LTC6909 Linear Dimensions Semiconductor, LTC6909 Datasheet - Page 8

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

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OPERATION
LTC6909
As shown in the Block Diagram, the LTC6909’s master
oscillator is controlled by the ratio of the voltage between
the V
(I
tion (SSFM) is disabled, I
by the (V
SSFM is enabled, I
dorandom noise (PRN) signal. Here the I
a random value uniformly distributed between (I
and (I
to produce an approximately fl at frequency spectrum,
centered about the set frequency with a bandwidth equal
to approximately 20% of the center frequency.
The voltage on the SET pin is forced to approximately
1.1V below V
voltage. This voltage is accurate to ±5% at a particular
input current and supply voltage (see Figure 1). The
LTC6909 is optimized for use with resistors between 20k
and 400k corresponding to master oscillator frequencies
between 500kHz and 10MHz. Accurate master oscillator
frequencies up to 20MHz (R
the supply voltage is greater than 4V. The R
connected between the V
the (V
the parts to attain excellent frequency accuracy regardless
of the precision of the SET pin. The master oscillation
frequency is:
8
MASTER
f
MASTER
+
SET
A and SET pins and the current entering the SET pin
+
A – V
). When the spread spectrum frequency modula-
+
+ 10%). In this way, the frequency is modulated
A – V
= 20MHz • 10k/R
Figure 1. V
1.4
1.3
1.2
1.1
1.0
0.9
0.8
SET
+
A by the PMOS transistor and its gate bias
0.1
T
SET
) voltage and the current I
A
= 25°C
MASTER
) voltage and the R
+
1
– V
+
SET
is modulated by a fi ltered pseu-
A and SET pins, locks together
MASTER
I
RES
V
+
Variation with I
SET
10
SET
= 3V
(μA)
V
+
= 5V
= 10k) are attainable if
is strictly determined
100
SET
MASTER
SET
6909 F01
resistor. When
RES
1000
. This allows
SET
SET
current is
resistor,
– 10%)
When the spread spectrum frequency modulation (SSFM)
is disabled, the master oscillator frequency is stationary.
When SSFM is enabled, the master oscillator frequency
varies from 0.9 • f
Output Frequency and Confi gurations
The output frequency of the LTC6909 is set by the R
resistor value and the connections of the PH0, PH1 and
PH2 logic input pins. The following formula defi nes the
relationship:
where PH = 3, 4, 5, 6, 7 or 8 and is defi ned as follows:
The PH0, PH1 and PH2 pins are standard logic input pins.
These pins do not have any active pull-up or pull-down
circuitry. As such, they cannot be left fl oating and must be
connected to a valid logic high or low voltage. The PH0,
PH1 and PH2 pin connections not only divide the master
oscillator frequency by the value PH but also determine
the phase relationship between the output signals. Figure
2 shows the output waveforms for each of the eight pos-
sible output confi gurations.
Note that 2-phase, complementary (180° phase shifted)
outputs are available in the 4-, 6- and 8-phase modes
by choosing the correct pair of signals. For example, in
4-phase mode, OUT1 and OUT3 (or OUT2 and OUT4) are
complementary.
PH2
f
OUT
0
0
0
0
1
1
1
1
= 20MHz • 10k/(R
PH1
0
0
1
1
0
0
1
1
MASTER
PH0
0
1
0
1
0
1
0
1
MODE
All Outputs Are Floating (Hi-Z)
All Outputs Are Held Low
3-Phase Mode (PH = 3)
4-Phase Mode (PH = 4)
5-Phase Mode (PH = 5)
6-Phase Mode (PH = 6)
7-Phase Mode (PH = 7)
8-Phase Mode (PH = 8)
SET
to 1.1 • f
• PH)
MASTER
.
6909f
SET