TDA9109 datasheet(24/30 Pages) STMICROELECTRONICS

24 / 30 page

IV - DC/DC CONVERTER PART

OPERATING DESCRIPTION (continued)

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.

ThisDC/DC converter must be configured in step-

up mode. It operates very similarly to the well

known UC3842.

IV.1 - Step-up Mode

Operating Description

- The power MOS is switched-on at the middle of

the horizontal flyback.

- The power MOS is switched-off when its current

reachesa predeterminedvalue. 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

4.8V and compared to the internal4.8V 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 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 I

2C bus.

When disabled, BOUT is driven to GND by a 0.5mA

current source. This feature allows to implement

externally a soft start circuit.

B+

1/3

400ns

Inhibit SMPS

12V

BOUT

SENSE

COMP

REGIN

95dB

± I

adjust

DAC

7bits

I2C

TDA9109/SN

Ω

22k

Ω

1.2V

4.8V

±20%

Inhibit

SMPS

Soft

Start

Figure 17 : DC/DC Converter

TDA9109/SN

24/30

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