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Rainbow Electronics ATA5746 User Manual

Page 15

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15

4596A–RKE–05/06

ATA5745/ATA5746 [Preliminary]

Figure 3-1.

Crystal Equivalent Circuit

With C

m

≤10 fF, C

0

≥ 1.0 pF, C

LN

= 9 pF and C

L1,2

= 16 pF ±1%, the pulling amounts to

P

≤±1 ppm.

The C

0

of the XTAL has to be lower than C

Lmin

/ 2 = 7.9 pF for a Pierce oscillator type in order to

not enter the steep region of pulling versus load capacitance where there is risk of an unstable
oscillation.

To ensure proper start-up behavior, the small signal gain and the negative resistance provided
by this XTO at start is very large. For example, oscillation starts up even in the worst case with a
crystal series resistance of 1.5 k

Ω at C

0

≤2.2 pF with this XTO. The negative resistance is

approximately given by

with Z

1

and Z

2

as complex impedances at pins XTAL1 and XTAL2, hence

Z

1

= –j / (2

× p × f

XTO

× C

L1

) + 5

Ω and Z

2

= –j / (2

× p × f

XTO

× C

L2

) + 5

Ω.

Z

3

consists of crystal C

0

in parallel with an internal 110-k

Ω resistor, hence

Z

3

= –j / (2

× p × f

XTO

× C

0

) / 110 k

Ω, gm is the internal transconductance between XTAL1 and

XTAL2, with typically 20 mS at 25°C.

With f

XTO

= 13.5 MHz, gm = 20 mS, C

L

= 9 pF, and C

0

= 2.2 pF, this results in a negative resis-

tance of about 2 k

Ω. The worst case for technology, supply voltage, and temperature variations

is then always higher than 1.4 k

Ω for C

0

≤2.2 pF.

Due to the large gain at start, the XTO is able to meet a very low start-up time. The oscillation
start-up time can be estimated with the time constant

τ .

After 10

τ to 20τ , an amplitude detector detects the oscillation amplitude and sets XTO_OK to

High if the amplitude is large enough; this activates the CLK_OUT output if it is enabled via the
pins CLK_OUT_CTRL0 and CLK_OUT_CTRL1. Note that the necessary conditions of the
DVCC voltage also have to be fulfilled.

It is recommended to use a crystal with C

m

= 3.0 fF to 10 fF, C

LN

= 9 pF, R

m

< 120

Ω and

C

0

= 1.0 pF to 2.2 pF.

C

0

C

L2

C

L1

C

m

L

m

R

m

C

L

= C

L1

× C

L2

/ (C

L1

+ C

L2

)

XTAL

Crystal Equivalent Circuit

Re Zxtocore

{

} Re

Z

1

Z

3

Z

2

Z

3

Z

1

Z

3

gm

Ч

Ч

+

Ч

+

Ч

Z

1

Z

2

Z

3

Z

1

Z

2

gm

Ч

Ч

+

+

+

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

=

τ

2

4

π

2

f

XTAL

2

C

m

Re Z

xtocore

(

) R

m

+

(

)

Ч

Ч

Ч

Ч

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

=