3

Complete schematic is at https://cdn.hackaday.io/files/1667667134916544/Evil%20Simon%20v1_2_1.pdf

This is a little game with a DAC output being used as a digital audio playback system. The DAC output goes into an LM4871 amplifier. As shown in the schematic, the same 3.3v boost converter (It's a XC9142B33, not a 9140) powers both the logic system and the audio amp. In this configuration, playing audio (the speaker is an 8Ω 1" magnetic loudspeaker) causes the controller to reset, presumably because the transient load overwhelms the boost converter's transient response abilities and browns-out the controller.

I've broken the trace that feeds power to the amp so that I can experiment with alternatives. What I at first attempted to do is add a decoupling inductor from the 3.3v rail followed by extra capacitance. The values I've tried are 47 µH and 47 µF (tantalum). That seemed to make things a little better, but not good enough. Upping the capacitance to 120 µF (electrolytic) didn't make things any better.

Am I barking up the wrong tree with this idea? Or do I just need to crank up the inductance until the thing stops browning out?

Another thing I tried is connecting the inductor up to the drain of the soft-power MOSFET. This works, but I am concerned that the audio amp is going to get an inconsistent voltage as the batteries age. I'd prefer the power be well regulated until the boost converter can't deal anymore and drops out.

EDIT: Someone asked to see the layout. Not shown in this (partial) screenshot is a ground flood on both sides of the board. Power to everything except the audio amp is on the blue line going down to the left. On the prototype the short trace from pin 6 of IC3 to the nearby via has been cut and a through-hole inductor tacked on to the + side of the nearby bypass cap. The other end has at various times been tacked directly to the bottom of C5 or the drain of Q8. enter image description here

nsayer
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  • What chip is `IC1`? Looks like a microcontroller, but I'm curious as to what. –  Aug 22 '19 at 22:16
  • It's an ATXMega32E5. – nsayer Aug 22 '19 at 22:30
  • How is it implemented? Custom PCB (can you upload the layout)? Perfboard or breadboard or something else (picture)? – Dampmaskin Aug 22 '19 at 22:43
  • if the boost converter can't deliver required power then adding decoupling caps and inductors won't help. Try with different psu. – Marko Buršič Aug 22 '19 at 22:47
  • The XC9142 is rated at 800 mA output at 3.3v - more than 2 1/2 watts. – nsayer Aug 22 '19 at 22:49
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    Do you have an oscilloscope that will let you look at the power rail while you switch audio on? – TimWescott Aug 22 '19 at 23:13
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    The best test would be to temporarily power the audio amp from a separate source altogether and make sure that this eliminates the problem. – Dave Tweed Aug 22 '19 at 23:17
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    You may also want to double-check that you're not sucking down the battery voltage coming in. All the transient protection on the output side won't help if it's on the input. – TimWescott Aug 22 '19 at 23:17
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    When the amp is powered from the soft-power MOSFET drain (Q8) everything works fine. – nsayer Aug 22 '19 at 23:25
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    "The XC9142 is rated at 800 mA output at 3.3v" - datasheet says max _input_ current is 800mA, and rated output at 1.8V in is 350mA. The LM4871 draws about 300mA peak on 3.3V with an 8 Ohm load. – Bruce Abbott Aug 23 '19 at 00:06
  • @BruceAbbott Hmm. That may be the answer. Maybe the TPS613221ADBVT might work better, but that means a new PCB. – nsayer Aug 23 '19 at 00:22
  • Have you considered using 4 individual pull-up resistors on the buttons, instead of diodes D1 and D2 ? – davidcary Aug 23 '19 at 00:30
  • I would just power the amp directly from the battery. Sound quality will diminish when the batteries get tired, which is good because it tells you that they need replacing. – Bruce Abbott Aug 23 '19 at 00:40
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    @davidcary Pull-ups for the switches are present in the microcontroller. The purpose of the diodes is to allow any one of them to turn the soft-power on while still allowing the controller to see 4 separate switches. – nsayer Aug 23 '19 at 00:40
  • @nsayer Put an empty cap, lets say 1000uF between the powerrails when turned on, tada your MCU will reset (everything). It must be something on the audio side, some cap that cause a short, inrush current that your system start to malfunction. That is why your soft-start solution works, it avoids a huge inrush spike, caps charge up slowly. Measure the current it use when turned on to verify/find the problem. – Codebeat Aug 23 '19 at 03:40

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