LTC6909HMS#PBF Linear Technology, LTC6909HMS#PBF Datasheet - Page 12

• Manufacturer Part Number: LTC6909HMS#PBF
• Description: IC OSCILLATOR SIL 5.5V 16-MSOP
• Manufacturer: Linear Technology
• Type: Oscillator, Siliconr

Specifications of LTC6909HMS#PBF

• Frequency: 6.67MHz
• Voltage - Supply: 2.7 V ~ 5.5 V
• Current - Supply: 2.4mA
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 16-MSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Count: -

LTC6909
ApplicAtions inForMAtion
Figure 5 shows the LTC6909 configured 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 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 4 circuit is less accurate than if a resistor controls
the output frequency.
12
+
R
R
R
and SET is approximately 1.1V ±5%, therefore, the
SET
SETMIN
SETMAX
SET
SET
=10k • 20MHz/f
resistance, a simple equation relates resistance
resistor tolerance will shift the output frequency
+
CONTROL
= 10k (5V supply), 20k (2.7V supply),
Figure 4. Current Controlled Oscillator
and the SET pin. Again, this circuit decouples
= 2M
f
MASTER
I
CONTROL
= 10k • (20MHz/1.13V) • I
V
, that is the voltage source connected
+
MASTER
C
BYP
CONTROL
V
GND
SET
OUT
+
A
CONTROL
6909 F04
. As already explained
, the resistor R
(A)
MASTER
CO
SET
. A volt-
resis-
SET
, will
is
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
the N = 64 setting. To disable the SSFM, connect 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 floated, 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
potentially 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 benefit 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
pseudorandom modulating signal where the frequency
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
+
6909fa
/N,
for