LTC6908CS6-2 datasheet(11/16 Pages) LINER

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LTC6908-1/LTC6908-2

690812fa

RSET (Ω)

100

1000

100k

10M

690812 F07

10k

10000

TA = 25°C

V+ = 3V

and load response are maintained while peak electromag-

netic 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 addi-

tional 0.2% of frequency error. In applications where a ﬁxed

frequency LTC6908 output clock is used to synchronize

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 approximately 260µs.

APPLICATIO S I FOR ATIO

Figure 7 shows start-up times for various RSET resistors.

An internal counter mutes the outputs 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

requirement is not met, the jitter will increase.

Figure 7. Start-Up Time

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