ADF4158 datasheet(32/35 Pages) AD

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ADF4158

Data Sheet

Rev. I | Page 32 of 35

FAST LOCK MODE

The ADF4158 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:

CLK1 × CLK2/fPFD = Time in Wide Bandwidth

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

In this example, the PLL has fPFD of 100 MHz and requires being

in wide bandwidth mode for 12 µs.

CLK1 × CLK2/fPFD = 12 µs

CLK1 × CLK2 = (12 × 10−6)(100 × 106) = 1200

Therefore, CLK1 = 12 and CLK2 = 100, which results in 12 µs.

Fast Lock Loop Filter Topology

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

reduction is required because the charge pump current is

increased by 16 in wide bandwidth mode, and stability must be

ensured.

To further enhance stability and mitigate frequency overshoot

during a frequency change in wide bandwidth mode, Resistor R3

is connected (see Figure 46). 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:

•

Divide the damping resistor (R1) into two values (R1 and

R1A) that have a ratio of 1:3 (see Figure 46).

•

Connect an extra resistor (R1A) directly from SW1 (see

Figure 47). 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.

For both topologies, the ratio R3:R2 must equal 1:4.

VCO

R1A

SW2

SW1

ADF4158

Figure 46. Fast-Lock Loop Filter Topology—Topology 1

VCO

R1A

SW2

SW1

ADF4158

Figure 47. Fast-Lock Loop Filter Topology—Topology 2

For more fast lock topologies, see ADIsimPLL.

ADF4158 Datasheet(PDF) 32 Page - Analog Devices

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