LTC6948 datasheet(28/38 Pages) AD

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LTC6948

6948fa

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applicaTions inForMaTion

Loop Filter Component Selection

Now set loop filter resistor RZ and charge pump current

ICP. Because the KVCO varies over the VCO’s frequency

range, using the KVCO geometric mean gives good results:

KVCO = 3.843 •109 • 0.047 • 0.07

= 220.4MHz / V

Using an ICP of 5.6mA, the FracNWizard uses Equation 12

to determine RZ:

RZ =

2• π •150k •76

5.6m•220.4M

RZ = 58.0Ω

For the 4th order Filter 3, FracNWizard uses modified

Equations 13 and 14 to calculate CI, CP:

CI =

4.5

2• π •150k •58

= 82.3nF

CP =

10.5• π •150k •58

= 3.5nF

FracNWizard calculates R1, L1, and C2 to be:

R1 = 58.0Ω

C2 = 2.3nF

L1 = 7.8µH

Status Output Programming

This example will use the STAT pin to alert the system

whenever the LTC6948 generates a fault condition. Pro-

gram x[5], x[4], x[3], x[1], x[0] = 1 to force the STAT pin

high whenever any of the UNLOCK, ALCHI, ALCLO, THI,

or TLO flags asserts:

Reg01 = h3B

Power Register Programming

For correct PLL operation all internal blocks should be

enabled. OMUTE may remain asserted (or the MUTE pin

held low) until programming is complete. For OMUTE = 1:

Reg02 = h02

VCO ALC, AUTOCAL, and AUTORST Programming

Set the ALC options (ALCMON = ALCULOK = ALCCAL = 1),

the auto reset options (AUTOCAL = AUTORST = 1), and

the Δ∑ modulator modes (DITHEN = 1, INTN = 0) at the

same time:

Reg03 = h7E

The ALC will only be active during a calibration cycle or

when the loop is unlocked, but the ALCHI and ALCLO

statusconditionswillbemonitoredcontinuously.TheVCO

will be calibrated and the Δ∑ modulator will be reset at the

end of the SPI write communication burst (assuming an

auto-increment write is used to write all registers).

LDO Programming

Use Table 13 and fPFD = 50MHz to determine V(LDO) and

LDOV[1:0]:

V(LDO) = 2.3V and LDOV[1:0] = 2

Use LDOV[1:0], LDOEN = 1 to enable the LDO, and the

previously determined BD[3:0] value to set Reg04. CPLE

should be set to 1 to reduce in-band noise and spurious

due to the Δ∑ modulator:

Reg04 = h5E

SEED Programming

The SEED[7:0] value is used to initialize the Δ∑ modulator

dither circuitry. Use the default value:

Reg05 = h11

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