Rainbow Electronics ATA5812 - Manual (Page 23)

23
ATA5811/ATA5812 [Preliminary]
4689B–RKE–04/04
With C
m
≤ 14 fF, C
0
≥ 1.5 pF, C
LN
= 9 pF and C
L
= 7.6 pF to 10.6 pF the pulling amounts
to P ≤
± 100 ppm and with C
m
≤ 7 fF, C
0
≥ 1.5 pF, C
LN
= 9 pF and C
L
= 7.4 pF to 10.6 pF the
pulling is P ≤
± 50 ppm.
Since typical crystals have less than ±50 ppm tolerance at 25° the compensation is not
critical.
C
0
of the XTAL has to be lower than C
Lmin
/2 = 3.8 pF for a Pierce oscillator type in order
to not enter the steep region of pulling versus load capacitance where there is a risk of
an unstable oscillation.
To ensure proper start-up behavior the small signal gain and thus the negative resis-
tance provided by this XTO at start is very large, for example oscillation starts up even in
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
, Z
2
as complex impedances at pin XTAL1 and XTAL2 hence
Z1 = -j/(2
× π ×
f
XTO
×
C
L1
) + 5
Ω
and Z2 = -j/(2
× π ×
f
XTO
×
C
L2
) + 5
Ω
.
Z
3
consists of crystals C
0
in parallel with an internal 110 k
Ω
resistor hence
Z3 = -j/(2
× π ×
f
XTO
×
C
0
) /110 k
Ω
, gm is the internal transconductance between
XTAL1 and XTAL2 with typically 19 ms at 25°C.
With f
XTO
= 13.5 MHz, gm = 19 ms, CL = 9 pF, C
0
= 2.2 pF this results in a negative
resistance of about 2 k
Ω
. The worst case for technological, supply voltage and tempera-
ture variations is then for C
0
≤
2.2 pF always higher than 1.5 k
Ω
.
Due to the large gain at start the XTO is able to meet a very low start-up time. The oscil-
lation start-up time can be estimated with the time constant
τ .
After 10
τ
to 20
τ
an amplitude detector detects the oscillation amplitude and sets
XTO_OK to High if the amplitude is large enough, this sets N_RESET to High and acti-
vates the CLK output if CLK_ON in control register 3 is High (see Table 20 on page 35).
Note that the necessary conditions of the VSOUT and DVCC voltage also have to be ful-
filled (see Figure 19 on page 24 and Figure 21 on page 26).
To save current in Idle and sleep mode, the load capacitors partially are switched off in
this modes with S1 and S2 seen in Figure 19 on page 24.
It is recommended to use a crystal with C
m
= 4.0 fF to 7.0 fF, C
LN
= 9 pF, R
m
< 120
Ω
and C
0
= 1.5 pF to 2.2 pF.
Re Z
xtocore
{ }
Re
Z
1
Z
3
Z
2
Z
3
Z
1
+ Z
2
×
Z
3
g
m
Ч Ч Ч
+
Ч
Z
1
Z
2
Z
3
Z
1
Z
2
×
g
m
×
+ + +
------------------------------------------------------------------------------------------------------
⎩ ⎭
⎨ ⎬
⎧ ⎫
=
τ
2
4
π 2 ×
f
m
2 ×
C
m
×
Re Z
xtocore
( )
R
m
+
( ) Ч
-------------------------------------------------------------------------------------------------------- =