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TDA8922 Datasheet(PDF) 9 Page - NXP Semiconductors |
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TDA8922 Datasheet(HTML) 9 Page - NXP Semiconductors |
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9 / 36 page  2003 Mar 20 9 Philips Semiconductors Objective specification 2 × 25 W class-D power amplifier TDA8922 8.2 Pulse width modulation frequency The output signal of the amplifier is a PWM signal with a carrier frequency of approximately 350 kHz. Using a 2nd-order LC demodulation filter in the application results in an analog audio signal across the loudspeaker. This switching frequency is fixed by an external resistor ROSC connected between pin OSC and VSSA. With the resistor value given in the schematic diagram of the reference design, the carrier frequency is typical 350 kHz. The carrier frequency can be calculated using the following equation: If two or more class-D amplifiers are used in the same audio application, it is advisable to have all devices operating at the same switching frequency. This can be realized by connecting all OSC pins together and feed them from a external central oscillator. Using an external oscillator it is necessary to force pin OSC to a DC-level above SGND for switching from the internal to an external oscillator. In this case the internal oscillator is disabled and the PWM will be switched on the external frequency. The frequency range of the external oscillator must be in the range as specified in the switching characteristics; see Chapter 13. In an application circuit: • Internal oscillator: ROSC connected between pin OSC and VSSA • External oscillator: connect the oscillator signal between pins OSC and SGND; delete ROSC and COSC. 8.3 Protections Temperature, supply voltage and short-circuit protections sensors are included on the chip. In the event that the maximum current or maximum temperature is exceeded the system will shut down. 8.3.1 OVERTEMPERATURE If the junction temperature Tj > 150 °C, then the power stage will shut down immediately. The power stage will start switching again if the temperature drops to approximately 130 °C, thus there is a hysteresis of approximately 20 °C. 8.3.2 SHORT-CIRCUIT ACROSS LOUDSPEAKER TERMINALS AND TO SUPPLY LINES When the loudspeaker terminals are short-circuited or if one of the demodulated outputs of the amplifier is short-circuited to one of the supply lines, this will be detected by the current protection. If the output current exceeds the maximum output current of 4 A, then the power stage will shut down within less than 1 µs and the high current will be switched off. In this state the dissipation is very low. Every 100 ms the system tries to restart again. If there is still a short-circuit across the loudspeaker load or to one of the supply lines, the system is switched off again as soon as the maximum current is exceeded. The average dissipation will be low because of this low duty cycle. 8.3.3 START-UP SAFETY TEST During the start-up sequence, when pin MODE is switched from standby to mute, the conditions at the output terminals of the power stage are checked. In the event of a short-circuit at one of the output terminals to VDD or VSS the start-up procedure is interrupted and the systems waits for open-circuit outputs. Because the test is done before enabling the power stages, no large currents will flow in the event of a short-circuit. This system protects for short-circuits at both sides of the output filter to both supply lines. When there is a short-circuit from the power PWM output of the power stage to one of the supply lines (before the demodulation filter) it will also be detected by the start-up safety test. Practical use of this test feature can be found in detection of short-circuits on the printed-circuit board. Remark: This test is only operational prior to or during the start-up sequence, and not during normal operation. During normal operation the maximum current protection is used to detect short-circuits across the load and with respect to the supply lines. f osc 910 9 × R OSC ------------------- Hz = |
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