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TL431 stability

The TI data sheet for the TL431 shows two different stability graphs, one for "All TL431 and TL431A devices except for SOT23-3, SC-70 and Q- temp devices" while the other is for "All TL431B, TL432, SOT23-3, SC70 and Q-temp devices". Both graphs are for 25 deg C only.

Are these really all the same device, or are there two die variants? The first group need less capacitance for stability at low output voltages.

Similarly, ST make a TL431 which appears to need less capacitance than the TI versions. Is this device really "better", or have they found a more favourable way of measuring it?

None of the data sheets mentions the esr of the capacitors used for stability measurement, nor do they discuss temperature effects on stability. I have tried adding between 1 and 2.2 Ohms series resistance to the load capacitor and this allows much lower value load capacitors than without any series resistance. (I am using ceramic capacitors and I can cope with a slight reduction in regulation accuracy. Operating voltages are 2.7 and 3.3V and I am using the SO8 package.)

I'm trying to minimize cost but I don't want a stability problem to crop up after 10k units have been built.

Any thoughts or painful past experiences?

John

Reply to
John Walliker
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Work in the low side of the capacitance range. Less than 4n7 gives no trouble, is cheaper and has fewer temperature considerations.

If you want filtering or load immunity, filter at the point of load - this works better, anyways, while allowing optimum low cost R and C.

RL

Reply to
legg

Be advised that every manufacturer has their own design; there are at least 3 different circuits being used. The TI design is completely different than the ST design.

Reply to
Robert Baer

That does sound a good idea when it is possible, but unfortunately I need more capacitance to absorb esd events and to cope with external devices being connected, each of which has 100nF of its own decoupling.

Thanks

John

Reply to
John Walliker

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Thanks. That is very useful to know. I will bench test with all makes that are likely to get used in production. Do you know whether TI are shipping two different designs or just specifying them in different ways?

John

Reply to
John Walliker

I do not KNOW if the designs are different, but i believe it is safe to say it is solely specsmanship. National has different TC specs depending on temperature range; looking at their curves (when you can find them) indicates the overall curve from lowest temp to highest temp looks like a parabola with a max at some temperature. Take that curve and slice off a large part of the upper end and some of the lower end,then run a line thru the net average on this new curve and wa-la; the TC has improved maybe an order of magnitude.

Reply to
Robert Baer

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