AK8186B Datasheet, PDF (28/69 Pages) Asahi Kasei Microsystems – Multi Output Clock Generator with Integrated 2.0GHz VCO

AK8186B
HOLDOVER
Some application requires holding the output frequency to be constant even though the REF input is lost out.
A holdover function is for such a requirement. In the AK8186B, the holdover function puts the charge pump
into high-impedance state so that the VCO keeps its frequency constant. However, any leakage could occur
at the charge pump output, which leads the unwanted VCO frequency shift. Adequate capacitive value in
the loop filter should be selected to avoid shifting the VCO frequency out of the required limit.
The AK8186B has two modes of holdover function, manual or automatic mode. Manual holdover is
activated by the SYNC pin. Automatic holdover is activated by the voltage of LD pin. Both holdover modes
are enabled with 0x1D[2:0].
Table 21 Setting Holdover
Mode
Manual Holdover
Automatic Holdover
Holdover Enable
0x1D[2]
1
1
Mannual/Automatic
0x1D[1]
1
0
Holdover Enable
0x1D[0]
1
1
Manual Holdover Mode
A manual holdover puts the charge pump into a high impedance state immediately when the SYNC pin is
asserted low. This is trigged by the falling edge of the SYNC.
Getting into the holdover
Condition
: the falling edge of the SYNC
Operation Timing : immediately
Operation
: puts the charge pump into a high impedance state
Leaving the holdover
Condition
: the SYNC = High
Operation Timing : synchronous with the first PFD rising edge after the SYNC goes high.
Operation
: puts the charge pump into a normal state, resets the B-counter.
Figure 18. Manual Holdover
Note: Set the channel divider to ignore the SYNC pin at least after an initial SYNC event. Otherwise, every
time SYNC is asserted low to invoke the manual holdover, the distribution outputs become DC output state.
Sep-2012
- 28 -
draft-E-02