FUNCTIONAL DESCRIPTION APPENDIX
5. SECAM L/L' sound switching circuit.
The microcontroller pin 4 is a three levels output. The voltage and configuration of this port is described below :
Sound modePort configurationVoltage
FMPush PullInternally shorted to ground
AM LPush PullPull up to 3.3V
AM LHigh ImpedanceFixed by R511, R156, R157
In FM mode the microcontroller is internally grounded to pin 4. The TDA9830 output is muted
6. Sound amplification
The device TDA7267A is a mono audio amplifier in powerDIP package specially designed for TV application. Thanks to
the fully complementary output configuration the device delivers a rail to rail voltage swing without need of bootstrap
capacitors. No external heat sink is needed as the Cu ground plane of the PCB is used as heat dissipation.
7. Vertical deflection
The vertical driver circuit is a bridge configuration. The deflection coil is connected between the output amplifiers, which
are driven in phase opposition. The differential input circuit is voltage driven. The input circuit is especially intended for
direc connection to driver circuits which deliver symmetrical current signals, but is also suitable for asymmetrical currents.
The output current of these devices is converted to voltages at the input pins via resistors R350 and R351. The differential
input voltage is compared with the output current through the deflection coils measured as voltage across R302, which
provides internal feed-back information. The voltage across R302 is proportional to the output current.
Flyback voltage
The flyback voltage is determined by an additional supply voltage Vfb. The principle of operation with two supply voltages
(class G) makes it possible to fix the supply voltage Vp optimum for the scan voltage and the second supply voltage Vfb
optimum for the flyback voltage. Using this method, very high efficiency is achieved. The supply voltage Vfb is almost
totally available as fly-back voltage across the coil, this being possible due to the absence of a coupling capacitor.
Protection
The output circuit has protection circuits for :
- Too high die temperature
- overvoltage of output stage A
Guard circuit
The guard signal is not used by the TDA9361/ TDA9381 to blank the screen in case of a fault condition.
Damping resistor
For HF loop stability a damping resistor (R305) is connected across the deflection coil.
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