LTC6909 datasheet(15/20 Pages) LINER

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LTC6909

6909f

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

the part. Connecting PH2 to V+D is also straightforward.

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 •

25μs

⎛

⎝

⎜

⎞

⎠

⎟ +10μs

For instance, with RSET = 100k, the LTC6909 will settle

to within 1% of its 1MHz ﬁnal value in approximately

260μs. Figure 7 shows the start-up time for various RSET

resistors.

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

to the LTC6909’s muting of the outputs to ensure ﬁrst cycle

accuracy, the PH0-PH1-PH2 codes 000 (all outputs are

APPLICATIONS INFORMATION

6909 F06

0.1μF

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

RSET

PH0

PH1

OUT1

OUT2

OUT3

OUT4

SET

PH2

MOD

V+D

OUT8

OUT7

OUT6

OUT5

RSET (Ω)

100

1000

100k

10M

6909 F07

10k

10000

TA = 25°C

V+ = 3V

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