I put together my first chip amp and tone ctrl boards. Both are working fine but big problems when I connected them!!
Anyways, I found the cause: The heat sink has a direct connection to vs - 38v existed between here and ground.
It would be simple to add an insulator but I suspect this is not right. I didn't see any mentions of precautions in the data sheet.
Which pins are supposed to link to metal tab on the TDA7295 chip? Could someone please have a look at my layout. I pretty much copied the example from the data sheet as my first attempt designing it directly from schematic suffered from ground loops. I've added on board fuses and rectifiers, other than that its the same (hopefully)
Thanks in advance any help would be much appreciated
Anyways, I found the cause: The heat sink has a direct connection to vs - 38v existed between here and ground.
It would be simple to add an insulator but I suspect this is not right. I didn't see any mentions of precautions in the data sheet.
Which pins are supposed to link to metal tab on the TDA7295 chip? Could someone please have a look at my layout. I pretty much copied the example from the data sheet as my first attempt designing it directly from schematic suffered from ground loops. I've added on board fuses and rectifiers, other than that its the same (hopefully)
Thanks in advance any help would be much appreciated
Pin 4 is connected to the surround or metal heatsink. See spec sheet from ST; http://www.st.com/content/ccc/resou...df/jcr:content/translations/en.CD00000199.pdf
ok, yeah pin 8 IS connected to heatsink. 4th in back row i guess you meant. And it is supposed to be connected to -vs. So insulator is in fact all I need. I still couldn't find how to determine that from the data sheet, but I found another device using a 7296 chip (same pinout) and checked
I do not see this fact in the cited datasheet.
I do know that "ALL" power devices solder the chip to the heat tab. Since the chip back is usually the most-negative point of the chip, the tab is at V-. They assume you can put a better insulator on the large outside of the tab than they could put on the tiny internal chip; or in some cases it is safe to float the heatsink at V- (protected against contact), or run single-supply with V- at Ground.
There are exceptions. Voltage regulators can be bought "fully encapsulated" in plastic. The now-old Sanken modules were not true chips and often the large heat tab was isolated.
I do know that "ALL" power devices solder the chip to the heat tab. Since the chip back is usually the most-negative point of the chip, the tab is at V-. They assume you can put a better insulator on the large outside of the tab than they could put on the tiny internal chip; or in some cases it is safe to float the heatsink at V- (protected against contact), or run single-supply with V- at Ground.
There are exceptions. Voltage regulators can be bought "fully encapsulated" in plastic. The now-old Sanken modules were not true chips and often the large heat tab was isolated.
Thanks PRR, A lesson learned. I'm familiar with the tabs on voltage regulators as its always clearly noted in the data sheets, but this one caught me by surprise. I have an alloy face plate transferring heat to fins on the other side as there is no ventilation otherwise - its mounted inside speaker cabinet. My problem occurred because the pots on the tone control are grounded and screw onto the face plate. The input ground track on the amp pcb burned through. But I have repaired, added insulator under the chip and all is well. Thankfully nothing else was damaged.
> familiar with the tabs on voltage regulators
Also nearly all Power Transistors.
On these, for structural reasons, the tab is usually the Collector. Depending NPN or PNP, this may be more positive or more negative, but always connected to a hefty power source, almost never Zero.
Also nearly all Power Transistors.
On these, for structural reasons, the tab is usually the Collector. Depending NPN or PNP, this may be more positive or more negative, but always connected to a hefty power source, almost never Zero.
Quite the opposite: tab at one rail voltage is NORMAL because of thermal considerations; so using an insulator is NORMAL.
Sometimes tab is gounded, but only because those devices are used with a single supply so -V rail and Gnd are the same, in both cases as PRR said above: "the most negative point".
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