TDA8924 datasheet(8/35 Pages) PHILIPS

8 / 35 page

2003 Jul 28

Philips Semiconductors

Objective speciﬁcation

× 120 W class-D power ampliﬁer

TDA8924

In an application circuit:

• Internal oscillator: ROSC connected from pin OSC to VSS

• External oscillator: connect oscillator signal between pin

OSC and SGND; delete ROSC and COSC.

8.3

Protections

Temperature, supply voltage and short-circuit protection

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

OVER-TEMPERATURE

If the junction temperature (Tj) exceeds 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 THE 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 12 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 the mode pin is

switched from standby to mute, the condition 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.

8.3.4

SUPPLY VOLTAGE ALARM

If the supply voltage falls below

±12.5 V the undervoltage

protection is activated and the system shuts down

correctly. If the internal clock is used, this switch-off will be

silent and without plop noise. When the supply voltage

rises above the threshold level the system is restarted

again after 100 ms. If the supply voltage exceeds

±32 V

the overvoltage protection is activated and the power

stages shut down. They are re-enabled as soon as the

supply voltage drops below the threshold level.

It has to be stressed that the overvoltage protection only

protects against damage due to supply pumping effects;

see Section 16.7. Apart from the power stages, the rest of

the circuitry remains connected to the power supply. This

means, that the supply itself should never exceed 30 V.

An additional balance protection circuit compares the

positive (VDD) and the negative (VSS) supply voltages and

is triggered if the voltage difference between them

exceeds a certain level. This level depends on the sum of

both supply voltages. An expression for the unbalanced

threshold level is as follows: Vth(unb) ~ 0.15 × (VDD +VSS).

Example: With a symmetrical supply of

±30 V the

protection circuit will be triggered if the unbalance exceeds

approximately 9 V; see also Section 16.7.

8.4

Differential audio inputs

For a high common mode rejection ratio and a maximum

of flexibility in the application, the audio inputs are fully

differential. By connecting the inputs anti-parallel the

phase of one of the channels can be inverted, so that a

load can be connected between the two output filters.

In this case the system operates as a mono BTL amplifier

and with the same loudspeaker impedance an

approximately four times higher output power can be

obtained.

The input configuration for mono BTL application is

illustrated in Fig.5; for more information see Chapter 16.

In the stereo single-ended configuration it is also

recommended to connect the two differential inputs in

anti-phase. This has advantages for the current handling

of the power supply at low signal frequencies.

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