LTC6902 datasheet(6/16 Pages) LINER

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LTC6902

6902f

QUICK DESIG GUIDE

Step 3. Calculating the RSET Resistor Value

The RSET resistor, the multiphase mode and the divider

setting set the output frequency (fOUT) for constant fre-

quency applications. For SSFM applications, the maxi-

mum frequency excursion (fMAX) is equal to fOUT.

MHz

NM f

Open

SET

OUT

=Ω • 



























100

••

DIV Pin

(4 - Phase Output) PH Pin

(3 - Phase Output) P Pin

(2 - Phase Output) P Pin

Step 4. Calculating the RMOD Resistor Value

(Note: For constant frequency applications RMOD is not

required. Disable SSFM by connecting the MOD pin to

GND)

Spreading Percentage

MOD

SET

= 20 •

where the Spreading Percentage is defined by the

following:

Spreading Percentage

MAX

MIN

MAX

= 100 •

–

where fMAX is the highest frequency excursion (set by the

RSET value calculated in Step 3) and fMIN is the lowest

frequency excursion.

Example

For a 4-phase, 250kHz clock with 40% spreading:

Connect PH Pin to V+

→ Selects 4-Phase Mode, M = 4

Leave DIV Pin Open

→ N = 10

RSET = 20k → Sets fOUT = fMAX = 250kHz

RMOD = 10k → Sets Spreading to 40%

PI FU CTIO S

V+ (Pin 1): Supply Voltage ( 2.7V

≤ V+ ≤ 5.5V). The supply

should be kept free from noise and ripple. It should be

bypassed directly to a ground plane with a 0.1

µF capacitor

placed as close to the pin as possible.

DIV (Pin 2): Divider Setting Input. This three-state input

selects among three divider settings determining the value

of N in the frequency equation. Pin 2 should be tied to GND

for the

÷1 setting, the highest frequency range. Floating

Pin 2, leaving it open, divides the master oscillator by 10.

Tie Pin 2 to V+ for the

÷100 setting, the lowest frequency

range. To detect a floating DIV pin, the LTC6902 places the

pin at the midsupply point with active circuitry. Therefore,

driving the DIV pin high requires sourcing approximately

µA. Similarly, driving the DIV pin low requires sinking

µA. When the DIV pin is floated, it should be bypassed by

a 1nF capacitor to GND or it should be surrounded by a

ground shield to prevent excessive coupling from other

PCB traces.

PH (Pin 3): Phase Setting Input. This three-state input

selects among three multiphase options. This sets the

outputs to produce 2-phase, 3-phase or 4-phase signals.

It also sets the value of M in the frequency equation. Pin 3

should be tied to GND for the 2-phase setting. This is the

highest frequency range with M set to 1. Floating Pin 3,

leaving it open, selects the 3-phase setting. This also sets

M to 3. Tie Pin 3 to V+ for the 4-phase setting. This is the

lowest frequency range as M is set to 4. To detect a floating

PH pin, the LTC6902 places the pin at the midsupply point

with active circuitry. Therefore, driving the PH pin high

requires sourcing approximately 2

µA. Similarly, driving

the PH pin low requires sinking 2

µA. When the PH pin is

floated, it should be bypassed by a 1nF capacitor to GND

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