LTC6909 Linear Dimensions Semiconductor, LTC6909 Datasheet - Page 12

• Manufacturer Part Number: LTC6909
• Description: Multiphase Silicon Oscillator
• Manufacturer: Linear Dimensions Semiconductor

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LTC6909
APPLICATIONS INFORMATION
Figure 5 shows the LTC6909 confi gured as a V
age source is connected in series with an external 10k
resistor. The master oscillator frequency, f
vary with V
between V
SELECTING THE FREQUENCY-SETTING RESISTOR
The LTC6909 has a master oscillator frequency range
spanning 100kHz to 20MHz depending on the R
tor value. However, accuracy may suffer if the oscillator
is operated at a master oscillator frequency greater than
10MHz with a supply voltage lower than 4V. With a linear
correspondence between the master oscillator period and
the R
with frequency.
Any R
by the same amount.
ALTERNATIVE METHODS OF SETTING THE OUTPUT
FREQUENCY OF THE LTC6909
The oscillator may be programmed by any method that
sources a current into the SET pin. The circuit in Figure 4
sets the oscillator frequency using a programmable cur-
rent source and in the expression for f
R
explained in the Operation section, the voltage difference
between V
fore, the Figure 4 circuit is less accurate than if a resistor
controls the output frequency.
12
SET
R
R
R
SET
SETMIN
SETMAX
SET
is replaced by the ratio of 1.1V/I
SET
=10k • 20MHz/f
resistance, a simple equation relates resistance
resistor tolerance will shift the output frequency
+
+
CONTROL
Figure 4. Current Controlled Oscillator
= 10k (5V supply), 20k (2.7V supply),
and the SET pin. Again, this circuit decouples
= 2M
and SET is approximately 1.1V ±5%, there-
f
MASTER
I
CONTROL
= 10k • (20MHz/1.13V) • I
V
+
, that is the voltage source connected
MASTER
C
BYP
V
GND
SET
+
A
CONTROL
6909 F04
CONTROL
OUT
(A)
, the resistor
MASTER
. As already
CO
SET
. A volt-
resis-
, will
the relationship between the input current and the volt-
age between V
degraded. The oscillator frequency, however, will increase
monotonically with decreasing V
SETTING THE MODULATION RATE OF THE LTC6909
The modulation rate of the LTC6909 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
one of the active 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.
When the MOD pin is fl oated, for the f
rate, it must be bypassed by at least a 1nF capacitor to
GND. Any AC signal coupling to the MOD pin could poten-
tially be detected and stop the frequency modulation.
DRIVING LOGIC CIRCUITS
The outputs of the LTC6909 are suitable for driving gen-
eral digital logic circuits. However, the form of frequency
spreading used in the LTC6909 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 LTC6909 uses a
+
for the N = 64 setting. To disable the SSFM, connect
Figure 5. Voltage Controlled Oscillator
f
OUT
V
+
CONTROL
= 10k • 20MHz/R
and SET; the frequency accuracy will be
V
+
+
–
R
C
SET
SET
BYP
(1 – V
V
GND
SET
CONTROL
CONTROL
+
A
6909 F05
OUT
/1.13V)
/32 modulation
.
OUT
6909f
/N,