9
Circuit description new circuits
33
L7.3A
The safety resistor R3525 limits the drive current to the gate of
FET 7518. In a stable situation Vpin 14 is typical 2V4.
9.3.13 Typical values for the L7.3 chassis
9.4
Protections
9.4.1
Over voltage protection of the secondary voltages
After start-up the supply voltage pin 1 will be take over by
positive winding 8 - 9 and after start up pin 1 is the measuring
point for the secondary output voltage. After start-up via an
internal switch pin 1 is internally tapped (voltage divider) to a
voltage which can be measured at pin 6; pin 6 is also a
measuring point for the secondary output voltages. If the
voltage at pin 6 > 2.5 Volts, the logic in IC 7520 will shut down
the output pin 3. This 2.5 Volts threshold at pin 6 is equivalent
to 16 Volts DC which is equivalent to a voltage at the supply
voltage (V BATT of approx. 108 Volts DC in normal operation
and 130 Volts DC in standby. After switching "off", the IC starts
up again because of over voltage protection; see slow-start.
REMARK: In case an overvoltage situation is sensed at the
secondary output voltages, the SMPS will go in protection; in
case the overvoltage situation remains present the SMPS will
start slow, etc.; this is a very good audible hick-up mode.
9.4.2
Under voltage protection of the secondary voltages
If the supply voltage at pin 1 < 9 Volts DC the output pulse at
pin 3 will be shut down.
Note: The DC voltage at pin 1 is equivalent to a voltage at V
BATT of approx. 70V DC in normal operation and the voltage
at pin 1 of 7.5 Volts is equivalent to a voltage at (V BATT of
approx. 55V DC in normal operation and 65V DC in standby.
In case an over voltage situation is sensed at the secondary
output voltages, the SMPS will first switch "off" the pulses and
later the complete IC 7520 will be switched "off". In case the IC
7520 is switched "off" also the SMPS will switch "off". In case
the under voltage situation remains present, the SMPS will give
under voltage protection like slow-start, etc; this is a very good
audible hick-up mode.
9.4.3
Unload protection
In case the load goes down e.g. the line goes down because of
stand-by mode or some failure in the line circuitry this will be
detected by IC7520 via I-prim and secondary output voltages
sensing. In case the load decreases below a certain threshold
the SMPS will switch in "reduced frequency mode" of about 20
kHz ; this threshold is determined by the voltage level at pin 12
of IC 7520.
REMARK: In case of an unload situation the set will switch to
"low frequency mode" or standby mode. Whether this unload
situation of the SMPS is caused by the standby command or by
a failure e.g. in the line circuitry this can be determined by
switching on the set again which the remote control. In case of
standby mode the TV will switch "on" again; in case of unload
situation the set will NOT switch "on".
9.4.4
Overload (short-circuit) protection
If the secondary load becomes too high, I-prim also becomes
too high; this is sensed by the current sense voltage at pin 7.
This voltage at pin 7 is not allowed to exceed 1 Volts DC and it
has current limiting. As the I-prim is limited, the secondary
output voltages will drop and supply voltage at pin 1 will also
drop. As soon as pin 1 < 9Volts DC the driving pulses at pin 3
will stop. As a result of these two mechanisms in case of an
overload the secondary voltages will drop very fast, this is
called the foldback mechanism. The foldback point can be
adjusted by pin 5 of IC7520 and this point is adjusted to a
maximum tolerable output power of 85W at 90 Volts AC and
165W at 276Volts AC. After this foldback the IC starts up again
(see slow-start); in case the overload situation remains
present, the SMPS will give foldback again, slow-start, etc.: as
a result in case of short-circuit or overload the TV will be in a
good audible hick-up mode.
9.5
Secondary side
9.5.1
Output voltages
See 9.1.2 for output voltages.
9.5.2
Regulator IC TDA 8319 (7541)
This is a low-dropout regulator. This IC has an output voltage
of +5 Volts , +8 Volts and RESET signal; the input is 11 Volts.
The Pin configuration of this IC is listed below:
•
Pin 1 of the IC is input 1 and the output 1 pin 9 that is +5
Volts DC.
•
Pin 2 of the IC is input 2 and the output 1 pin 8 that is +8
Volts DC.
•
Pin 3 of the IC is a timing capacitor input and controls the
turn-on delay of the reset signal.
•
Pin 4 of the IC is a disable pin connected to standby pin of
the microcontroller, if the voltage below 0.5 Volts the +8
Volts output will be disabled.
•
Pin 5 of the IC is a common ground .
•
Pin 6 of the IC is a reset voltage output connected to the
microcontroller. This reset output is developed with respect
+5 Volts output level of 4.95 Volts.
•
Pin 7 of the IC is a program pin for output 2, in this circuit
this output pin 8 is programmed to +8.2 Volts by resistors
3552, 3553 and 3554.
Other features of this IC:
•
Built-in thermal protection.
•
Built-in over current and short circuit protection for +5 Volts
and +8Volts.
•
Low dropout voltage
Mains Voltage:
90 - 276V multi voltage
150 - 276V single voltage
Mains frequency: 50 Hz , 60 Hz
Power consumption
in normal mode:
25"
75W + /- 10%
29"
90W +/- 10%
Power Consumption in stand-by mode: 11W
