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Problem with TDA7295 oscillating

Discussion in 'Electronic Design' started by Adrian Tuddenham, Mar 14, 2013.

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  1. 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)?
     
  2. Klaus Bahner

    Klaus Bahner Guest

    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
     
  3. Phil Allison

    Phil Allison Guest

    "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
     
  4. 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.
     
  5. 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.
     
  6. Klaus Bahner

    Klaus Bahner Guest

    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
     
  7. 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.
     
  8. 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.
     
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