LTC6908-1/LTC6908-2

11

Rev C

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RSET (Ω)

100

1000

1k

100k

1M

10M

690812 F07

10

10k

10000

TA = 25°C

V+ = 3V

the bandwidth of the switching regulator is sufficient,

not a difficult requirement in most cases, the regulator’s

regulation, efficiency and load response are maintained

while peak electromagnetic radiation (or conduction) is

reduced. Output ripple may be somewhat increased, but

its behavior is very much like noise and its system impact

is benign.

HIGH FREQUENCY REJECTION

Using the LTC6908 in spread spectrum mode naturally

eliminates any concerns for output frequency accuracy

and stability as it is continually hopping to new settings.

In fixed frequency applications however, some attention to

V+ supply voltage ripple is required to minimize additional

output frequency error. Ripple frequency components on

the supply line near the programmed output frequency

of the LTC6908 in excess of 30mVP-P could create an

additional 0.2% of frequency error. In applications where

a fixed frequency LTC6908 output clock is used to syn-

chronize the same switching regulator that provides the

V+ supply to the oscillator, noticeable jitter of the clock

may occur if the ripple exceeds 30mVP-P .

START-UP TIME

The start-up time and settling time to within 1% of the

final value can be estimated by tSTART ≈ RSET • (2.5µs/k)

+ 10µs. For instance, with RSET = 100k, the LTC6908 will

settle to within 1% of its 1MHz final value in approxi-

mately 260µs. Figure 7 shows start-up times for various

RSET resistors. An internal counter mutes the outputs

(OUT1 = Low, OUT2 = High) for the first 64 clock cycles

after power-up, ensuring that the first clock cycle is close

to the desired operating frequency.

JITTER

The Peak-to-Peak Jitter vs Output Frequency graph, in the

Typical Performance Characteristics section, shows the

typical clock jitter as a function of oscillator frequency and

power supply voltage. These specifications assume that

the capacitance on SET is limited to less than 10pF, as

suggested in the Pin Functions description. If this require-

ment is not met, the jitter will increase.

Figure 7. Start-Up Time

APPLICATIONS INFORMATION