LTC6952 datasheet(19/80 Pages) AD

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LTC6952

6952f

For more information www.analog.com

Preliminary Technical Data

Advance Product Information Subject to Change

Rev PrA

OPERATION

Larger values of COUNTS lead to more accurate and stable

lock indications at the expense of longer lock indication

times. Set LKCT[1:0] = 0 to disable the lock indicator. See

Table 5 for LKCT[1:0] programming and the Applications

Information section for examples.

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 4 below for more details.

Figure 4. UNLOCK, LOCK, and LOCK Timing

Figure 5. Simplified Charge Pump Schematic

+tLWW

–tLWW

UNLOCK FLAG

LOCK FLAG

t = COUNTS/fPFD

6952 F04

PHASE

DIFFERENCE

AT PFD

VCP

+/2

VCP

ICP

CPMID

CPUP

CPDN

DOWN

6952 F05

Table 6 for programming specifics and the Applications

Information section for loop filter examples.

Table 6. CP[4:0] Programming

CP[4:0]

ICP

420µA

500µA

592µA

700µA

842µA

1.00mA

1.18mA

1.40mA

1.68mA

2.00mA

2.36mA

2.81mA

3.37mA

4.00mA

4.72mA

5.61mA

6.73mA

8.02mA

9.43mA

11.20mA

20 to 31

Invalid

Charge Pump Functions

The charge pump contains additional features to aid in

system startup. See Table 7 for a summary.

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 ICPmaybesetfrom420µAto

11.2mA using the CP[4:0] bits found in serial port register

h0A. A larger ICP can result in lower in-band noise due to

the lower impedance of the loop filter components. See

LTC6952 Datasheet(PDF) 19 Page - Analog Devices

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