TDA9109N datasheet(24/32 Pages) STMICROELECTRONICS

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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.

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.

IV.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

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 internal 4.8V reference (IVREF). The

differenceis amplifiedby 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.

IV.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-onas 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 fre-

quency,

- B+ voltage always lower than the DC source,

- No current limitation.

IV.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.

- Fora P-channelMOS,thegateis controlleddirectly

fromPin28througha capacitor(this allowstospare

a Transformer).In thiscase,a negative-goingpulse

is neededto makethe 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 trans-

former can be easily adapted to the negative-

going control pulse.

B+

1/3

Horizontal Dynamic

Focus Sawtooth

down

HDF Disc

400ns

down

Inhibit SMPS

12V

Command step-up/down

BOUT

SENSE

COMP

REGIN

95dB

± I

adjust

DAC

7bits

I2C

TDA9109/N

Ω

22k

Ω

1.2V

4.8V

±20%

Figure 18 : DC/DC Converter

TDA9109/N

24/32

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