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TDA4857

I2C-bus autosync deflection controller for PC monitors

Philips Electronics 

Philips Semiconductors

I2C-bus autosync deflection controller for

PC monitors

Product specification

TDA4857PS

Frequency-locked loop

The frequency-locked loop can lock the horizontal

oscillator over a wide frequency range. This is achieved by

a combined search and PLL operation. The frequency

range is preset by two external resistors and the

recommended maximum ratio is -ff-m-m---a-i-n-x = 6---1-.--5-

This can, for instance, be a range from 15.625 to 90 kHz

with all tolerances included.

Without a horizontal sync signal the oscillator will be

free-running at fmin. Any change of sync conditions is

detected by the internal coincidence detector. A deviation

of more than 4% between horizontal sync and oscillator

frequency will switch the horizontal section into search

mode. This means that PLL1 control currents are switched

off immediately.

The internal frequency detector then starts tuning the

oscillator. Very small DC currents at HPLL1 (pin 26) are

used to perform this tuning with a well defined change rate.

When coincidence between horizontal sync and oscillator

frequency is detected, the search mode is first replaced by

a soft-lock mode which lasts for the first part of the next

vertical period. The soft-lock mode is then replaced by a

normal PLL operation. This operation ensures smooth

tuning and avoids fast changes of horizontal frequency

during catching.

In this concept it is not allowed to load HPLL1.

The frequency dependent voltage at this pin is fed

internally to HBUF (pin 27) via a sample-and-hold and

buffer stage. The sample-and-hold stage removes all

disturbances caused by horizontal sync or composite

vertical sync from the buffered voltage. An external

resistor connected between pins HBUF and HREF defines

the frequency range.

Out-of-lock indication (pin HUNLOCK)

Pin HUNLOCK is floating during search mode if no sync

pulses are applied, or if a protection condition is true.

All this can be detected by the microcontroller if a pull-up

resistor is connected to its own supply voltage.

For an additional fast vertical blanking at grid 1 of the

picture tube a 1 V signal referenced to ground is available

at this output. The continuous protection blanking

(see Section “Video clamping/vertical blanking generator”)

is also available at this pin. Horizontal unlock blanking can

be switched off, by control bit BLKDIS via the I2C-bus,

while vertical blanking is maintained.

Horizontal oscillator

The horizontal oscillator is of the relaxation type and

requires a capacitor of 10 nF to be connected at HCAP

(pin 29). For optimum jitter performance the value of 10 nF

must not be changed.

The minimum oscillator frequency is determined by a

resistor connected between pin HREF and ground.

A resistor connected between pins HREF and HBUF

defines the frequency range.

The reference current at pin HREF also defines the

integration time constant of the vertical sync integration.

Calculation of line frequency range

The oscillator frequencies fmin and fmax must first be

calculated. This is achieved by adding the spread of the

relevant components to the highest and lowest sync

frequencies fsync(min) and fsync(max). The oscillator is driven

by the currents in RHREF and RHBUF.

The following example is a 31.45 to 90 kHz application:

Table 1 Calculation of total spread

spread of

IC

CHCAP

RHREF, RHBUF

Total

for fmax

±3%

±2%

±2%

±7%

for fmin

±5%

±2%

±2%

±9%

Thus the typical frequency range of the oscillator in this

example is:

fmax = fsync(max) × 1.07 = 96.3 kHz

fmin = -f-s---y-1--n--.c-0--(--9m----i-n--) = 28.9 kHz

The TV mode is centred around fmin with a control range of

±10%. Activation of the TV mode is only allowed between

15.625 and 35 kHz.

The resistors RHREF and RHBUFpar can be calculated using

the following formulae:

RHREF = -f-m----i-n----+---7--0--8-.--0-×--0---k-1---H2-----z×----×-f--m2--k--i-nΩ--[---k---H-----z----] = 2.61 kΩ

RHBUFpar = -f-m----a---x---+---7--0--8--.-0--×--0--k-1---H2-----z×----×-f--m2--k--a--Ωx---[--k----H-----z---]- = 726 Ω

2000 Jan 31

7

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