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we made a design with 1520 Dual PCMCIA Controller from TI. We are using TPS2226ADB for power switching. The design is made as PCI add-in card, so we are using only PCI parallel interrupts.
At least, this is not a very special or even complex design. That I was thinking before we started production of initial batch. Now, after a few days testing, we have failure rate of 25%. We found out, that the 2226ADB is burning away. This chip is showing the same burning areas more or less the same on all burned chips. This areas are corrosponding to both 12V input pins. BUT, there are no cards inserted into the cardbus slots. So no current should be used by this input pins.
We double checked schematics, every thing is ok, regarding datasheets and also reference design. So we went deeper and tried to check for the serial protocol between 1520 and TPS2226A, but we also found a note in the datasheet, that definition of D0 to D10 can be seen at control logic table, but this table is not part of the datasheet ...
Maye somebody has seen that before?
Burned devices are typically due to negative voltage or overvoltage on a pin of the device. In power switching devices I see this problem alot and it is usually due to too much stray inductance in the layout of the board. When the device (the TPS2226A in this case) turns the current on or off to the load, then the increasing/decreasing current acts through the stray inductance to generate voltages that are beyond the absolute maximum of the IC. The first cure (and prevention) is to route the board with the lowest inductance traces as possible in the power path. This means short, fat wide traces (both power and ground) that are closely spaced together.
Adding ceramic capacitors across the outputs of the TPS2226A help to reduce any overvoltages during switching by countering the stray inductance. The datasheet recommends 0.1uF capacitors (located very close to the IC) between VOUT and GND. The input voltage should also be bypassed right at the pins of the IC with a 4.7uF ceramic capacitor in parallel with a0.1uF capacitor.
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In reply to Scot Lester:
thank you for your comments. I know all of this about the issue when switching voltages. We also have capacitors as they are recommended. Power lines to the switch are coming direct from plane or with fat traces.
But my problem is, that we do not have any cardbus/pcmcia cards inserted in the slot, so no switching of voltages is occouring. At least there should (and is) not voltage on all outputs of the switch. The switch should stay in reset or default condition.
The chip is just burning by itself. Regarding the the internal switch schema, we have seen, that the switch is able to short the outputs to GND. Our fear is now, that in this state, the ship is switching on 12V and shorting down to GND. We do not see any other reason, why else the chip should burn on the 12 V inputs without any other load on the outputs.
We tried to verify the chip programming over the serial interface, but we do not find any information about the control logic table, we would need to verify.
There is a control logic table in the TPS2226 datasheet.
Regarding the parts burning: can you verify that this does not occur during power-up?
In reply to Bob Kando:
Thank you Bob,
I will check for the datasheet for the TPS2226.
Concerning the time off burning, we can not say for sure, when this is happening. All boards have passed initial testing without any issue, but at this test, cards are inserted in the slots. We have seen the chip burning one time live direct in front of us. The unit was running (with no PCMCIA cards inserted) for about 3-4 minutes before it starts smoking. Also, the current for this must be low, since we did not see dramatic increase of input current to the complete unit at that time. Some of the chips are showing dramtaic destruction of the housing, some are only showing very small bubbles. We are also monitoring 12V on this units, but we can not see any problem there,
In reply to Jens Guessregen:
finally I found the datasheet for the TPS2226. TI hided this as TPS2224, so if somebody is looking for that, search for TPS2223 or TP2224 ;-)
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