Data Sheet
ADF4159
Fast Lock Loop Filter Topology
FAST LOCK MODE
To use fast lock mode, an extra connection from the PLL to the
loop filter is needed. The damping resistor in the loop filter must
be reduced to ¼ of its value in wide bandwidth mode. This reduc-
tion is required because the charge pump current is increased
by 16 in wide bandwidth mode, and stability must be ensured.
The ADF4159 can operate in fast lock mode. In this mode, the
charge pump current is boosted and additional resistors are
connected to maintain the stability of the loop.
Fast Lock Timer and Register Sequences
When fast lock mode is enabled (Register R4, DB[20:19]), after
a write to Register R0, the PLL operates in a wide bandwidth
mode for a selected amount of time. Before fast lock is enabled,
the initialization sequence must be performed after the part is
first powered up (see the Initialization Sequence section). The
time in bandwidth mode is set by:
To further enhance stability and mitigate frequency overshoot
during a frequency change in wide bandwidth mode, Resistor R3
is connected (see Figure 53). During fast lock, the SW1 pin is
shorted to ground, and the SW2 pin is connected to CP (set
Bits DB[20:19] in Register R4 to 01 for fast lock divider).
The following two topologies can be used:
CLK1 × CLK2 / fPFD = Time in wide bandwidth
•
Divide the damping resistor (R1) into two values (R1 and
R1A) that have a ratio of 1:3 (see Figure 53).
Connect an extra resistor (R1A) directly from SW1 (see
Figure 54). The extra resistor must be selected such that the
parallel combination of an extra resistor and the damping
resistor (R1) is reduced to ¼ of the original value of R1.
where:
CLK1 = Register R2, DB[14:3].
CLK2 = Register R4, DB[18:7].
•
fPFD = the PFD frequency.
Note that the fast lock feature does not work in ramp mode.
Fast Lock Example
For both topologies, the ratio R3:R2 must equal 1:4.
In this example, the PLL has fPFD of 100 MHz and requires being
in wide bandwidth mode for 12 µs.
R3
SW2
R2
VCO
CP
CLK1 × CLK2 / fPFD = 12 µs
CLK1 × CLK2 = (12 × 10−6)(100 × 106) = 1200
C1
C2
R1
C3
ADF4159
SW1
Therefore, CLK1 = 12 and CLK2 = 100, which results in 12 µs.
R1A
Figure 53. Fast Lock Loop Filter Topology 1
R3
SW2
R2
VCO
CP
C1
C2
R1
C3
ADF4159
R1A
SW1
Figure 54. Fast Lock Loop Filter Topology 2
For more fast lock topologies, see ADIsimPLL™.
Rev. E | Page 33 of 36