LTC6908IDCB-2#PBF Linear Technology, LTC6908IDCB-2#PBF Datasheet - Page 9

• Manufacturer Part Number: LTC6908IDCB-2#PBF
• Manufacturer: Linear Technology
• Type: Silicon Oscillatorr

Specifications of LTC6908IDCB-2#PBF

• Mounting Style: Surface Mount
• Screening Level: Industrial
• Product Depth (mm): 3mm
• Lead Free Status / RoHS Status: Compliant

APPLICATIO S I FOR ATIO
Figure 6 shows the LTC6908 confi gured as a VCO. A voltage
source is connected in series with an external 10k resis-
tor. The output frequency, f
that is the voltage source connected between V
SET pin. Again, this circuit decouples the relationship
between the input current and the voltage between V
SELECTING THE FREQUENCY-SETTING RESISTOR
The LTC6908 has an output frequency range spanning
50kHz to 10MHz. However, accuracy may suffer if the
oscillator is operated at a frequency greater than 5MHz with
a supply voltage lower than 4V. With a linear relationship
correspondence between oscillation period and resistance,
a simple equation relates resistance with frequency.
Any resistor, R
f
ALTERNATIVE METHODS OF SETTING THE OUTPUT
FREQUENCY OF THE LTC6908
The oscillator may be programmed by any method that
sources a current into the SET pin. The circuit in Figure 5
sets the oscillator frequency using a programmable current
source and in the expression for f
replaced by the ratio of 1.1V/I
in the Operation section, the voltage difference between
V
Figure 5 circuit is less accurate than if a resistor controls
the output frequency.
OUT
+
R
R
R
and SET is approximately 1.1V ±5%, therefore, the
I
.
SET
SETMIN
SETMAX
CONTROL
V
f
=10k • 10MHz/f
OUT
+
= 10k • (10MHz/1.13V) • I
Figure 5. Current Controlled Oscillator
= 2M
= 10k (5V supply), 20k (3V supply),
SET
C
BYP
, tolerance will shift the output frequency,
U
V
GND
SET
+
OUT
U
CONTROL
690812 F05
OUT1
OUT2
OUT
MOD
CONTROL
, will vary with V
(A)
OUT
W
. As already explained
, the resistor R
V
FLOAT
+
U
+
CONTROL
and the
OUT1
OUT2
SET
is
+
,
and SET; the frequency accuracy will be degraded. The
oscillator frequency, however, will increase monotonically
with decreasing V
SETTING THE MODULATION RATE OF THE LTC6908
The modulation rate of the LTC6908 is equal to f
where N is the modulation rate divider setting, which is
determined by the state of the MOD pin. The MOD pin should
be tied to ground for the N = 16 setting. Floating the MOD
pin selects N = 32. The MOD pin should be tied to V
the N = 64 setting. To disable the SSFM, connect one of
the outputs to the MOD pin. An AC detector circuit shuts
down the modulation circuitry if a frequency that is close
to the output frequency is detected at the MOD pin.
DRIVING LOGIC CIRCUITS
The outputs of the LTC6908 are suitable for driving gen-
eral digital logic circuits. However, the form of frequency
spreading used in the LTC6908 may not be suitable for
many logic designs. Many logic designs have fairly tight
timing and cycle-to-cycle jitter requirements. These sys-
tems often benefi t from a spread spectrum clocking system
where the frequency is slowly and linearly modulated by a
triangular waveform, not a pseudorandom waveform. This
type of frequency spreading maintains a minimal difference
in the timing from one clock edge to the next adjacent
clock edge (cycle-to-cycle jitter). The LTC6908 uses a
pseudorandom modulating signal where the frequency
transitions have been slowed and the corners rounded
by a fi rst order lowpass fi lter with a corner frequency set
to the modulation rate (f
tion rate divider setting, which is determined by the state
V
CONTROL
f
OUT
= 10k • 10MHz/R
V
+
+
–
Figure 6. Voltage Controlled Oscillator
LTC6908-1/LTC6908-2
R
C
SET
BYP
SET
CONTROL
(1 – V
V
GND
SET
+
CONTROL
OUT
.
690812 F06
OUT1
OUT2
MOD
/N), where N is the modula-
/1.13V)
V
FLOAT
+
OUT1
OUT2
OUT
690812fa
+
9
/N,
for