LTC6906

10

6906fa

APPLICATIO S I FOR ATIO

Start-Up Time

When the LTC6906 is powered up, it holds the OUT pin

low. After the master oscillator has settled, the OUT pin is

enabled and the first output cycle is guaranteed to be

within specification. The time from power-up to the first

output transition is given approximately by:

The digital divider ratio, N, does not affect the start-up

time.

Power Supply Rejection

The LTC6906 has a very low supply voltage coefficient,

meaning that the output frequency is nearly insensitive to

the DC power supply voltage. In most cases, this error

term can be neglected.

potential to interfere with the LTC6906’s master oscillator.

Periodic noise, such as that generated by a switching

power supply, can shift the output frequency or increase

jitter. The risk increases when the fundamental frequency

or harmonics of the noise fall near the master oscillator

frequency. It is relatively easy to filter the LTC6906 power

supply because of the very low supply current. For ex-

ample, an RC filter with R = 160

Ω and C = 10µF provides

a 100Hz lowpass filter while dropping the supply voltage

only about 10mV.

Operating the LTC6906 with Supplies Higher

Than 3.6V

The LTC6906 may also be used with supply voltages

between 3.6V and 5.5V under very specific conditions. To

ensure proper functioning above 3.6V, a filter circuit must

be attached to the power supply and located within 1cm of

the device. A simple RC filter consisting of a 100

Ω resistor

and 1

µF capacitor (Figure 16) will ensure that supply

resonance at higher supply voltages does not induce

Figure 12. Test Circuit with an Inductive Power Supply

Figure 13. Output Waveforms with

an Inductive Supply (See Figure 12)

Figure 14. Test Circuit with a Resistive Power Supply

Figure 15. Output Waveforms with

a Resistive Supply (See Figure 14)

GND

DIV

OUT

GRD

SET

LTC6906

100k

330nH

3.3V

6906 F12

1MHz

GND

DIV

OUT

GRD

SET

LTC6906

100k

100

Ω

3.3V

6906 F14

1MHz

TIME (

µs)

4.65

2

3

5.05

6906 F13

1

0

4.75

4.85

5.15

3.5

4.95

TIME (

µs)

0.4

2

3

1.0

6906 F15

1

0

0.7

0.6

0.5

0.8

1.1

3.5

0.9