TDA7298 datasheet(9/11 Pages) STMICROELECTRONICS

Teilenummer	TDA7298

Bauteilbeschribung	28W Hi-Fi AUDIO POWER AMPLIFIER WITH MUTE / STAND-BY

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SHORT-CIRCUIT PROTECTION

The TDA7298 has an original circuit which pro-

tects the device during accidental short-circuit be-

tween output and GND / -Vs / +Vs, taking it in

STAND-BY mode, so limiting also dangerous DC

current flowing throught the loudspeaker.

If a short-circuit or an overload dangerous for the

final transistors are detected, the concerned SOA

circuit sends out a signal to the latching circuit

(with a 10

µs delay time that prevents fast random

spikes from inadvertently shutting the amplifier

off) which makes Q1 and Q2 saturate (see Block

Diagram). Q1 immediately short-circuits to ground

the A point turning the final stage off while Q2

short-circuits to ground the external capacitor

driving the pin 3 (Mute/Stand-by) towards zero

potential.

Only when the pin 3 voltage becomes lower than

1V, the latching circuit is allowed to reset itself

and restart the amplifier, provided that the short-

circuit condition has been removed. In fact, a win-

dow comparator is present at the output and it is

aimed at preventing the amplifier from restarting if

the output voltage is lower than 0.35 Total Supply

Voltage or higher than 0.65 Total Supply Voltage.

If the output voltage lies between these two

thresholds, one may reasonably suppose the

short-circuit has been removed and the amplifier

may start operating again.

The PLAY/MUTE/STAND-BY function pin (pin 3)

is both ground- and positive supply-compatible

and can be interfaced by means of the R5,C3 net

either to a TTL or CMOS output (

µ-Processor) or

to a specific application circuit.

The R5,C3 net is fundamental, because connect-

ing this pin directly to a low output impedance

driver such as TTL gate would prevent the correct

operation during a short-circuit. Actually a final

stage overload turns on the protection latching

circuit that makes Q2 try to drive the pin 3 voltage

under 0.8 V. Since the maximum current this pin

can stand is 3 mA, one must make sure the fol-

lowing condition is met:

R5

≥

(VA − 0.7V)

3mA

that yields: R5, min = 1.5 K

Ω with VA=5V.

In order to prevent pop-on and -off transients, it is

advisable to calculate the C3,R5 net in such a

way that the STAND-BY/MUTE and MUTE/PLAY

threshold crossing slope (positive at the turn-on

and vice-versa) is less than 100 V/sec.

Figure 16: Thermal Protection Block Diagram

THERMAL PROTECTION

The thermal protection operates on the 125

µA

current generator, linearly decreasing its value

from 90

°C on. By doing this, the A voltage slowly

decreases thus switching the amplifier first to

MUTE (at 145

°C) and then to STAND-BY

(155

°C).

Figure 17: Maximum Allowable Power Dissipa-

tion vs. Ambient Temperature.

The maximum allowable power dissipation de-

pends on the size of the external heatsink (ther-

mal resistance case-ambient); figure 17 shows

the dissipable power as a function of ambient

temperature for different thermal resistance.

TDA7298

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