I cannot identify pin 1 on the TLV3011 SOT-23-6 device with 100% certainty.
My prototype simple battery over-voltage protection circuit with hysteresis using the tlv3011 simply refuses to work. Maybe I have misidentified pin 1 on the package.
The data sheet shows a chamfered area at the corner of the package rectangle that identifies pin 1. However, the delivered device has no such chamfering. Printed on top of the device is "ALR". I chose pin 1 as being near the first leg of the "A". MAYBE there is a small divit in the opposite corner to identify pin 1. Maybe my loupe is not strong enough. Should there be a divit and I just can not see it?
Pin behavior behaves SOMEWHAT correctly (default position Vref pin is at 1.248) volts, but still, the simple circuit (based on the data sheet "Battery Good" circuit) absolutely refuses to operate correctly. I am very confused.
Silly, I know. But can someone please help me find this. The next step, if I was correct in finding pin 1, is to post the circuit and ask for help.
Thank you, in advance.
In reply to SirCut:
The TLV3011 is well-established. It's had a few quirky issues through the years but I don't think they relate in any way to what you're seeing. It's rarely used as an ADC reference. It's more likely used the way you do, including high resistance components for low power. I don't expect that the slowly changing supply voltage is an issue. I don't see a bypass cap on the supply in your circuit. I think that would be good practice, though the battery impedance is probably low enough. I know better than to assure you that it can't be due to the basic behavior of the part, but I still think it's unlikely.
Sounds like you are probing with a scope. Look for oscillations or other weirdness, including on the reference pin.
We can continue on the forum unless you are uncomfortable with it.
In reply to Bruce Trump:
Another thought: It is showing some symptoms of a disconnected V+ pin. The reference circuitry could be powered from the voltage applied to the inverting input pin flowing through the internal protection clamp diode. This might allow the reference to appear normal while the comparator fails to operate.
Hi Bruce, thanks for hangin' with me ...
OK, so I had 2 tlv3011 chips left (Thank you, TI, for samples).
Just because I am now becoming superstitious, I put one chip in "backwards" to be sure I got pin 1 identified. We had pin 1 identified correctly from the get go. Pin 1 is at the lower-left leg of the "A" in the "ALR" marking. That is, the "backward" test showed it was all wrong.
So I put other tlv back in, forward direction (lower-left leg of "A" is pin 1). fixed a solder bridge (a few years of age and SOT-23 pin pitch do not mix well). I get my 1.244 ref voltage out of the ref pin (good). I checked V+, it is correctly pinned to the target supply/batt voltage (2.7 V). However, I still get 0.56x volts at IN- (same node/potential as common side of R1 and R4. Checked my smt resistors (visual read of values ... correct). Current is sneaking out from that node somehow (IN- node).
Yes, I have a low ESR bypass cap (batt is actually cap ... bit proprietary).
Unfortunately, no scope here. Using a pretty low level fluke dmm.
I'll see if I can get some Hz reading or significant rms power reading on the board (theoretically all AC should be quiet).
I agree, probability, Murphy's law, (and its corollary - the embarrassment law) suggest the TLV3011 is probably NOT at fault. It is probably implementation (bad solder) or bad circuit (somehow?). However, like you I cannot rule out the chip input impedance being lower than spec'd. I also keep building this and similar circuit to same end result.
Was thinking of moving high side of both R7 and Q1 to other side of diode D1. Theoretically, I don't think this should matter when driven by current source, however, and I think would be exercising superstition. It would cost a little bit in the way of leakage power and time and energy. Also, not enough schmart boards or adapters to put SMD's onto 0.1" breadboard. I'd rather understand just what the heck is happening.
I will look for other paths on the board that current can get from IN- to ground. There don't seem to be many except through the TLV chip. In the meantime, if you have a line to the design engineers and might be able to ask them if it is crazy what I see on pin "IN-" (like does it ever have lower impedance than expected) that would be soooooo nice.
Bruce, simply reciting your short list of possible causes (incl. contamination) made me clean the board again. After very tedious and careful cleaning, heating the board to 85 deg. C for a while, the voltage in question (IN-, or voltage divider) near doubled. AHA! Still Cleaning and checking, but things look better (not solved, but maybe). I am not used to SMT (older stuff and little of it) and was not ready for board surface conductivity with such a high impedence/low current board.
My PCB layout for the TLV3011 (.png file attached) had a trace passing under the small SOT-23 chip. I think maybe some flux residue under the chip, combined with the nearness of conductors, allowed leakage current between pins that was comparable to design parameters.
I'll send another update, but I think the take-away, at a minimum, is:
1) Use more care in circuit layout for high impedance parts. (incl., maybe guards)
2) Cleanliness is next to ... well, you know who
3) It is usually your (my) fault
3) the corollary to Murphy's law is again verified in the wild.
I'll probably have to start with a virgin board and rebuild, with much attention to cleanliness. I might even have to create a new layout.
You are awesome, Bruce
I'll post a final, soon (assuming that all observations fit our hypothesis and the board behaves as predicted).
(note traces near high impedence inputs on SOT-23-6)
Looks like you are homing in on the problem. A very minor layout change could eliminate some vulnerability. The blue trace shown below would avoid some possible leakage under the IC. Don't know whether the other trace could be as easily re-routed. Many layout folks go to great lengths to avoid traces under a surface-mount IC, even when impedances are low. Even aggressive production cleaning processes can leave some gunk under ICs.
For a discussion of some unrelated PCB layout issues with some helpful links, see this blog:
BTW... I accidentally checked the box indicating that the question was answered (too much wine). Feel free to un-check it.
You suggest a good, no-cost, layout mod.
Thank you, Bruce
The best to you!
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