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I am trying to make a high-power audio amplifier circuit consisting of the TPS55340 as a boost converter IC to boost a battery's voltage from 14.4 (12-16.6 V) to ~19 V, to allow higher output power from the audio amplifier and keep a steady voltage throughout the total battery discharge cycle.

However, I cannot pull more than 1.4 A from the battery before the output voltage on the TPS55340 begins to drop significantly (~16.2 V where Vin = 15.52 V). The effect is even worse as the battery discharges more.

Requirements for the boost converter design:

  • 19 V Vout
  • 3 A peak output current
  • Smallest PCB footprint design and layout
  • Lowest possible ripple on Vout

The battery is a custom made 4S/1P 21700 pack consisting of the Samsung 50E cells. The associated BMS is able to output 6 A of continuous and 8 A non-continuous current, confirmed using low-value resistor loads. The cutoff voltage for overdischarge is 12 V and the cutoff for overcharge is 16.6 V. I'm certain that this battery pack is not the issue. I've even used a bench power supply rated up to 30 V, 6 A on Vin and the voltage drop on Vout still persists.

I've cleaned the PCB from flux and other residue using IPA alcohol, as I was informed that this could have an impact on the circuit performance. Sadly, Vout still drops significantly.

I've attempted to increase the C10 capacitor value, all the way up to 2200 μF and still no luck.

BoM:

  • D1
  • L1
  • All other parts are generic. All the capacitors are rated for at least 25 V and are low ESR. The ceramic caps have XR7 quality. The resistors are all within 1% of their rated values. Both resistors and ceramic caps are 0603 (imperial) packaging.

Schematic:

enter image description here

PCB layout (C9 footprint is open and not used in this design):

enter image description here

After countless hours of head-scratching, I feel that I screwed up what appears to be a simple and straightforward boost converter design. Any help is much appreciated.

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    \$\begingroup\$ Have you checked input current vs output current? It is a good bet that the inductor is saturating or not operating at a high enough frequency to transfer sufficient energy to the output. Or it could be too high frequency. Try changing the FREQ resistor. \$\endgroup\$
    – PStechPaul
    Dec 31, 2022 at 5:51
  • \$\begingroup\$ I would indeed double check the inductor and the higher ESR caps. \$\endgroup\$
    – RemyHx
    Dec 31, 2022 at 6:01
  • \$\begingroup\$ You should have a little over 5A peak inductor current with 3 A out, which should be OK (Isat on this inductor is 6 A). The average input current should be about 4.9 A, also OK. Have you looked at the switch node waveform to see if you have excessive jitter or instability when the output voltage drops? \$\endgroup\$
    – John D
    Dec 31, 2022 at 6:07
  • \$\begingroup\$ @PStechPaul I was able to monitor both in and out current, and the output current spikes as the Vout voltage drops (as expected). I’ve attempted different switching frequencies above 600KHz but all appear pose the same result, with slightly different ripple on Vout. I can’t select a frequency too low as per the data sheet it would require a higher value inductor and decoupling capacitors which isn’t suitable for the layout restrictions of this design. \$\endgroup\$
    – goose_ader
    Dec 31, 2022 at 6:31
  • \$\begingroup\$ @John D I haven’t probed the switch pins but I did probe the outputs of the audio amplifier powered by the boost converter to analyze the clipping. When Vin voltage was above 15.6 V the clipping was controlled and there didn’t appear to be any instabilities from the amp or the boost converter (the Vout still dropped to ~17.52 V where Vin was ~15.95 V). When Vin was under ~15.6V, the boost converter was starting to oscillate when the amp was clipping. As a result, the amp just went into a shutdown state and restarted when Vout increased to the expected value. \$\endgroup\$
    – goose_ader
    Dec 31, 2022 at 6:43

1 Answer 1

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I did an LTspice simulation of a simple boost converter to see what the waveforms might look like. This is at 1MHz and 30% duty cycle. Everything looks OK. Easiest to check might be at the switch, making sure the signal goes solidly to GND without desaturation. That is the violet v(n002) trace.

Simple Boost Circuit

Can you get a cleaner trace for your switch node? Can't really tell anything with so much noise.

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  • \$\begingroup\$ Thanks for the breakdown. I tried scoping different nodes connected to the SW pin and this appeared to be the best I could get when Vout drops: ibb.co/z49rmMf. Now, I also discovered that changing the load reduced the voltage drop on Vout until the inductor began to saturate. So this issue may be specific to the load of the boost converter (2x TPA3128D2 Class D amps). I will have to perform some more tests to be sure but if that is the case I will likely open a new question about it. \$\endgroup\$
    – goose_ader
    Jan 1 at 3:12
  • \$\begingroup\$ It looks like there is some power line AC noise, and the converter seems to be operating in some sort of burst mode. It would help to run a sweep frequency high enough to see details of the switching waveform. And you may need to rig up a special clip for the scope probe. What do the input and output DC waveforms look like? And also try a plain resistive load. \$\endgroup\$
    – PStechPaul
    Jan 2 at 4:32

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