Datasheet

LTC6905
10
6905fd
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.
APPLICATIONS INFORMATION
Figure 3 shows a method to control the frequency of the
LTC6905 using a current source. R
SET
, in this case, sets a
maximum frequency according to the regular expression
for f
OSC
. 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, R
SET
sets a constant current into the SET pin, and R
CNTRL
will
subtract from this current in order to change the frequency.
Increasing V
CNTRL
will increase the output frequency.
f
N
MHz k
VV
R
I
OSC
SET
SET
CNTRL
=
Ω
+
1
168 5 10.•
+
+
VV
MHz
SET
.15
I
CNTRL
Frequency ≤ 100kHz
Example (Figure 3): V
SET
= (V
+
– 1V), R
SET
= 10k, N = 1
f
OSC
= [168.5MHz • (1 – 10kΩ • I
CNTRL
) + 1.5MHz
f
N
MHz k
VV
R
VV
OSC
SET
SET
SET CNTR
=
Ω
+
1
168 5 10.•
LL
CNTRL
SET
R
VV
MHz
+
+
.15
V
CNTRL
Frequency ≤ 100kHz
Example (Figure 4): V
SET
= (V
+
– 1V), R
SET
= 10k, R
CNTRL
= 33.2k,
N = 1, V
+
= 3V
fMHzk
k
VV
k
OSC
CNTRL
= Ω
ΩΩ
168 5 10
1
10
2
33 2
.•
.
+
15.MHz
V
+
1
2
3
5
f
OSC
69.8MHz TO 170MHz
V
+
0.1μF
I
CNTRL
0μA TO 60μA
R
SET
10k
6905 F03
4
GND
V
+
N = 1
LTC6905
SET
OUT
DIV
Figure 3. Current Controlled Oscillator
V
+
1
2
3
5
V
+
= 3V
0.1μF
R
SET
10k
V
CNTRL
0V TO 2V
6905 F04
4
GND
V
+
N = 1
LTC6905
SET
OUT
DIV
+
R
CNTRL
33.2k
f
OSC
69.8MHz TO 170MHz
Figure 4. Voltage Controlled Oscillator