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LTC6909
15
6909fa
Figure 7. Start-Up Time
Figure 6. Supply Bypassing and PCB Layout
9. The connections for PH0, PH1 and PH2 are not shown
in Figure 6. These pins are connected to either GND or
V+D depending on the output phasing required for the
application. Connection to ground is done underneath
thepart.ConnectingPH2toV+Disalsostraightforward.
Connecting PH0 or PH1 to V+D may require one or
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.
START-UP ISSUES AND CONSIDERATIONS
The start-up time and settling time to within 1% of the
final value is estimated by the following equation:
tSTART ≈RSET
25s
1k
+10s
For instance, with RSET = 100k, the LTC6909 will settle to
within 1% of its 1MHz final value in approximately 260s.
Figure7showsthestart-uptimeforvariousRSETresistors.
To assist in an orderly start-up sequence, the LTC6909’s
outputs are in a high impedance state for the first 128
master clock cycles after power-up. This ensures that
the first 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
totheLTC6909’smutingoftheoutputstoensurefirstcycle
accuracy, the PH0-PH1-PH2 codes 000 (all outputs are
ApplicAtions inForMAtion
6909 F06
C1
0.1F
C2
0.1F
DIRECT, LOW IMPEDANCE
CONNECTION TO THE V+
SUPPLY
V+A
LTC6909
GND
GROUND
PLANE
GROUND
PLANE
PH0
PH1
OUT1
SET
PH2
MOD
V+D
OUT8
RSET
V+A
GND
C1
RSET
C2
PH0
PH1
OUT1
OUT2
OUT3
OUT4
SET
PH2
MOD
V+D
OUT8
OUT7
OUT6
OUT5
RSET (Ω)
100
ST
AR
T-UP
DELA
Y(s)
1000
1k
100k
1M
10M
6909 F07
10
10k
10000
TA = 25°C
V+ = 3V
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