STW81101ATR datasheet(20/48 Pages) STMICROELECTRONICS

20 / 48 page

Circuit description

STW81101

20/48

5.5

Phase frequency detector (PFD)

The PFD takes inputs from the reference and the VCO dividers and produces an output

proportional to the phase error. The PFD includes a delay gate that controls the width of the

anti-backlash pulse. This pulse ensures that there is no dead zone in the PFD transfer

function.

Figure 17 is a simplified schematic of the PFD.

Figure 17.

PFD diagram

5.6

Lock detect

This signal indicates that the difference between rising edges of both UP and DOWN PFD

signals is found to be shorter than the fixed delay (roughly 5 ns). The Lock Detect signal is

high when the PLL is locked and low when the PLL is unlocked. Lock Detect consumes

current only during PLL transients.

5.7

Charge pump

This block drives two matched current sources, IUP and IDOWN, which are controlled

respectively by UP and DOWN PFD outputs. The nominal value of the output current is

controlled by an external resistor (to be connected to the REXT input pin) and a 3-bit word

that allows selection among 8 different values.

The minimum value of the output current is: IMIN = 2*VBG/REXT (VBG~1.17 V)

DFF

VDD

DFF

VDD

Delay

Down

ABL

Fref

ref

STW81101ATR データシート(PDF) 20 Page - STMicroelectronics