TDA9111 datasheet(33/43 Pages) STMICROELECTRONICS

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TDA9111

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3.7 Dynamic Horizontal Phase Control

IOUT=K4 (VOUT - VDCMID) + K5 (VOUT - VDCMID)

K4 is adjustable by the parallelogram I2C register.

K5 is adjustable by the side pin balance I

2C regis-

ter.

4 DC/DC CONVERTER PART

This unit controls the switch-mode DC/DC con-

verter. It converts a DC constant voltage into the

B+ voltage (roughly proportional to the horizontal

frequency) necessary for the horizontal scanning.

This DC/DC converter can be configured either in

step-up or step-down mode. In both cases it oper-

ates very similarly to the well known UC3842.

4.1 Step-up Mode

Operating Description

– The power MOS is switched ON during the fly-

back (at the beginning of the positive slope of the

horizontal focus sawtooth).

– The power MOS is switched OFF when its cur-

rent reaches a predetermined value. For this pur-

pose, a sense resistor is inserted in its source.

The voltage on this resistor is sent to Pin16

(ISENSE).

– The feedback (coming either from the EHV or

from the flyback) is divided to a voltage close to

5.0V and compared to the internal 5.0V refer-

ence (IVREF). The difference is amplified by an

error amplifier, the output of which controls the

power MOS switch-off current.

Main Features

– Switching synchronized on the horizontal fre-

quency,

– B+ voltage always higher than the DC source,

– Current limited on a pulse-by-pulse basis.

The DC/DC converter is disabled:

– when VCC or VDD are too low,

– when X-Ray protection is latched,

– directly through I2C bus.

When disabled, BOUT is driven to GND by a

0.5mA current source. This feature allows to im-

plement externally a soft start circuit.

4.2 Step-down Mode

In step-down mode, the ISENSE information is not

used any more and therefore not sent to the

Pin16. This mode is selected by connecting this

Pin16 to a DC voltage higher than 6V (for example

VREF-V).

Operating Description

– The power MOS is switched ON as for the step-

up mode.

– The feedback to the error amplifier is done as for

the step-up mode.

– The power MOS is switched OFF when the HFO-

CUSCAP voltage gets higher than the error am-

plifier output voltage.

Main Features

– Switching synchronized on the horizontal fre-

quency,

– B+ voltage always lower than the DC source,

– No current limitation.

4.3 Step-up and Step-down Mode Comparison

In step-down mode the control signal is inverted

compared with the step-up mode.This, for the fol-

lowing reason:

– In step-up mode, the switch is a N-channel MOS

referenced to ground and made conductive by a

high level on its gate.

– In step-down, a high-side switch is necessary. It

can be either a P- or a N-channel MOS.

• For a P-channel MOS, the gate is controlled

directly from Pin 28 through a capacitor (this

allows to spare a Transformer). In this case,

a negative-going pulse is needed to make

the

MOS

conductive.

Therefore

necessary to invert the control signal.

• For a N-channel MOS, a transformer is

needed to control the gate. The polarity of

the transformer can be easily adapted to the

negative-going control pulse.

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