Problems with voltage references - Page 1
Forum section: electronics / projects

author: EngineerNate
author_rank: Newbie
timestamp: April 14, 2018, 02:09:02 pm
content: Hi all, new here. I'm an Aero engineer by day but I dabble in some electronics as a hobby, mainly repair of vintage audio gear.I have a turntable which uses a zener shunt to set a reference voltage and to supply a constant input voltage to the main servo IC on the board. You can see it here just to the right of my red warning callout box, it's D3,4,5 with series resistor R10:I was having some speed drift over time and it seemed to be related to temperature, so I started looking into replacing the zener shunt with something more stable/modern. I measured the voltage drop through R10 at ~3.2v nominal so at 180 ohms the circuit seemed to be pulling a little over 15mA of current. I started looking into. Vref ICs that could do that as a minimum and chose the Maxim MAX6174 and MAX6175, as they stated they could source 30mA of current and I liked their high temp stability at 3ppm/*C.I built this little circuit:It places the two references in series with 0.1 uF filter caps for each and the pot acting as a voltage divider between vout and trim as shown in the datasheet. I used a 47k cermet pot I've used for other purposes in the past just out of familiarity. Imeasured and tuned vout with the pot to 8.8v +-1mV and load tested it to 15mA on the bench and everything seemed gravy. The pots weren't too close to either extreme so I didn't expect any stability issues.To wire it up, I removed R10, D3, D4, and D5, connected vout to the high side of the D5 board connection, Vin to the 12v rail where R10 previously connected, and ground to the low side pad for D3. I didn't see any need to make multiple connections to the board beyond that and though this was the simplest way to convert to a series reference topology without board modification.In circuit though, the "upper" vref seems to cut out and go into some protection mode that results in a 0.8 volt drop across it instead of a ~4v increase. Both refs are tied to a 12v rail on the Vin side, so even the upper one should have ~3v deltaV to work with to keep the output at the set point.What am I missing? I was able to replicate the behavior intermittently on the bench by adding a cap across the output while it was running, the voltage would sometimes drop immediately from 8.8 to a little over four volts. The only way to get the correct value back is to completely unload the output, even resetting by removing power doesn't restore the correct output voltage if any load is still applied. I'm stumped!Cheers, and thanks for any help you can give.Nathan

author: iMo
author_rank: Super Contributor
timestamp: April 14, 2018, 04:26:47 pm
content: Those chips have got an Input (2) and Output (6) and GND (4). From your pictures it is not clear how did you wire the 2 chips together. Can you provide a schematics?QuoteThat means your output which may source up to 30mA is the pin 6 (the input pin 2 is the operating voltage of the chip, not the stabilized output voltage, and, the input pin 2 voltage has to be Vout+2V). I doubt therefore you can stack up 2 chips in series to get 8.8V and source 30mA..

author: EngineerNate
author_rank: Newbie
timestamp: April 14, 2018, 05:09:26 pm
content: Here's a schematic with a simplified representation of the load condition:Vin-Vref1 = 7v so I should have plenty of headroom (3v, 1v over the minimum value) for Vref2 to operate within it's specs. Is there some reason they would not be able to source the full 30mA they're specced for when placed in a series circuit like this?

author: EngineerNate
author_rank: Newbie
timestamp: April 14, 2018, 05:13:30 pm
content: I got the idea from this circuit in the AD REF194 datasheet. I liked that the maxim didn't need an output capacitor and had a much broader range of voltage adjustment. I thought that as long as (Vin-Vref1)>(Vref2+2) I'd be okay as far as source voltage goes and I hadn't found anything that made me think stacking them would degrade the current source capability.I fully admit I'm not an EE and am learning as I go here.I'm currently looking into potentially moving back to the shunt topology but using an AZ431A instead of the zener setup. I had hoped to get better than the 20ppm "typical" advertised by Diodes Incorporated for that device though and I'm somewhat dubious on how it's specced so much better than any of the other 431 based shunt references. I'd also looked into putting two LT1634 shunt references in series, or going to the LT1461 to get more current capability and sticking with the stacked series reference topology.I'm wondering if there's something about the circuit that doesn't respond well to transient changes in load. I can hook up a 470ohm resistor to the output and maintain voltage at ~18mA, but adding an uncharged cap across that resistor can tank the output. Also, at startup, the supply rail is a bit slow to stabilize and there may be some transient loads from the servo IC startup that are too sudden/peaky without R10 limiting the current through the reference.

author: iMo
author_rank: Super Contributor
timestamp: April 14, 2018, 05:18:18 pm
content: QuoteFor example the lower-side 6175 can sink max 2mA.. If the upper-side 6174 would require more (including the pot) it could be an issue..Try to put an ~1k resistor from 6175 output to ground - like on the REF scheme above. That allows more regulation current for the upper 6175..

author: EngineerNate
author_rank: Newbie
timestamp: April 14, 2018, 05:22:18 pm
content: Quote from: imo on April 14, 2018, 05:18:18 pmIs there something I'm missing in the source/sink definition here? There should be no current sinking going on aside from the miniscule amount from the 6174, something in the mu-Amp range. I thought that the load current would all be a "source" condition. Load from the pot should be 0.008mA.

author: BNElecEng
author_rank: Regular Contributor
timestamp: April 14, 2018, 05:23:18 pm
content: imo has got it spot on. The MAX617 series cannot sink the current needed for your circuit. The short answer is that not every voltage reference chip can be connected in series.

author: iMo
author_rank: Super Contributor
timestamp: April 14, 2018, 05:24:01 pm
content: Try to put an ~1k resistor from 6175 output to ground - like on the REF scheme above. That allows more regulation current for the upper 6174..

author: EngineerNate
author_rank: Newbie
timestamp: April 14, 2018, 05:24:43 pm
content: Quote from: BNElecEng on April 14, 2018, 05:23:18 pmThanks for the confirmation. What exactly about the circuit is requiring it to sink current? I thought that both would be in a source condition once under load.

author: iMo
author_rank: Super Contributor
timestamp: April 14, 2018, 05:30:27 pm
content: Try to put an ~1k resistor from 6175 output to ground - like the R1 on the REF scheme above. That allows more regulation current for the upper 6174..The lower 6175 sinks, the upper 6174 sources..

author: BNElecEng
author_rank: Regular Contributor
timestamp: April 14, 2018, 05:33:17 pm
content: The cause of the problem is the MAX6174. Take a look at your schematic and try to picture the current and voltage loops. Remember that the MAX6174 is trying to regulate the voltage between its output and pin 4 (its internal ground) , but your load is referenced to GND.As an experiment, remove your test load and try putting two separate loads into the circuit, one for each of the voltage references, between their own output and ground pins.imo's idea of the 1k resistor is a good one, give it a try as well.

author: EngineerNate
author_rank: Newbie
timestamp: April 14, 2018, 05:40:29 pm
content: So is the short answer pick a Vref that can sink/source equivalent amounts of current.Results of adding 1k resistor:If load connected at startup, no change. 6174 still drops out.If load connected after vout has stabilized:voltage remains steady. This is an improvement but not overly useful for my application.Still struggling to wrap my brain around the mentioned current loops. With individual loads, I guess all of the current through the 6174 would have to sink through the 6175, and it has some sort of over current protection that's cutting off the 6174 when it attempts to do so?Connecting ground directly to the 6174 makes it behave. Even with loads only on it, it crashes if it's connected in circuit.Why would a load connected straight to ground result in it trying to sink too much current through the 6175?

author: iMo
author_rank: Super Contributor
timestamp: April 14, 2018, 05:43:49 pm
content: QuoteIt may happen there is an in-rush transient >60mA current while switching the circuit on, most probably into some discharged higher value capacitors, which fires the short-circuit protection on..

author: iMo
author_rank: Super Contributor
timestamp: April 14, 2018, 05:58:42 pm
content: Try to put a 100 ohm resistor in series with the capacitor see below..PS: that will eliminate the decoupling capability of that capacitor, however..

author: EngineerNate
author_rank: Newbie
timestamp: April 14, 2018, 06:09:34 pm
content: I actually removed the capacitor from the circuit in the table and had the same result. There's no other major capacitance that I know of. I think it's somewhat unnecessary with this topology but something else was tanking the output in circuit. I'm unsure if removing it and adding the sinking resistor will be enough. That's a combo I have not attempted.