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TLC274AID Drive Capability

Eric Dahl
Intellectual 1310 points

Other Parts Discussed in Thread: TLC274, TLV2374

Hello,

I had the following question come in about the TLC274AID and would appreciate the help.

"I’m analyzing a cct that someone no longer with the company designed for us because I’m not sure it has sufficient drive capability.

We use a TLC274AID quad op amp where each output drives a 350 ohm resistive load worst case. The signal is unmodulated IRIG and jumps between 5.8V for a mark and 0V for a space. The length of the marks and spaces are 2, 5 and 8ms. More detail here if needed: http://www.cyber-sciences.com/documents/TN-102_IRIG-B.pdf.

The signal spends 50% of the time high and 50% low. All 4 inputs are tied together so all channels drive high and low simultaneously. The amp power rails are +10V and -6V.

So, when the signal is a mark then 5.8V/350ohms = 16.6mA per output. The datasheet states that the max current per output is +/-30mA. 16.6mA is < 30mA so no worries there. However, Vdd max input current is 45mA and since 4 x 16.6mA =66.4mA > 45mA I’m suspecting I may have a problem. However, given that the signal is only driving 5.8V 50% of the and 0V 50% of the time I wonder if it’s safe to only consider the DC component of 2.9V for this analysis. If I do this then 2.9V/350 ohms = 8.3mA and 4 x 8.4mA = 33.2mA which is < 45mA and all is well.

Could you help a digital guy figure out if he’s got a problem or not?"

Let me know if you need any more information!

Thanks,


Eric

  • 0
    TI__Guru* 93105 points

    Hello Eric,

    The Absolute Maximum TLC274 total current into the Vdd, and total current out of GND specifications are a bit uncommon and I had to do some research as why they are set to 45 mA maximum. 

    The individual amplifier outputs are capable of an output current of about 30 mA when VDD is 16 V; less at lower VDD levels. Then of course if all 4 output were putting out the maximum current the total through a supply pin could be as high as 120 mA, plus the device operating current, exceeding the 45 mA maximum by a quite a margin. It turns out the current limit is dictated by the power busing metal on the integrated circuit die. If the current density becomes too high the metal acts like a fuse and opens.

    Even when duty cycle is taken into account that doesn't negate the fact that the peak current could still hit the same high value as when it is operated in a dc condition. The average output power with a 50 % duty cycle would be 50 % of the dc power and that would result in less heating of the bus metal. I suspect this does increase the peak current the metal bus will tolerate, but just how it relates is difficult to predict. 

    If you want to be safe it may be best to consider an amplifier that doesn't have the VDD and GND current maximums specified as an absolute maximum. The TLC274 is an older design and this kind of limitation has necessarily carried through to newer designs, on newer processes. I suggest you have a look at the TLV2374. It looks similar to the TLC274 in terms of electrical performances, but doesn't list VDD and GND current maximums in the Absolute Maximums table. You can see the data sheet at:

    http://www.ti.com/lit/ds/slos270d/slos270d.pdf

    Regards, Thomas

    PA- Linear Applications Engineering