Datasheet LTC6902 (Analog Devices) - 6

LTC6902
U QUICK DESIG GUIDE Step 3. Calculating the RSET Resistor Value Step 4. Calculating the RMOD Resistor Value
The RSET resistor, the multiphase mode and the divider (Note: For constant frequency applications RMOD is not setting set the output frequency (fOUT) for constant fre- required. Disable SSFM by connecting the MOD pin to quency applications. For SSFM applications, the maxi- GND) mum frequency excursion (fMAX) is equal to fOUT. R R SET MOD = 20 • SpreadingPercentage MHz 10  R = k SET 20 Ω • N•M• f where the Spreading Percentage is defined by the OUT   following: 100 DIV Pin = + V  N = 10 DIV Pin = Open  f – f MAX MIN =  Spreading Percentage 100 • 1 DIV Pin = 0V  fMAX 4 (4-Phase Output) PHPin = + V where fMAX is the highest frequency excursion (set by the  R M = 3 (3 - H Phase Output) P Pin = Open  SET value calculated in Step 3) and fMIN is the lowest  frequency excursion. 1 (2 - H Phase Output) P Pin = 0V 
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%
U U U PI FU CTIO S V+ (Pin 1):
Supply Voltage ( 2.7V ≤ V+ ≤ 5.5V). The supply ground shield to prevent excessive coupling from other should be kept free from noise and ripple. It should be PCB traces. bypassed directly to a ground plane with a 0.1µF capacitor
PH (Pin 3):
Phase Setting Input. This three-state input placed as close to the pin as possible. selects among three multiphase options. This sets the
DIV (Pin 2):
Divider Setting Input. This three-state input outputs to produce 2-phase, 3-phase or 4-phase signals. selects among three divider settings determining the value It also sets the value of M in the frequency equation. Pin 3 of N in the frequency equation. Pin 2 should be tied to GND should be tied to GND for the 2-phase setting. This is the for the ÷1 setting, the highest frequency range. Floating highest frequency range with M set to 1. Floating Pin 3, Pin 2, leaving it open, divides the master oscillator by 10. leaving it open, selects the 3-phase setting. This also sets Tie Pin 2 to V+ for the ÷100 setting, the lowest frequency M to 3. Tie Pin 3 to V+ for the 4-phase setting. This is the range. To detect a floating DIV pin, the LTC6902 places the lowest frequency range as M is set to 4. To detect a floating pin at the midsupply point with active circuitry. Therefore, PH pin, the LTC6902 places the pin at the midsupply point driving the DIV pin high requires sourcing approximately with active circuitry. Therefore, driving the PH pin high 2µA. Similarly, driving the DIV pin low requires sinking requires sourcing approximately 2µA. Similarly, driving 2µA. When the DIV pin is floated, it should be bypassed by the PH pin low requires sinking 2µA. When the PH pin is a 1nF capacitor to GND or it should be surrounded by a floated, it should be bypassed by a 1nF capacitor to GND 6902f 6