STUW81300 datasheet(28/63 Pages) STMICROELECTRONICS

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Circuit description

STuW81300

28/63

DS11314 Rev 7

7.7

Charge pump

This block consists of two matched current sources, Iup and Idown, which are controlled

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

(ICP) is controlled by selecting one of 32 values by a 5-bit word.

The minimum value of the output current (ICP) is 158 µA.

The charge pump also includes compensation circuitry to take into account variation of

KVCO with VCO control voltage, which changes with temperature and process for a

specified frequency. The KVCO compensation block adjusts the nominal ICP value,

minimizing the variation of the product ICP x KVCO to keep the PLL bandwidth constant for

the specified frequency. In order to compensate the change of KVCO with frequency, the

user should manually adjust the ICP value to keep the PLL bandwidth constant.

In addition, the charge-pump output stage can operate with a 3.3 V to 4.5 V supply voltage.

The LDO_4V5, programmable at 3.3 V and 4.5 V can be used for this purpose.

7.8

Fast lock mode

The fast-lock feature can be enabled to trade fast settling time against spurs rejection,

performance parameters which generally require different settings of PLL bandwidth

(narrow for better spurs rejection and wide for fast settling time).

A narrow bandwidth for low spurs can be designed for the lock state while a wide bandwidth

can be designed for the PLL transients.

The wide bandwidth is achieved during the transient by increasing the charge pump current

and reducing accordingly the dumping resistor value of the loop filter in order to keep the

phase margin of the PLL constant. The duration of the PLL wide band mode, in terms of

number of PFD cycles, is set by programming the fast-lock 13 bit counter.

Table 8. Current value versus selection

CPSEL4

CPSEL3

CPSEL2

CPSEL1

CPSEL0

Current

Value

000

IMIN

158 µA

000

3*IMIN

316 µA

...

111

29*IMIN

4.58 mA

111

30*IMIN

4.74 mA

111

31*IMIN

4.9 mA

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