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Problem with TDA7295 oscillating

I am using a TDA7295 to drive a nominal 12 - 15 ohm load. On the test bench the load is a chunk of electric fire element with connections made by adjustable straps, so it is slightly inductive. The load in the final version will be a loudspeaker voice coil on the end of some unavoidably long wiring.

The TDA7295 data sheet claims "No Boucherot cell" but does not elaborate any further. I am using the recommended circuit and the gain is set by a divider of 22k and 680 ohms, exactly as specified.

An oscilloscope shows bursts of H.F. oscillation at two points on the output waveform as it approaches +Vcc and -Vcc. These only occur on load and they disappear if the signal amplitude is reduced below approximately half the maximum peak-to-peak voltage. The power supply decoupling is as recommended by the manufacturers and is mounted within

0.25 " of the chip pins.

To eliminate the possibility of earth loops or inductive coupling from the output currents by temporarily connecting the load across different signal and earth points in the layout. I have connected the signal generator direct to the chip input. Neither of these tests made any difference. The effect is independent of signal frequency below 25 Kc/s; but above that frequency it rapidly fades out.

The only thing which has removed the oscillation is the addition of a Boucherot/Zobel network across the output terminals.

1) Have I missed something?

2) Is this one peculiar faulty chip?

3) Is the manufacturer making a false claim about needing no Boucherot Cell (or a claim which is only true under exceptional circumstances)? 
--
~ Adrian Tuddenham ~ 
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www.poppyrecords.co.uk
Reply to
Adrian Tuddenham
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On a sunny day (Thu, 14 Mar 2013 12:01:31 +0000) it happened snipped-for-privacy@poppyrecords.invalid.invalid (Adrian Tuddenham) wrote in :

I use that other chip (TDA7294 IIRC) and I have never seen any oscillations. Are you sure your power decoupling caps are of good quality? Perhaps bypass those with a few hundred nF to test?

No, at least in my case, with this chip, it clean as it can get without thet RC to ground. Wiring, layout?

Reply to
Jan Panteltje

Where have you found that statement? The datasheet says "The Boucherot cell R6, C10 is normally not necessary for a stable operation. It could be needed in presence of particular load impedances at Vs < +/- 25V"

Sounds like an artifact from the MOSFET output stage. Perhaps your bootstrap is not working properly, starving the MOSFETs for sufficient gate drive, causing the typical MOSFET bursts of oscillations due to their non-linear gate capacitance.

Klaus

Reply to
Klaus Bahner

"Adrian Tuddenham"

** SFA - in reality.

** Have you tried that ??

** Might mean with typical loudspeaker loads.

Or it might mean that there are no damaging CONTINUOUS oscillations.

What you have described is hardly likely to be audible on any programme - at all !!

I have used the TDA7295 and added the standard 0.1 uF plus 8.2 ohms as a matter of course.

** You a man or a mouse ?

** A bushel of common sense maybe?

... Phil

Reply to
Phil Allison

[...]
[...]

There are two 63v 220nf capacitors mounted within 6mm of the chip. Further away there are two good-quality 220uf electrolytic capacitors.

The layout is 'difficult' becaue of the circumstances. The IC is screwed to the inside of the large die-cast box which houses the equipment. It has a small PCB mounted directly on its pins which carries the H.F. decoupling, the bootstrap capacitor, the input D.C. blocking capacitor, the input bias resistor and the two feedback resistors.

There is then a long hank of cabling which connects this to the main PCB by means of a set of header pins and sockets. I was worried that inductive or capacitive cross-talk in this cable might be part of the problem, but I have eliminated that by connecting the load and a low impedance source directly to the chip pins.

--
~ Adrian Tuddenham ~ 
(Remove the ".invalid"s and add ".co.uk" to reply) 
www.poppyrecords.co.uk
Reply to
Adrian Tuddenham

We must have different data sheets. Mine is by STMIcroelectronics; the reference to Boucherot cells is in the list of bullet points on the first page and there is no other mention of them anywhere else in the document. There are no components R6 or C10 shown on any of the circuits.

It seems as though you have found the answer. My supply voltage is +/-

30v and I have no idea what the load impedance will be when the circuit is in use. it looks as though I ought to incorporate a Boucherot/Zobel network

I'm not aware of anything wrong with the bootstrap, but I will check it.

--
~ Adrian Tuddenham ~ 
(Remove the ".invalid"s and add ".co.uk" to reply) 
www.poppyrecords.co.uk
Reply to
Adrian Tuddenham

Mine is from ST, too. Dated April 2003. The Zobel network and quoted text are included in figure 1 "Typical Application and Test Circuit".

In fact the bullet point regarding the Boucherot cell is also in my datasheet - I seldom read these feature lists, because they are way too often "marketing highlighting", which is proofed again by this datasheet :-)

Klaus

Reply to
Klaus Bahner

The date on mine is January 2003 and there is definitely no Zobel network shown in Fig,1. It looks as though they had discovered their mistake by April (the Zobel components on your circuit diagram are numbered at the end of the numbering range, which suggests that they were added last).

Exactly - but I searched for further information and couldn't find any.

--
~ Adrian Tuddenham ~ 
(Remove the ".invalid"s and add ".co.uk" to reply) 
www.poppyrecords.co.uk
Reply to
Adrian Tuddenham

On a sunny day (Thu, 14 Mar 2013 15:25:33 +0000) it happened snipped-for-privacy@poppyrecords.invalid.invalid (Adrian Tuddenham) wrote in :

The TDA7294 only mentions 'NO BOUCHEROT CELL' on page 1, and seems from 1998. It is possible they changed production processes since then, renamed the chip to TDA7295, and then found out later it needed the RC network. I have done pretty strange things to the TDA7294, and usualy with a scope connected, never a problem. 'Strange things' include driving capacitive loads (lots of piezos parallel at >

20 kHz), and power transformers (step up) at 60 Hz.

Just add that RC, and be done with it:-)

Reply to
Jan Panteltje

That seems to be working. I've got it on a thermal soak test at the moment, but will try an aasortment of loads and signals tomorrow.

--
~ Adrian Tuddenham ~ 
(Remove the ".invalid"s and add ".co.uk" to reply) 
www.poppyrecords.co.uk
Reply to
Adrian Tuddenham

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