Philips CEM5000/00 Service Manual page 27

TENTATIVE
4. Self-diagnosis function (pin 25)
By detecting the unusual state of the IC, the signal is output to the pin 25.
Also, by controlling the standby switch after the signal of the pin 25 is detected by the microcomputer,
the burnout of the speaker can be prevented.
1) Shorting to VCC / Shorting to ground : The pin 25 becomes the low level.
2) Load shorting : The pin 25 is alternated between the low level and the high level according to the output signal.
3) Output offset detection : when the output offset voltage exceeds the detection threshold voltage,
* Note: The output offset abnormality is thought of as the leakage current of the input capacitor.
In addition, the pin 25 has become the NPN open collector output (active low).
The pin 25 must be left open when this function is not used.
5. Sound Quality (low frequencies)
By varying the value of input capacitor, low-frequency characteristic can be improved.
However, it is concerned the shock noise. Please confirm in each set when the capacitance value varies.
6. Pop noise
For pop noise prevention, it is recommended to use the muting function at the same time.
- Please turn on the muting function simultaneously with power supply on when the amplifier is
turned on. Next, turn off the muting function after the output DC potential stabilization.
- When the amplifier is turn off, turn off the power supply after turning on the muting function.
. Oscillation Stability
Pay due attention on the following points because parasitic oscillation may occur due to effects of the capacity
load, board layout, etc.
(1) Capacity load
When the capacitor is to be inserted between each output pin and GND so as to prevent electric mirror noise,
select the capacitance of maximum 1500 pF. (Conditions: Our recommended board, RL = 4 )
(2) Board layout
- Provide the VCC capacitor of 0.1µF in the position nearest to IC.
- PREGND must be independently wired and connected to the GND point that is as stable as possible,
such as the minus pin of the 2200µF VCC capacitor.
In case of occurrence of parasitic oscillation, any one of following parts may be added as a countermeasure.
Note that the optimum capacitance must be checked for each set in the mounted state.
- Series connection of CR (0.1µF and 2.2 ) between BTL outputs
- Series connection of CR(0.1µF and 2.2 ) between each output pin and GND.
Note : Information in this document is subject to change without notice.
the pin 25 becomes the low level.
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