PLL BUILDING BLOCK

MDS 673-01 L

7

Revision 051310

www.idt.com

ICS673-01

Determining the Loop Filter Values

The loop filter components consist of CS, CP, and RS.

Calculating these values is best illustrated by an

example. Using the example in Figure 1, we can

synthesize 20 MHz from a 200 kHz input.

The phase locked loop may be approximately

described by the following equations:

Bandwidth

Damping factor,

where:

KO = VCO gain (Hz/V)

Icp = Charge pump current (A)

N = Total feedback divide from VCO,

including the internal VCO post divider

CS = Loop filter capacitor (Farads)

RS = Loop filter resistor (Ohms)

As a general rule, the bandwidth should be at least 20

times less than the reference frequency, i.e.,

In this example, using the above equation, bandwidth

should be less than or equal to 10 kHz. By setting the

bandwith to 10kHz and using the first equation, RS can

be determined since all other variables are known. In

the example of Figure 1, N = 200, comprising the divide

by 2 on the chip (VCO post divider) and the external

divide by 100. Therefore, the bandwidth equation

becomes:

and RS = 26 kΩ

Choosing a damping factor of 0.7 (a minimal damping

factor than can be used to ensure fast lock time),

damping factor equation becomes:

and CS = 1.32 nF (1.2 nF is the nearest standard

value).

The capacitor CP is used to damp transients from the

charge pump and should be approximately 1/20th the

size of CS, i.e.,

Therefore, CP = 60 pF (56 pF nearest standard value).

To summarize, the loop filter components are:

CS = 1.2 nf

CP = 56 pf

RS = 26 kΩ

When choosing either CLK1 or CLK2 to drive the

feedback divider, IDT recommends that CLK2 be used

so that the rising edges of CLK1, CLK2, and REFIN are

all synchronized. If CLK1 is used to feedback, CLK2

may be either a rising or falling edge when compared to

CLK1 and REFIN.

NBW

R

S

K

O

I

CP

⋅⋅

2

πN

--------------------------------

=

ζ

R

S

2

------

K

O

I

CP

C

S

⋅⋅

N

-----------------------------------

=

BW

REFIN

() 20

⁄

≤

0,000

R

S

190

10

6

2.5

10

–

⋅⋅

⋅

⋅

2

π

200

⋅

--------------------------------------------------------------------

=

0.7

25 000

,

2

----------------

190

10

6

2.5

10

6

–

C

S

⋅⋅

⋅

⋅

200

--------------------------------------------------------------------------

=

C

P

C

S

20

⁄

≅