LTC6905C データシートの表示（PDF） - Linear Technology
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LTC6905C Datasheet PDF : 12 Pages
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LTC6905
APPLICATIONS INFORMATION
ALTERNATIVE METHODS OF SETTING THE OUTPUT
FREQUENCY OF THE LTC6905
The LTC6905 may be programmed by any method that
sources a current into the SET pin (Pin 3). The accuracy
of the programming is best with a simple resistor because
the LTC6905 takes into account both the voltage at the SET
pin and the current into the SET pin when generating the
output frequency. Since the voltage at the SET pin can
vary by as much as 5%, setting the frequency using a
current rather than a resistor will result in as much as 5%
additional inaccuracy in the output frequency.
Figure 3 shows a method to control the frequency of the
LTC6905 using a current source. RSET, in this case, sets a
maximum frequency according to the regular expression
for fOSC. The current source will subtract current from the
SET pin to lower the frequency.
Figure 4 shows a method for controlling the frequency
of the LTC6905 using a voltage source. In this case, RSET
sets a constant current into the SET pin, and RCNTRL will
subtract from this current in order to change the frequency.
Increasing VCNTRL will increase the output frequency.
V+
1
0.1μF
2
RSET
10k
3
ICNTRL
0μA TO 60μA
V+ OUT
LTC6905
GND
5
fOSC
69.8MHz TO 170MHz
4
SET DIV
6905 F03
V+
N=1
Figure 3. Current Controlled Oscillator
V+ = 3V
0.1μF
RSET
10k
RCNTRL
33.2k
+–
VCNTRL
0V TO 2V
1 V+
5
OUT
2 LTC6905
GND
fOSC
69.8MHz TO 170MHz
3
4
SET DIV
6905 F04
V+
N=1
Figure 4. Voltage Controlled Oscillator
fOSC =
1
⎡
⎢
⎢
168.5MHz
•
10kΩ
•
⎡
⎢
⎣⎢
V+ – VSET
RSET
N
⎢
⎢
V+ – VSET
⎤
– ICNTRL ⎥
⎦⎥
⎤
⎥
+ 1.5MHz⎥⎥
⎥
⎣⎢
⎦⎥
ICNTRL Frequency ≤ 100kHz
Example (Figure 3): VSET = (V+ – 1V), RSET = 10k, N = 1
fOSC = [168.5MHz • (1 – 10kΩ • ICNTRL) + 1.5MHz
fOSC =
1
⎡
⎢
⎢
168.5MHz
•
10kΩ
•
⎡
⎢
⎣⎢
V+ – VSET
RSET
N
⎢
⎢
V+ – VSET
–
VSET – VCNTRL
RCNTRL
⎤
⎥
⎦⎥
⎤
⎥
+ 1.5MHz⎥⎥
⎥
⎣⎢
⎦⎥
VCNTRL Frequency ≤ 100kHz
Example (Figure 4): VSET = (V+ – 1V), RSET = 10k, RCNTRL = 33.2k,
N = 1, V+ = 3V
fOSC
=
⎡
⎢168.5MHz
⎣
•
10kΩ
•
⎡1
⎢
⎣
10kΩ
–
2V – VCNTRL
33.2kΩ
⎤
⎥
⎦
+
⎤
1.5MHz⎥
⎦
6905fd
10
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