| Electronics > Projects, Designs, and Technical Stuff |
| PSA: do not use the TPS61099 boost reg in your designs |
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| amyk:
--- Quote from: T3sl4co1l on January 22, 2019, 11:28:37 pm ---It's a bloody switcher, what does it need all that logic for? Do they start with an MCU and strip out unused bits? Just look: https://zeptobars.com/en/read/Ti-TPS62321-3-MHz-Step-Down-Chip-Scale-CSP --- End quote --- It's more like "start with an MCU and add power transistors"... I wouldn't be surprised if there was some sort of primitive microcode and CPU-ish logic in them. I'd be interested in a die photo of a simpler switching regulator, for comparison. |
| T3sl4co1l:
I mean, this might be a little too simple, but it illustrates the point: https://zeptobars.com/en/read/MC34063 I've synthesized similar functionality to the UC3842 with a mere seven transistors, lacking some functions like low-current startup and UVLO, which would hardly take a dozen more. It's not hard to do, they just don't care about analog design anymore. Tim |
| DaJMasta:
Even if there are some ways to reduce the likelihood of a faulty start to near zero, they should probably be described in the reference circuit in the datasheet and much more importantly: this kind of over-boosting failure mode shoudln't ever be acceptable in such a chip. If you have a faulty start and need extra stuff to clean up power, fine, it may not be a treat to use in your design but it will work well enough.... if a potential failure mode for a boost converter is to overdrive the target voltage by like 50%.... that's a design failure. Maybe they can update their datasheet to include clamping circuitry, but they really should be looking into respinning it and considering recalling the parts - it's not going to be a converter of choice if the BoM inflates with larger clamping diodes. |
| radioactive:
What values did you use for R1/R2 feedback? Did you see this in the datasheet? --- Quote ---For the best accuracy, the current following through R2 should be 100 times larger than FB pin leakage current. Changing R2 towards a lower value increases the robustness against noise injection. Changing R2 towards higher values reduces the FB divider current for achieving the highest efficiency at low load currents. 1-MΩ and 249-kΩ resistors are selected for R1 and R2 in this example. High accuracy resistors are recommended for better output voltage accuracy. --- End quote --- Also, another thing that would be worth trying is switching to the fixed 3.6V (TPS610995)? Another last ditch type effort would be to have an external voltage ref / comparator monitor for over voltage and the drive enable pin low. Probably not worth the cost unless you really need to use this part. |
| jeremy:
I am using the --- Quote from: radioactive on January 23, 2019, 03:41:07 am ---What values did you use for R1/R2 feedback? Did you see this in the datasheet? --- Quote ---For the best accuracy, the current following through R2 should be 100 times larger than FB pin leakage current. Changing R2 towards a lower value increases the robustness against noise injection. Changing R2 towards higher values reduces the FB divider current for achieving the highest efficiency at low load currents. 1-MΩ and 249-kΩ resistors are selected for R1 and R2 in this example. High accuracy resistors are recommended for better output voltage accuracy. --- End quote --- Also, another thing that would be worth trying is switching to the fixed 3.6V (TPS610995)? Another last ditch type effort would be to have an external voltage ref / comparator monitor for over voltage and the drive enable pin low. Probably not worth the cost unless you really need to use this part. --- End quote --- I am using the fixed 3.6V option partially because I didn't want to be bothered with problems like this :-DD . I have been playing with various voltage supervisors, etc and I just can't seem to get the circuit right. It's fairly easy to delay the startup of the converter with one, but the overvoltage condition is what is stumping me. Really, you need a circuit which de-asserts the enable pin if it goes too high *and* discharges the capacitors on the output stage. A comparator with wide hysteresis and a voltage ref is probably what is needed, but cost and complexity starts to be a problem there. Also it has to be crazy low Iq. Right now my circuit uses 4 times the sleep current of the entire rest of the system. --- Quote from: DaJMasta on January 23, 2019, 02:21:45 am ---Even if there are some ways to reduce the likelihood of a faulty start to near zero, they should probably be described in the reference circuit in the datasheet and much more importantly: this kind of over-boosting failure mode shoudln't ever be acceptable in such a chip. If you have a faulty start and need extra stuff to clean up power, fine, it may not be a treat to use in your design but it will work well enough.... if a potential failure mode for a boost converter is to overdrive the target voltage by like 50%.... that's a design failure. Maybe they can update their datasheet to include clamping circuitry, but they really should be looking into respinning it and considering recalling the parts - it's not going to be a converter of choice if the BoM inflates with larger clamping diodes. --- End quote --- I suspect this is a marketing problem perhaps (though I suppose Hanlon's razor says that its a case of nobody who can do anything about it knows about it). This is clearly designed for a tiny board area (otherwise who would bother to use the CSBGA), and clamping would make that a lot worse. You can also kiss the ~1uA Iq goodbye as well. At that point, this converter basically has no market; who wants a low Iq boost converter that isn't really that low Iq, takes up a bunch of board space but still requires you to solder and inspect a CSBGA, and still might get into a glitch loop where it draws too much current. |
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