ADF4377 datasheet(24/79 Pages) AD

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Data Sheet

ADF4377

THEORY OF OPERATION

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Figure 64. Simplified PFD Schematic

Charge Pump

The charge pump, controlled by the PFD, forces sink (down) or

source (up) current pulses onto the CP pin, which must be connect-

ed to an appropriate loop filter. See Figure 65 for a simplified

schematic of the charge pump.

Figure 65. Simplified Charge Pump Schematic

The output current magnitude (ICP) may be set from 0.79 mA to

11.1 mA using the CP_I bits. A larger ICP can result in lower

in-band noise (LNORM) due to the lower impedance of the loop filter

components. A smaller ICP can result in better spurious perform-

ance. See Table 12 for programming specifics and the Applications

Information section for information on designing a loop filter.

Table 12. CP Programming

CP_I

ICP

0.79 mA

0.99 mA

1.19 mA

1.38 mA

1.59 mA

1.98 mA

2.39 mA

2.79 mA

3.18 mA

3.97 mA

4.77 mA

5.57 mA

6.33 mA

7.91 mA

Table 12. CP Programming

CP_I

ICP

9.51 mA

11.1 mA

Charge Pump Test Mode

When the EN_CPTEST bit is set to 1, the CP_UP bit and

CP_DOWN bit can be programmed to force a constant ICP source

or sink current, respectively, on the CP pin. These bits are com-

monly used as an aid to debug PLL related issues during the

hardware and software development phase of a project. For normal

operation, set EN_CPTEST, CP_UP, and CP_DOWN to 0. Refer to

Figure 65 and Table 13.

Table 13. Charge Pump Debug Functions

EN_CPTEST

CP_UP

CP_DOWN

CP Pin State Debug Test

High-Z

VCO open loop

~VV5_CP

Charge pump

output voltage

verification

~GND

Normal oper-

ation

Not applicable

Charge Pump Bleed Current

A small programmable constant charge pump current, known as

bleed current, can be used to either increase or decrease the

propagation delay from the reference input pins to the clock output

pins.

To enable the bleed current, set the EN_BLEED bit to 1. When

the BLEED_POL bit is set to 1, a small constant source current

is forced onto the CP pin. When the BLEED_POL bit is set to 0,

a small constant sink current is forced onto the CP pin. Refer to

Figure 65.

The bleed current LSB step size is 536 nA. The bleed current delay

step size (tIDEL-STEP) is a function of the bleed current step size

(ICP) and fPFD, as shown in Equation 9. Figure 66 provides a quick

reference of tIDEL-STEP vs. several common ICP and fPFD values.

tIDEL−STEP= 536 nA

ICP×fPFD

(9)

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