LTC6946 datasheet(14/30 Pages) LINER

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LTC6946

6946fa

OPERATION

Table 3. LKWIN[1:0] Programming

LKWIN[1:0]

tLWW

fPFD

3ns

>5MHz

10ns

≤5MHz

30ns

≤1.7MHz

90ns

≤550kHz

The PFD phase difference must be less than tLWW for the

LOKCNT number of successive counts before the lock

indicator asserts the LOCK flag. The LKCNT[1:0] bits found

in register h09 are used to set LOKCNT depending upon

the application. See Table 4 for LKCNT[1:0] programming

and the Applications Information section for examples.

Table 4. LKCNT[1:0] Programming

LKCNT[1:0]

COUNTS

032

128

512

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 4

for more details.

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. See

Figure 5 for a simplified schematic of the charge pump.

The output current magnitude ICPmaybesetfrom250μAto

11.2mA using the CP[3:0] bits found in serial port register

h09. A larger ICP can result in lower in-band noise due to

the lower impedance of the loop filter components. See

Table 5 for programming specifics and the Applications

Information section for loop filter examples.

Table 5. CP[3:0] Programming

CP[3:0]

ICP

250μA

350μA

500μA

700μA

1.0mA

1.4mA

2.0mA

2.8mA

4.0mA

5.6mA

8.0mA

11.2mA

12 to 15

Invalid

The CPINV bit found in register h0A should be set for

applications requiring signal inversion from the PFD,

such as for complex external loops using an inverting op

amp. A passive loop filter as shown in Figure 14 requires

CPINV = 0.

+tLWW

–tLWW

UNLOCK FLAG

LOCK FLAG

t = COUNTS/fPFD

6946 F04

PHASE

DIFFERENCE

AT PFD

Figure 4. UNLOCK and LOCK Timing

THI

0.9V

VCP

TLO

–

0.9V

6946 F05

–

VCP

+/2

CPMID

CPUP

CPDN

DOWN

Figure 5. Simplified Charge Pump Schematic

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