 and Equation (3)
(V)
SET
OSC
= 143k (1%) and R
R
R
V
N = 1
2
IN
SET
+
/20 or 25kHz, which-
= 3v
= 182k
IN(MAX)
= 143k
SET
2.5
= 2MHz, when the
6900 F11
= 181.8k. For
OSC
IN
3
= 2.5V and
at a rate up
vs V
OSC
IN
/20).
. For
IN
6900f
=