TDA9109N View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'TDA9109N' PDF : 32 Pages View PDF
TDA9109/N
OPERATING DESCRIPTION (continued)
IV - 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.
IV.1 - Step-up Mode
Operating Description
- The powerMOS 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
differenceisamplifiedby 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).
Figure 18 : DC/DC Converter
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,thegateiscontrolleddirectly
fromPin28througha capacitor(this allowstospare
a Transformer).In thiscase,a negative-goingpulse
isneededto 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.
I2C
DAC
7bits
8V
± Iadjust
4.8V ±20%
95dB
A
Horizontal Dynamic
Focus Sawtooth
C1
Σ
1/3
C2
1.2V
1.2V
C3
8V
HDF Disc
400ns
6V C4
down
up
S
Q
R
Inhibit SMPS
Command step-up/down
down
up
12V
BOUT
28
REGIN
15
COMP
14
ISENSE
16
TDA9109/N
1MΩ
L
22kΩ
+
VB+
24/32
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