[SOLVED] OPA462 HV op amp - Status/Flag pin voltage - Page 1
Forum section: electronics / projects

author: Unixon
author_rank: Frequent Contributor
timestamp: October 26, 2020, 09:27:24 pm
content: I have a project with TI's high voltage op amp OPA462 and I came across a little issue with it.The problem is closed state current of Status/Flag pin.This OPA462 is powered from +160V positive rail and -5V negative rail to allow clear 0V on the output, E/D Com is GND (0V).I use its Status/Flag pin for monitoring overload condition with 3.3V powered STM32 (as well as setting voltage with DAC and monitoring op amp output with ADC).A piece of circuit between OPA462 and STM32 input is a current-limiting 100R resistor + a 10k pull-up to 3.3V bus + a 3.3V zener diode.Even during normal operation I register 2.0V-2.1V at Status/Flag pin instead of expected 3.2V-3.3V and this reads as a very unstable logic '1'.Datasheet for OPA462 mentions 10k pull-up in characteristics table plus 1M pull-up to +90V bus in an example (which means 180uA in active state) plus a 15mA capability figure at 5V.But all these numbers are for open state! I wonder if I should just pull harder on Status/Flag pin? Then how this thing works at 180uA in the example circuit?

author: thm_w
author_rank: Super Contributor
timestamp: October 26, 2020, 10:07:50 pm
content: Is the STM32 input set to internal pullup or pulldown? If you remove the 100R resistor and manually pull up to 3.3V or 5V with 10k does it do the same thing?Yeah doesn't seem right that its pulling in 130uA into Status Flag.

author: Unixon
author_rank: Frequent Contributor
timestamp: October 26, 2020, 10:48:47 pm
content: Quote from: thm_w on October 26, 2020, 10:07:50 pmNo, no input pulling from MCU side.Quote from: thm_w on October 26, 2020, 10:07:50 pmWell, I can play a little bit with pull-up resistor value by changing it to 1k/100k/1M to check for leakage current.Removing current limiter completely may be very bad idea... if OPA462 shorts internally

author: langwadt
author_rank: Super Contributor
timestamp: October 26, 2020, 11:19:10 pm
content: Quote from: Unixon on October 26, 2020, 10:48:47 pmremove it, so the opamp isn't connector the STM , that way you can see if 10k is sufficient pulling up on theSTM pin or that you maybe have a pulldown enabled

author: Unixon
author_rank: Frequent Contributor
timestamp: October 26, 2020, 11:42:34 pm
content: Quote from: langwadt on October 26, 2020, 11:19:10 pmOK, I'll check that, but I'm pretty sure this side is unaffected.My main concern is why 10k might be not enough to pull Status/Flag pin up when the datasheet shows that this and less pull-up current should be sufficient.So, there must be one of two things happening:1) pull-up is too weak and op amp is fine;2) pull-up is too strong and op amp is already overloaded and I'm just overpowering the status output;(overheating - definitely no, it's cold; overcurrent - it shouldn't, but I have to check that)

author: magic
author_rank: Super Contributor
timestamp: October 27, 2020, 06:14:05 am
content: Quote from: Unixon on October 26, 2020, 09:27:24 pmTry without. Low voltage zeners are very leaky below their knee voltage.Besides, you could have measured voltage drop across 100Ω to see whether it's the opamp side or the MCU side which is sinking the pullup's current.Quote from: Unixon on October 26, 2020, 11:42:34 pmWithout reading the datasheet I wager that this status output is rated to sink at least a few mA

author: Unixon
author_rank: Frequent Contributor
timestamp: October 27, 2020, 08:23:03 am
content: Quote from: magic on October 27, 2020, 06:14:05 amI know, but they aren't that bad. This is true for precision circuits, I got this issue with 4-20mA current receiver and I had to replace 3.3V zener with 5.1V one.In this particular project I've put 3.3V zeners on digital lines and 3.6V zeners on analog inputs before preamps so maybe 300mV headroom is enough to hide leakage effects.The working point is around Vcc/2 I'm less worried about zeners affecting my measurement.Quote from: magic on October 27, 2020, 06:14:05 amThis resistor drops about 100mV (2.1V on MCU side and 2.0V on op amp side) which means it is about 10mA and this is really high current for Status/Flag pin.OPA462 datasheet specifies Status/Flag pin current for pull-ups up to 5V, and from that figure is must be 15mA at 5V, and 10mA corresponds quite precisely to 3.3V @25C.(See figure 59 in datasheet)So I guess it is (2) but then the next big question is why.

author: Unixon
author_rank: Frequent Contributor
timestamp: October 27, 2020, 08:42:04 am
content: Correction: Status/Flag pin is open drain type, but active high so it is actually 3rd case:3) pull-up is too strong and op amp is not overloaded.Oh, this diagonal reading looks impossible without skipping important things...

author: langwadt
author_rank: Super Contributor
timestamp: October 27, 2020, 08:42:39 am
content: Quote from: Unixon on October 27, 2020, 08:23:03 amhmm. 0.1V/100R = 1mA

author: magic
author_rank: Super Contributor
timestamp: October 27, 2020, 08:43:18 am
content: 100mV through 100Ω is 1mA, not 10mA. And to get 1mA from a 10kΩ pullup at 2V, you would need to pull to 12V.Something is not right with your measurements.I would still try without the zener. 2V means there is some 130µA flowing through 10kΩ, I think it isn't inconceivable that a 3.3V zener may sink 130µA with 2V across it. They really are nasty.

author: Unixon
author_rank: Frequent Contributor
timestamp: October 27, 2020, 08:50:06 am
content: QuoteOh, sh.. I goofed my math. I was thinking about 100mA and 10mV

author: Unixon
author_rank: Frequent Contributor
timestamp: October 27, 2020, 08:52:24 am
content: Just a little note: both pull-up resistor and zener diode are on the MCU side of 100R limiter.

author: Unixon
author_rank: Frequent Contributor
timestamp: October 27, 2020, 08:59:39 am
content: According to Figure.59 1mA is nearly nothing, it looks like 0.5mA is quiescent current for Status/Flag pin, so it almost doesn't draw any significant current.This means that op amp is indicating fault condition by releasing Status/Flag pin to closed state (logic high), but pull-up isbecause 3.3V cannot drop to 2.0V at 1mA via 100R.2.1V can drop to 2.0V via 100R at 1mA but then who the hell is dropping my 3.3V to 2.1V?So, things to check:1) internal pull-down circuit is fighting with the external pull-up;2) this particular 3.3V zener thinks it is a 2.1V zener;

author: Unixon
author_rank: Frequent Contributor
timestamp: October 27, 2020, 09:31:30 am
content: Quote from: Unixon on October 27, 2020, 08:42:04 amCorrection to correction:Can anybody explain to me, what is going on in this datasheet?In "PIN FUNCTIONS" table it is written than Status/Flag pin is active low, but in section "6.5 Electrical Parameters" in "STATUS FLAG PIN (Referenced to E/D Com)" table section it is written that "Device thermal shutdown : Alarm (Status Flag high); Return to normal operation (Status Flag low)".It looks to me that these two statements are contradicting each other.What state of Status/Flag pin indicates the fault condition?

author: langwadt
author_rank: Super Contributor
timestamp: October 27, 2020, 10:58:51 am
content: Quote from: Unixon on October 27, 2020, 09:31:30 amand figure 68 shows it as active high

author: Unixon
author_rank: Frequent Contributor
timestamp: October 27, 2020, 11:30:17 am
content: Quote from: langwadt on October 27, 2020, 10:58:51 amAnd in the actual device I see OPA462 perfectly scaling up DAC output and holding Status/Flag high at the same time

author: Unixon
author_rank: Frequent Contributor
timestamp: October 27, 2020, 12:43:58 pm
content: OK, here's what I found.MCU - not responsible, zener - yes, this sucker leaks as expected.OPA462 dosn't draw any current through Status/Flag in normal operation, pin is active low, and it is low in overload state.I had to change pull-up first to 4.7k and eventually to 1k and zener to 3.6V but that didn't help much (100mV up with 10k).Now status sense MCU pin is at 2.7V which is pretty good for Vcc=3.3V, I could probably stay with 4.7k pull-up that produced 2.4V.

author: magic
author_rank: Super Contributor
timestamp: October 27, 2020, 01:42:30 pm
content: Schottky diode from that signal to VCC may be better. Perhaps some transil on VCC too if you expect really bad things.Schottkys also leak, but under reverse bias, and then it will be the opamp's job to sink that current. If you use a small diode like BAT82/BAS82 it will be less than 1mA even at high temperature.

author: Unixon
author_rank: Frequent Contributor
timestamp: October 27, 2020, 03:09:16 pm
content: Quote from: magic on October 27, 2020, 01:42:30 pmNot perhaps, but definitely. Can't use this architecture here because of complex and dense layout.Even if I could use one big TVS plus many Schottky diodes type of protection circuit, that TVS would still affect all protected lines.Putting potentially high voltage on a low voltage power rail even with TVS protection is bad idea IMO, this may disturb a lot of components.This is like 3rd major revision of the board and every time I had to integrate new HV driver schematic in existing layout, so it is not as nice as it could be.First it was DRV2700 in flyback configuration, then it was DAC controlled LM3488, now it is DAC controlled OPA462 and LM3488 is still there but only provides HV power rail.HV rail is not particularly strong, but it can supply few tens of mA and this is enough to kill low voltage stuff on the board.Quote from: magic on October 27, 2020, 01:42:30 pmI look forward to using special low leakage TVS instead of regular zener diodes, but don't have them in yet.

author: langwadt
author_rank: Super Contributor
timestamp: October 27, 2020, 07:13:08 pm
content: Quote from: Unixon on October 27, 2020, 03:09:16 pmsomething like ,https://www.ti.com/product/ESDS314?and remember if you are using the 5V tolerant inputs on the STM32, you are allowed Vdd +4V

author: Unixon
author_rank: Frequent Contributor
timestamp: October 28, 2020, 11:52:25 pm
content: Quote from: langwadt on October 27, 2020, 07:13:08 pmWhen we discussed current loop receiver problem somebody had suggested uClamp series (e.g. UCLAMP3301).Hopefully I'll get them soon and see if they work better than a regular zener diodes.Quote from: langwadt on October 27, 2020, 07:13:08 pmOK. Digital lines are fine, analog inputs are not 5V tolerant and this is the real problem.