LTC6909 Linear Dimensions Semiconductor, LTC6909 Datasheet - Page 15

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

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APPLICATIONS INFORMATION
9. The connections for PH0, PH1 and PH2 are not shown
START-UP ISSUES AND CONSIDERATIONS
The start-up time and settling time to within 1% of the
fi nal value is estimated by the following equation:
For instance, with R
to within 1% of its 1MHz fi nal value in approximately
260μs. Figure 7 shows the start-up time for various R
resistors.
GROUND
PLANE
in Figure 6. These pins are connected to either GND or
V
application. Connection to ground is done underneath
the part. Connecting PH2 to V
Connecting PH0 or PH1 to V
both traces to go down a layer. If you are dynamically
changing one or all of the PH pins, place a 10k resis-
tor in series with the signal line. Locate the resistor
fairly close to the PH pin. This signal typically comes
from a microcontroller or the power good signal from
a switching regulator and is usually quite noisy. The
series resistor provides some isolation between the
noisy signal and the LTC6909.
t
+
START
0.1μF
D depending on the output phasing required for the
C1
≈ R
V
GND
PH0
PH1
OUT1
SET
+
A
LTC6909
R
SET
•
⎛
⎜
⎝
SET
25μs
OUT8
MOD
PH2
SET
V
1k
+
D
= 100k, the LTC6909 will settle
GROUND
⎞
⎟ +10μs
⎠
PLANE
+
D is also straightforward.
C2
0.1μF
+
D may require one or
DIRECT, LOW IMPEDANCE
CONNECTION TO THE V
SUPPLY
Figure 6. Supply Bypassing and PCB Layout
+
SET
To assist in an orderly start-up sequence, the LTC6909’s
outputs are in a high impedance state for the fi rst 128
master clock cycles after power-up. This ensures that
the fi rst clock cycle is very close to the desired operating
frequency.
Powering up and down complex multiphase switching
regulator circuits is always chaotic and can have serious
system consequences if it is not done carefully. In addition
to the LTC6909’s muting of the outputs to ensure fi rst cycle
accuracy, the PH0-PH1-PH2 codes 000 (all outputs are
C1
10000
1000
100
OUT1
OUT2
OUT3
OUT4
GND
PH0
PH1
10
V
+
A
1k
T
V
A
+
Figure 7. Start-Up Time
= 25°C
= 3V
10k
R
R
SET
SET
100k
(Ω)
1M
OUT8
OUT7
OUT6
OUT5
MOD
PH2
V
SET
6909 F07
+
D
LTC6909
10M
C2
15
6909f
6909 F06