TDA7540N Datasheet, PDF (35/76 Pages) STMicroelectronics – AM/FM car radio tuner IC with stereo decoder and intelligent selectivity system (ISS)

TDA7540N
Functional description
4.4.2
Frequency generation for phase comparison
The RF signals applies a two modulus counter (32/33) pre-scaler, which is controlled by a 5-
bit A-divider. The 5-bit register (PC0 to PC4) controls this divider. In parallel the output of the
prescaler connects to an 11-bit B-divider. The 11-bit PC register (PC5 to PC15) controls this
divider
Dividing range behind VCO divider:
f_VCOdiv = [33 x A + (B + 1 - A) x 32] x f_REF
f_VCOdiv = (32 x B + A + 32) x f_REF
Important: For correct operation: A ≤ 32; B ≥ A
Three state phase comparator
The phase comparator generates a phase error signal according to phase difference
between f_SYN and f_REF. This phase error signal drives the charge pump current generator.
Charge pump current generator
This system generators signed pulses of current. The phase error signal decides the
duration and polarity of those pulses. The current absolute values are programmable by A
register for high current and B register for low current.
Inlock detector
Switching the chargepump in low current mode can be done either via software or
automatically by the inlock detector, by setting bit LDENA to "1".
After reaching a phase difference about lower than 40nsec the chargepump is forced in low
current mode. A new PLL divider alternation by I²C-Bus will switch the chargepump in the
high current mode.
Low noise CMOS op-amp
An internal voltage divider at pin VREF2 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. The negative input is
switchable to three input pins, to increase the flexibility in application. This feature allows two
separate active filters for different applications.
While the high current mode is activated LPHC output is switched on.
IF counter block
The aim of IF counter is to measure the intermediate frequency of the tuner for AM and FM
mode. The input signal for FM and AM upconversion is the same 10.7MHz IF level after
limiter. AM 450KHz signal is coming from narrow filtered IF2 before demodulation. A switch
controlled by IF counter mode (IFCM) is choosing the input signal for IF counter.
The grade of integration is adjustable by eight different measuring cycle times. The
tolerance of the accepted count value is adjustable, to reach an optimum compromise for
search speed and precision of the evaluation.
The IF-counter mode
The IF counter works in 3 modes controlled by IFCM register.
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