Voltage divider for PMIC enable input

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jars121
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Posted by jars121: Thu. Mar 7, 2019 - 09:03 PM
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G'day,

 

I'm overlooking something really simple, and figured some fresh eyes will make short work of this issue!

 

I've just assembled the first prototype of my device, which is a 4-layer board, populated one both sides, with a total of ~800 components. As this is the first attempt at this design, I've come across a few small errors and areas for optimisation (as I would expect). One such area is in how the power management of the device is enabled. I'm using a TPS51120 PMIC, which manages two power rails (3V3 and 5V). The TPS51120 has a number of enable pins, and without going into detail, I have EN2 and EN5 connected together, which need to be driven high (they're both 5V tolerant). EN2 enables the 3V3 rail (EN5 has a role as well, but it's not as critical as EN2), whilst the 5V rail is enabled via EN1, which is toggled by a pin on my MCU.

 

As part of this original design, I incorporated a pushbutton control IC, the output of which was connected to EN2 and EN5. The pushbutton IC is connected to a switched power source, such that when the switched source is turned on, the pusbutton IC would send the required enable pins high to turn on the 3V3 rail. When the switched source was turned off, the pushbutton IC would send an interrupt to the MCU to allow proper powering down, before turning off the device. I've now removed these components from the prototype board, and would like to manually turn on/off the 3V3 rail using a simple rocker switch connected to the 12V source. As such, I've got the following:

 

The values of the resistors chosen are based on both bringing the 12V source voltage down to less than 5V, and what I had available. The voltage divider output is ~4.4V, which is a suitable HIGH logic level for both EN2 and EN5. Here's where I become thoroughly confused. With the 12V source applied, I have verified that the voltage divider output is indeed 4.4V. However, when I connected a wire from the voltage divider output node to the EN5 pin (which in turn is connected to EN2 via a trace), the voltage drops to ~0.78V. I even replaced R2 with a second 680Ohm resistor, which correctly produced a ~6.04V output when the EN2/EN5 wire was disconnected, but again dropped to ~0.8V when connected.

 

What am I missing here?

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Learning and Information, General Electronics
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ki0bk
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Posted by ki0bk: Thu. Mar 7, 2019 - 10:04 PM
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So this is a new board, until now it has not been powered on, correct.

I would begin by looking for a solder bridge or circuit trace from EN2/EN5 pins to gnd, as you enable pins are drawing way too much current then they should.

 

Jim

PS: do you have a bare pcb that has not yet been populated, if so, check continuity between ENA pins and gnd!

 

 

 

(Possum Lodge oath) Quando omni flunkus, moritati.

"I thought growing old would take longer"

 

Last Edited: Thu. Mar 7, 2019 - 10:06 PM
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jars121
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Posted by jars121: Thu. Mar 7, 2019 - 10:14 PM
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Correct, this is a brand new board, that has only been powered on a handful of times to test and troubleshoot why I'm unable to get ~4.4V to the EN2/5 pins.

 

I did some extensive continuity testing yesterday, but have just re-checked continuity between the EN2/5 pins and ground, and there is definitely no issue there. I've also checked on 2 other, unpopulated boards and had the same result.

 

EDIT: I've just done some further continuity testing, and it looks like I am in fact getting some continuity to GND, although somewhat sporadically. I'll desolder a few things, triple check all the connections and try again. Thanks for picking that up!

Last Edited: Thu. Mar 7, 2019 - 10:43 PM
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jars121
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Posted by jars121: Thu. Mar 7, 2019 - 11:29 PM
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I've desoldered and re-soldered the wire between the voltage divider output and the EN5 pad, making absolutely certain there's no continuity between the EN5 pad and GND. I'm still getting ~0.78V when the board is powered. Is it safe to assume I've blown the TPS51120 IC?

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avrcandies
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Posted by avrcandies: Fri. Mar 8, 2019 - 12:02 AM
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Maybe your TPS51120  is not getting its power (Vcc=0)...Maybe then any ENx input pin will diode clamp to the positive rail (at 0V ) ...you have 0.7V.

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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jars121
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Posted by jars121: Fri. Mar 8, 2019 - 12:11 AM
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I hadn't thought of that, that could definitely make sense. Unfortunately I've just tested the Vin and have a rock solid 12V sad

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avrcandies
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Posted by avrcandies: Fri. Mar 8, 2019 - 01:23 AM
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Show a real schematic of EN2 & EN5...somewhere they are connected to a diode (real or otherwise).  You must list EVERYWHERE they go.

Remember, power fets have diodes in them, between Source and Drain.

 

The TPs datasheet also mentions the ENx pins can charge a cap for slow start, so they do a bit of weak driving.  However, you should be overcoming that with you low value resistors (like 400 ohms)

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Fri. Mar 8, 2019 - 01:25 AM
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jars121
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Posted by jars121: Fri. Mar 8, 2019 - 01:51 AM
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Well this is embarrassing and highly frustrating.

 

It looks like I've built the footprint for the TPS51120 in my EDA software upside down, so the 4.4V isn't actually interfacing with the EN pins after all.

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Kartman
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Posted by Kartman: Fri. Mar 8, 2019 - 02:15 AM (Reply to #8)
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You're not the first one to do this, nor the last! On the upside, you've now got another thing on your checklist before issuing a pcb for manufacture.

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jars121
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Posted by jars121: Fri. Mar 8, 2019 - 02:17 AM
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It's not even the first instance on this board! I've been able to go without certain functions, but I'm struggling to see how I can get this board up and running now without power management. I'm looking at discrete 3V3 step down converters I can use to hack it together, as assembling this board took 2 days by hand, so I'm reluctant to just give up on it and push ahead on the next revision.

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Kartman
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Posted by Kartman: Fri. Mar 8, 2019 - 02:35 AM
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Unfortunately, mechanical defects will be the death knell of a pcb. If something doesn't fit, has a wrong footprint, backwards etc, with fine pitch devices there's little way out of it. A wrong track here or there is generally recoverable (except if it is on the inner layers). Personally, if you can get a board of your complexity viable on the first spin, then there's cause for celebration.

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avrcandies
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Posted by avrcandies: Fri. Mar 8, 2019 - 03:59 AM
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It looks like I've built the footprint for the TPS51120 in my EDA software upside down, so the 4.4V isn't actually interfacing with the EN pins after all.

Did you spend serious time checking your layout?  Been there, done that.  Spending 5 hours checking and finding nothing and spending 5 hours checking and finding 2 "oops" are both well worth it.   I now look carefully at transistors in 5 pin packages (like sot23-5), since EBC between slightly different part suffixes, can map to completely different physical pins.  Even saying which pins are 1 2 3 4 5 isn't followed exactly the same by everyone.  So check & check & when you are finished...check again.    

 

You need to check that the upside down part has not allowed high current 12V to wrongly flow where it will damage other parts...so then you end up wondering why things still don't work, even after your "fix"

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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jars121
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Posted by jars121: Fri. Mar 8, 2019 - 04:02 AM
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Yes very valid point, thanks avrcandies. I have made every effort to check and re-check each component as I go, but unfortunately a couple out of the several hundred discrete components has slipped through. I'll see how I go with making use of this board, knowing that the outcome may be to leave it be, focus on version 2 and make sure I don't make the same mistakes again :)

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N.Winterbottom
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Posted by N.Winterbottom: Fri. Mar 8, 2019 - 08:28 AM
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If I had a penny for each time an engineer had done that I would be posting from my private island in the Caribbean.

 

Time to test your dexterity with the soldering iron and mount the regulator upside down attaching wires to each pin/pad. I.e. dead spider mounting.

 

Last Edited: Fri. Mar 8, 2019 - 08:27 AM
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DocJC
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Posted by DocJC: Fri. Mar 8, 2019 - 02:21 PM
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Dead bug prototyping the board was also my first though, but I think the chip is a VQNF package, so that would be very challenging.

Additionally, I am not sure how critical the layout is for operation of the chip.

Dead bug style my not work at all...

 

That said, one would truly like to be able to test more of the PCB and its circuitry than just the power supply before you re-spin the PCB.

 

One might substitute an external, alternative, power supply and route the V+ and Ground rails to the main PCB so that one could continue to test the remainder of the board, even if not quite being able to fully test your power up and power down sequences.

 

Another option would be to spin a very small PCB with just the dual power supply chip and its part, and then connect its power and ground rails into the main PCB, (and remove the chip's support circuitry from the main PCB).

 

JC

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jars121
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Posted by jars121: Sat. Mar 9, 2019 - 07:03 AM
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JC, this is exactly what I plan on doing. I've started working on revision 2 today, making the required changes to the power management circuitry, but will endeavour to piece together the required rails using external sources to at least try and test some of the other functions before retiring this board. I've just collected a really nice rework station, which should make removal of incorrect components much easier than my current process (soldering iron and wick).

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