LTC6948 datasheet(16/38 Pages) AD

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LTC6948

6948fa

For more information www.linear.com/LTC6948

operaTion

Figure 4. Simplified Charge Pump Schematic

Figure 3. UNLOCK and LOCK Timing

+tLWW

–tLWW

UNLOCK FLAG

LOCK FLAG

t = COUNTS/fPFD

6948 F03

PHASE

DIFFERENCE

AT PFD

ICP

THI

0.9V

VCP+

TLO

+–

0.9V

6948 F04

+–

VCP+/2

CP LINEARIZER

CONTROL

ENABLE

CPMID

CPUP

ILIN

CPDN

CP[2:0]

CPINV

DOWN

ND[9:0]

INTN

CPLE

VCP+

Table 5. LKCT[1:0] Programming

LKCT[1:0]

COUNTS

Lock Indicator Disabled

256

2048

When the PFD phase difference is greater than tLWW, the

lock indicator immediately asserts the UNLOCK status

flag and clears the LOCK flag, indicating an out-of-lock

condition. The UNLOCK flag is immediately de-asserted

when the phase difference is less than tLWW. See Figure 3

for more details.

Note that fREF must be present for the LOCK and UNLOCK

flags to properly assert and clear.

to the lower impedance of the loop filter components,

although currents larger than 5.6mA typically cause worse

spurious performance. See Table 6 for programming

specifics and the Applications Information section for

loop filter examples.

Table 6. CP[2:0] Programming

CP[2:0]

ICP

1.0mA

1.4mA

2.0mA

2.8mA

4.0mA

5.6mA

8.0mA

11.2mA

The CPINV bit found in register h0D should be set for ap-

plications requiring signal inversion from the PFD, such as

for complex external loops using an inverting op amp. A

passiveloopfilterasshowninFigure13requiresCPINV=0.

CHARGE PUMP

The charge pump, controlled by the PFD, forces sink

(DOWN) or source (UP) current pulses onto the CP pin,

which should be connected to an appropriate loop filter.

SeeFigure4forasimplifiedschematicofthechargepump.

The output current magnitude ICP may be set from 1mA to

11.2mA using the CP[2:0] bits found in serial port register

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