TDA7294 bootstrap and svr pins

[youyoung21147]

Hello,

I have bought some TDA7294 chip amps and found two strange pins on them : the boostrap and svr pins.

I've read the datasheet of this component but I didn't understand the purpose of these two pins. Maybe I've read bad, or it isn't clearly explained... If I'm right, they don't exist on ICs like LM3886 and LM4780, which I have already played with in the past.

Can I leave them floating ? What is their precise purpose ?

My application is to power a tweeter in an active setup, and cables will be no longer than 2.5 meters.

TDA7294 datasheet

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If you did look at the first page (fiure 1) of the datasheet you will see that the bootstrap pin is connected to the output via a polarized cap, and SVR (pin 4) is shall be connected to earth.

Here is an updated datasheet for the TDA7294


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[youyoung21147]

Thanks for the updated datasheet !

I've understood the purpose of SVR : it seems to serve as signal reference when in mute mode, so should be grounded.

But I don't understand the role of the bootstrap pin.

I know they propose to place a 22µF cap between this pin and out, but it doesn't explain me what is the function on this pin.

Can I leave it floating ? Or replace the cap by a wire ? They say a smaller value degrades signal at LF, I assume it is a form of feedback, so any component in this path must degrade audio quality, especially if an electrolytic is used...

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No you must provide a bootstrap cap. If you don't the amp will not be able to swing 100% positive and you will get a negative DC at the output when you play at loud volume.

The TDA7294 uses two n-channel mosfet at its output. The output mosfets need a voltage at its gate some volts above the output voltage. If the upper mosfet is to supply an output voltage at or near the supply voltage, the gate voltage has to be some volts above the supply voltage.
It uses the clever bootstrap trick that uses the alternating output signal and rectifies it internally to a voltage above the supply voltage.

If you search the net for bootstrapping in amplifiers I'm sure you'll get better understanding of what the bootstrap cap does.


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[youyoung21147]

OK thanks for the explanation !

So if I understand you right, I need a 22µF or more cap in this place, and its quality is not a huge concern so a large electrolytic should do the trick.

I've just tried using a wire in place of the cap, and I get -32V DC offset at the output. Didn't try leaving it disconnected, I've read somewhere it could blow the IC...