10

INDUSTRIALTEMPERATURERANGE

IDT5V9885T

3.3V EEPROMPROGRAMMABLECLOCKGENERATOR

Example

Since the spread is center, the SS_OFFSET can be set to '0'. Solve for the nominal M value; keep in mind that the nominal M should be chosen to maximize

the VCO. Start with D = 1, using Eq.10 and Eq.11.

Using Eq.4, we arbitrarily choose N = 22, A = 3. Now that we have the nominal M value, we can determine TSSC and NSSC by using Eq.12.

Nssc * Tssc = 25MHz / (33KHz * 4) = 190

However, using Eq. 7 and Eq.8, we find that the closest value is when TSSC = 14 and NSSC = 6. Keep in mind to maximize the number of samples used

to enhance the profile of the spread spectrum waveform.

Tssc = 14 + 2 = 16

Nssc = 6 * 2 = 12

Nssc * Tssc = 192

Use Eq.14 to determine the value of the sigma-delta-encoded samples.

±2% = Σ∆ * 100

64 * 48

Σ∆ = 61.44

Either round up or down to the nearest integer value. Therefore, we end up with 61 or 62 for sigma-delta-encoded samples. Since the sigma-delta-encoded

samples must not exceed 63 with SS_OFFSET set to '0', 61 or 62 is well within the limits. It is the discretion of the user to define the shape of the profile that

is better suited for the intended application.

Using Eq.14 again, the actual spread for the sigma-delta-encoded samples of 61 and 62 are ±1.99% and ±2.02%, respectively.

Use Eq.10 to determine if the X2 bit needs to be set;

Amplitude = 48 * (1.99 or 2.02) / 100 = 0.48 < 1

2

Therefore, the X2 = '0 '. The dither bit is left to the discretion of the user.

The example above was of a center spread using spread spectrum. For down spread, the nominal M value can be set one integer value lower to 43.

Note that the 5V9885T should not be programmed with TSSC > '0', SS_OFFSET = '0', and SD = '0' in order to prevent an unstable state in the modulator.

ThePLLloopbandwidthmustbeatleast10xthemodulationfrequencyalongwithhigherdamping(larger ωuz)topreventthespreadspectrumfrombeingfiltered

and reduce extraneous noise. Refer to the LOOP FILTER section for more detail on ωuz. The A[3:0] must be used for spread spectrum, even if the total multiplier

value is an even integer.

FRACTIONAL DIVIDER

There is the option for the feedback-divider to be programmed as a fractional divider for only PLL0 and PLL. By setting TSSC > '0' and SD bits to '0', the

SS_OFFSET bits would determine the fractional divide value. See the SPREAD SPECTRUM GENERATION section for more details on the TSSC, SD, and

SS_OFFSET bits. The following equation governs how the fractional divide value is set.

M = 2*N[11:0] + A[3:0] + 1 + SS_OFFSET[5:0] *1/64