TDA9109A datasheet(37/47 Pages) STMICROELECTRONICS

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TDA9109A

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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

flyback (at the beginning of the positive slope

of the horizontal focus sawtooth).

• The power MOS is switched-off when its

current reaches a predetermined value. For

this purpose, 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

reference (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

frequency

• B+ voltage always higher than the DC source

• Current limited on a pulse-by-pulse basis

The DC/DC converter is disabled:

• when V

CC 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 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

HFOCUSCAP voltage get higher than the

error amplifier output voltage

Main Features

• Switching synchronized on the horizontal

frequency

• 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.

The reason for this, is the following:

• 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 it is 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 neg-

ative-going control pulse.

TDA9109A Datasheet(PDF) 37 Page - STMicroelectronics

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