TDA7427 datasheet(8/21 Pages) STMICROELECTRONICS

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DIVIDER FROM VCO FREQUENCY TO

REFERENCE FREQUENCY

This divider provides a low frequency fSYN which

phase is compared with the reference frequency

fREF. It is controlled by the registers PC0 to PC4

and PC5 to PC15

OPERATING MODES

Four operating modes are available fo PLL; they

are user programmable with the Mode PM regis-

ters (see table):

PM0

PM1

Operating Mode

Standby

AM (swallow)

AM (direct)

- Standby mode: in this mode all device func-

tions are stopped. This allows low current

consumption without loss of information in all

registers. The pin LP-OUT is forced to 0V,

and all data registers are set to EFH. The os-

cillator keeps running.

- FM and AM (SW) Swallow Mode (SW):

in this mode the FM or AM signal is applied to

a 32/33 prescaler, which is controlled by a 5

bit divider ’A’.The 5 bit register (PC0 to PC4)

controls this divider. In parallel the output of

the prescaler is connected to a 11 bit divider

’B’. (PC5 to PC15).

fOSC = (R+1)

⋅ fREF

Dividing range calculation :

fVCO =[ 33

⋅ A + (B + 1 - A) ⋅ 32 ] ⋅ fREF

fVCO = (32

⋅ B + A + 32) ⋅ fREF

Important:for correct operationA

≤ 32,B ≥ A,with

A andB variable values of the dividers).

- AM direct mode: the AM signal is applied di-

rectly to the 16 bit static divider ’C’. (PC0 to

PC15)

fOSC =(R + 1)

⋅ f REF

Dividing range:

fVCO =(C + 1)

⋅ fREF

THREE STATE PHASE COMPARATOR

The phase comparator generates a phase error

signal according to phase difference between

fSYN and fREF. This phase error signal drives the

charge pump current generator (fig. 3)

CHARGE PUMP CURRENT GENERATOR

This stage generates signed pulses of current.

The phase error signal decides the duration and

polarity of those pulses.

The current absolute values are programmable by

A0, A1, A2 registers for high current and B0, B1,

registers for low current.

LOW NOISE CMOS OP-AMP

An internal voltage divider at pin VREF connects

the positive input of the low noise Op-Amp. The

charge pump output connects the negative input.

This internal amplifier in cooperation with external

components can provide an active filter.

OSC IN

RC0 ... RC15

PREDIVIDER

PHASE

DETECTOR

AM IN

FM IN

PC0 ... PC15

PRESCALER

fref

fsyn

D95AU376A

∆ϕ

TO CHARGE

PUMP

Figure 2. AM direct mode operation for SW, MW and LW

TDA7427

8/21

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