LTC6946 datasheet(21/30 Pages) LINER

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LTC6946

6946fa

APPLICATIONS INFORMATION

INTRODUCTION

A PLL is a complex feedback system that may conceptually

be considered a frequency multiplier. The system multiplies

the frequency input at REF± and outputs a higher frequency

at RF±. The PFD, charge pump, N divider, and external VCO

and loop filter form a feedback loop to accurately control

the output frequency (see Figure 14). The R and O divider

are used to set the output frequency resolution.

Using the above equations, the output frequency resolu-

tion fSTEP produced by a unit change in N is given by

Equation 6:

fSTEP =

fREF

R• O

(6)

LOOP FILTER DESIGN

A stable PLL system requires care in selecting the external

loop filter values. The Linear Technology PLLWizard ap-

plication, available from www.linear.com, aids in design

and simulation of the complete system.

The loop design should use the following algorithm:

1. Determine the output frequency, fRF, and frequency step

size, fSTEP, based on application requirements. Using

Equations 3, 4, 5 and 6, change fREF, N, R, and O until

the application frequency constraints are met. Use the

minimum R value that still satisfies the constraints.

Then calculate B using Equation 1 and Table 7.

2. Select the open-loop bandwidth, BW, constrained by

fPFD. A stable loop requires that BW is less than fPFD

by at least a factor of 10.

3. Select loop filter component RZ and charge pump cur-

rent ICP based on BW and the VCO gain factor, KVCO.

BW (in Hz) is approximated by the following equation:

BW ≅

ICP •RZ •KVCO

2• π •N

(7)

RZ =

2• π •BW •N

ICP •KVCO

where KVCO is in Hz/V, ICP is in Amps, and RZ is in Ohms.

KVCO is obtained from the VCO tuning sensitivity in the

Electrical Characteristics. Use ICP = 11.2mA to lower in-

band noise unless component values force a lower setting.

R_DIV

N_DIV

÷R

÷N

O_DIV

÷O

fPFD

LTC6946

REF±

(fREF)

fVCO

KPFD

KVCO

RF±

(fRF)

LOOP FILTER

LF(s)

6946 F14

TUNE

ICP

Figure 14. PLL Loop Diagram

OUTPUT FREQUENCY

When the loop is locked, the frequency fVCO (in Hz)

produced at the output of the VCO is determined by the

reference frequency, fREF, and the R and N divider values,

given by Equation 3:

fVCO =

fREF •N

(3)

Here, the PFD frequency fPFD produced is given by the

following equation:

fPFD =

fREF

(4)

and fVCO may be alternatively expressed as:

fVCO = fPFD • N

The output frequency fRF produced at the output of the O

divider is given by Equation 5:

fRF =

fVCO

(5)

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