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Old 30th April 2008, 01:27 AM   #1
Minion is offline Minion  Canada
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Default Looking for basic TDA7293 PCB and Parts list!!

Hi folks, I was lucky enough to find a 5 CH amp at the salvation army for $5 , The amp has Six TDA7293 in it and a bunch of Big heatsinks and some big 10,000Uf 105c caps and a Big 500Va Torroid in it...

I wanted to use a few of these Chips in an amp and was wondering if there was a better layout available than the one in the datasheet (which i hear isn"t the best)??....
Single sided would be Nice....

I have designed a few LM3886 PCB"s before but I don"t think I am up to the task of designing a TDA7293 PCB because I have no Idea at all what a lot of the Pins are for.... Like I don"t know what the Bootstrap pin is or the Bootstrap Loader pin is or the Buffer driver pin , the datasheet don"t go into enough detail about these pins for me to really understand what they are for and if I can just leave them out of the equation of not...

Any help here would be Great.....

Thanx a Lot.....
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Old 30th April 2008, 04:33 AM   #2
Bearman is offline Bearman  United States
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Try this post....

My GIFT for you: TDA7293 board + extras
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Old 30th April 2008, 05:06 PM   #3
Minion is offline Minion  Canada
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Hi, Yes I looked at that design but I didn"t like the way it was layed out, Many of the offboard connections are in the center of the PCB and not on the outside the way I like them which makes it a bit to messy for my likeing as I am usually messy enough with my projects as it is.....

Maybe if someone could explain the Things I don"t understand about the Chip and how to do the layout then I can design my own PCB....

Actually after looking through the Datasheet again I think I understand amost of it accept for a couple things so maybe someone could help me with them??

The Clip detector:
There doesn"t seem to be much Info in the datasheet about it or how to use it....Do I just connect a LED through a resistor in series with ground to the Clip pin??

Mute/Standby Pins:
Can I just use a STDP switch to use this Function?? The datasheet says something about a CMOS switch but I know nothing about them....

I believe these are the only things that are confuseign me about this chip right now...


Thanx
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Old 30th April 2008, 05:59 PM   #4
teemuk is offline teemuk  Finland
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I think the datasheet does excellent job in explaining the function of various pins and how they are arranged in practical circuits. The text does not explain all but the figures are very informative.

The mute and stand-by are disabled when they receive a voltage that is higher than about 3V. There is a recommended sequence for their switching and a delay circuit devised to generate that sequence (see figure 5). Separate switch is not needed if you only wish to use the feature for creating a startup delay. The datasheet explains all this very well. Take a look at figure 6 and see how the mute and stand-by functions are arranged in a practical circuit. It’s really simple. Stand-by ground is the ground pin for the concerned CMOS logic circuitry.

The bootstrap is a bootstrap, there’s been enough discussion what that means and I will not start rephrasing it. The differences between the ordinary bootstrap configuration (from bootstrap pin to output) and the “boot loader” configuration are explained in the datasheet.

The signal ground pin (#4) is sort of a “jumper” that determines whether the chip is run in master or in slave mode. It’s a very clever feature. In master mode you hook the pin to signal common, in slave mode you hook it to negative rail. See figure 7.

Buffer driver is a pin that bypasses the internal voltage amplifier and feeds the output buffer directly. It’s needed in the modular application circuit. Again, refer to figure 7.

There are also separate supply pins for the current and voltage amplifier sections. You can utilize them, for example, to build class-G –type of applications as in figure 6. In a typical application you just hook both pins together. If I remember correctly, the chip is prone to blowing if the voltage difference between the pins gets too high. This issue has been discussed on this forum quite thoroughly.

The clipping detector/short circuit indicator is not discussed in the datasheet but it looks like a JFET switch. (Refer to figure 1). The switch is either “closed” or “open” when the output is clipping or short circuiting. The other end of the switch connects to common and there is a slight hint that the output is typically tied to 5V logic rail with a 10K pull-up resistor. This makes sense considering how the switch is arranged. I suppose the common application is that the pin gives logic output of “1” (about 5V) when the amp clips. That would mean the switch is normally closed (pin tied to common).
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Old 30th April 2008, 07:10 PM   #5
Minion is offline Minion  Canada
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Yes the datasheet might explain things so a EE or Techie can understand them but I don"t.....

I understand the Bootstrap consept to some degree, since my Rails are going to be +/-42v I don"t think I need to use the Bootstrap....

Clipping Indicator...
Still don"t understand it, So I think I will just not use it....

What I don"t understand still is the "Mute/Standby" Function...
Like I said before I don"t understand what CMOS is or Logic or anything like that is.....
So In Laymans terms can someone explain it to me??
Do I need to use this feature?? Can I just not use it?? How would I do that??

Thanx
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Old 30th April 2008, 10:04 PM   #6
teemuk is offline teemuk  Finland
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You don’t need to be an EE or a techie to grasp how those features work. I am neither. Let’s see if I can shed some light to these:

First, the operation of bootstrap. Without understanding the concept of "current source loading" it is pretty difficult to understand the concept of bootstrap circuit, which is a constant current source load. Fortunately, Rod Elliott does a good job in explaining these things. I suggest you read the whole article from:

http://sound.westhost.com/ism.htm

I believe that you do need the bootstrap capacitor, as it is quite important for the overall function of the amplifier. But you can simply put the capacitor between the bootstrap- and output pins. Using the bootstrap loader instead is not neccessary.

---

Then the clipping indicator pin: Let’s look at a basic transistor switch, such as this one:

http://www.crystal-netbook.info/site...156782/p37.gif

When the base of the transistor is grounded (having a voltage potential of zero volts) the transistor is in cut-off state and does not conduct (think that it has an infinite resistance). Therefore the Vout node must have the voltage potential of the supply rail. Now, when transistor’s base is hooked to a voltage potential that exceeds the transistor’s forward voltage the transistor begins to conduct. (It’s internal resistance decreases). Now, in switching applications we just use enough voltage to downright saturate the transistor and make it conduct fully. (Think that its resistance becomes zero ohms). In this case Vout node practically becomes connected to ground and you begin to have some voltage drop over the collector resistor. This is how a transistor acts as a switch.

The chip already has the transistor (except that it’s a FET instead of a BJT) and the logic to drive it. You just need to provide the “pull-up” resistor and a proper supply rail. So, you can use this type of circuit to drive other logic circuits or transistor switches, or you could replace the resistor with an indicator LED, a relay coil or something suitable depending on the application and the current sinking capability of the chip’s logic. If you don’t need the feature I think the pin can be simply left floating.

---

CMOS logic is a name for a family of MOSFET-based switching circuits that perform certain logic functions and operate at some standardized input-output “threshold” voltages that represent the digital 1 and 0. You can always use Google if you need to know more. In modern CMOS logics the threshold levels are typically 5V for 1 and zero volts for 0. To provide some “hysteresis” the state actually changes to 1 already with a voltage of about 3V and to 0 with a voltage of about 1V.

For convenience, the mute and stand-by circuits of the TDA7293 chip are compatible with the CMOS logic’s threshold voltage levels. But they also work with pretty much any other voltage levels as well. In essence, the mute and stand-by functions become enabled when the pin voltage is below the threshold voltage level of the 0 state, that is, below about 1V. To acquire this state it is convenient to simply hook the pins to ground. The functions become disabled when their pin voltage exceeds about 3V (that being the threshold voltage of digital “1” state). So if you want to permanently disable those functions just hook the pins to the positive supply rail. Simple, crude and your amp will pop annoyingly when you power it up.

Thus, I see no point in the described practice and I’d follow the configuration presented in the datasheet, as it’s almost as simple, very easy to implement and provides the convenient start-up delay that eliminates the popping. I think it’s more reliable too.

See figure 5: It depicts a RC-based delay circuit that creates the switching sequence presented in figure 4. In essence, the resistors define how fast the capacitors charge (or discharge) and therefore the voltage potential at the pins changes gradually and creates a delay in operation. Now look at the circuit in figure 6, and notice that it utilizes the very same RC circuit. The switch is needed only if you need the muting feature but primarily the input of the RC circuit just hooks to the positive supply rail. Now, when the amp is powered up (or power is turned off) the RC-circuit in between provides a delay in turning off the mute and stand-by and the popping of the amplifier is prevented. If you use the switch and ground the input of the RC circuit the same delay works “backwards” following the sequence proposed in figure 4.

I don’t know how important following the proper sequence of switching is but, for example, Fender guitar amplifiers utilizing this chip have a delay circuit that is similar to that proposed by the datasheet and they seem to be quite reliable. Marshall’s guitar amplifiers using the chip permanently disable the mute function (the corresponding pin is just connected to Vcc) and these chips keep failing in them. I don’t know if that is due to not following the proposed sequential operation of switching the mute and stand-by functions but I see no point in pushing your luck and testing if that is true. I believe there must be a good reason why the datasheet features the figure 4 and the design for the delay circuit.
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Old 30th April 2008, 10:36 PM   #7
Minion is offline Minion  Canada
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Thanx Teemuk...After reading the datasheet a bit more i am understanding the Mute and standy features and from the Datasheet schematci it seems to indicate I can enable/Disable these fueatres by useing a switch to switch these pins to ground....

I"m not going to use the Clip feature.....

I have decided I would try the datasheet PCB Layout for now .....


Thanx a Lot....Cheers
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Old 30th April 2008, 11:10 PM   #8
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The document for K106, a TDA7294 kit, may provide some additional explanation. http://electronics123.net/amazon/datasheet/k106.pdf
That's not, TDA7293, but its quite similar for the support circuit.

However, there's a "shortcut" with the mute and standby pins. You can run both mute and standby switches together from a single 10k resistor. Or you can use a resistor per each (standby 22k, but mute with 10k).
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Old 30th April 2008, 11:14 PM   #9
teemuk is offline teemuk  Finland
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Just to clarify, enabling and disabling those functions requires an arrangement that switches between voltage potentials of

Mute or stand-by off: pin voltage > 3.5V (conveniently the positive supply rail)
Mute or stand-by on: pin voltage < 1.5V (conveniently the ground rail)
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Old 30th April 2008, 11:32 PM   #10
Minion is offline Minion  Canada
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Thanx Teemuk and Daniel....

So so enable the Mute/standby Function I just connect the pins to the Possitive supply and to disable to short them to ground?!! Gotcha....

Thanx
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