ltc6902cms-trpbf Linear Technology Corporation, ltc6902cms-trpbf Datasheet - Page 9

• Manufacturer Part Number: ltc6902cms-trpbf
• Description: Multiphase Oscillator With Spread Spectrum Frequency Modulation
• Manufacturer: Linear Technology Corporation

THEORY OF OPERATIO
divider goes through further division. In addition to fur-
ther division, the duty cycle of the output depends on the
multiphase mode selected. Figure 4 shows the waveform
at each output for 2-, 3- and 4-phase modes.
2-Phase Mode
In 2-phase mode, all outputs are nominally 50% duty
cycle. OUT1 and OUT2 are 180 degrees out of phase.
Stated differently, OUT2 is OUT1 inverted. However, OUT2
10000
1000
Figure 3. R
(PH = GND, 2-Phase, M = 1)
100
OUT1
OUT2
OUT3
OUT4
OUT1
OUT2
OUT3
OUT4
OUT1
OUT2
OUT3
OUT4
2-PHASE, PH = GND
3-PHASE, PH = OPEN
4-PHASE, PH = V
10
1
1k
DESIRED OUTPUT FREQUENCY (Hz)
10k
100
SET
+
vs Desired Output Frequency
100k
10
1M
1
10M
U
69012 F03
Figure 4. Mulitphase Output Waveforms
100M
is not simply OUT1 routed through a standard logic
inverter. This would lead to substantial delay for OUT2’s
transitions from OUT1’s transitions. OUT1 and OUT2 are
created by a delay matched inverting circuit. Apart from
the basic inversion, the delay matching is determined by
analog circuit parameters. With this type of design, OUT1
and OUT2 transitions are typically within 100ps. OUT3 and
OUT4 are replications of OUT1 and OUT2 respectively.
Since the two phases are generated via delay matched
inverters, there is not any further division and the param-
eter M in the frequency setting equation is 1 (M = 1).
3-Phase Mode
In 3-phase mode, OUT1, OUT2 and OUT3 are active and all
three outputs have a 33.3% duty cycle. OUT4 is not active
and is at a logic low state. The three active outputs are all
120 degrees out of phase. OUT2 lags OUT1 by 120 degrees
and OUT3 lags OUT2 by 120 degrees. The signals are
generated by a shift register. The output frequency is the
programmable divider’s output further divided 3 (M = 3).
4-Phase Mode
In 4-phase mode, all outputs have a 50% duty cycle. The
outputs are all 90 degrees out of phase. OUT2 lags OUT1
69012 F04
M = 1
DUTY CYCLE = 50%
M = 3
DUTY CYCLE = 33%
(OUT4 = LOGIC LOW)
M = 4
DUTY CYCLE = 50%
LTC6902
9
6902f