1
\$\begingroup\$

I "designed" a simple buck converter, which is basically a clone of the 5V reference design in the TPS6293x datasheet. Unfortunately, my converter is only putting about 1V (closer to 0.95V). This voltage is independent of the input voltage, and I've reproduced it on two separate boards. I used a multimeter to verify the two resistors in the voltage divider are accurate.

I'm unsure what else to check. The voltage being regulated, just at the wrong level, makes me think something is wrong with the resistor divider, but the values match the example reference design almost exactly.

Here is my schematic and board layout:

schematic

layout

\$\endgroup\$
8
  • \$\begingroup\$ Have you checked the waveform at the inductor, and at the output, with an oscilloscope? \$\endgroup\$
    – BeB00
    Commented Nov 21, 2023 at 22:03
  • \$\begingroup\$ With the scope connected to pin 1 of the inductor and ground, I see the inductor ringing (a decaying sinewave). Connected between pin 2 and ground, I see a flat line, with some 60Hz ripple on top of it. \$\endgroup\$
    – Stephen D.
    Commented Nov 21, 2023 at 22:22
  • \$\begingroup\$ I don't see anything obviously wrong. Is the FB pin sitting at 0.8 V as you change the line and load? As @BeB00 said looking at the switch node on a 'scope might provide a clue also. Do you just have 1 board acting this way? Can you swap out the IC? \$\endgroup\$
    – John D
    Commented Nov 21, 2023 at 22:34
  • \$\begingroup\$ @StephenD. The inductor ringing that you see could be normal for discontinuous mode (light loads). Does the ringing go away as you increase the output current toward full load? The ringing should start after the switch node goes low for some period and the inductor "runs out" of current. This happens when the part is operating in "diode emulation mode". \$\endgroup\$
    – John D
    Commented Nov 21, 2023 at 22:39
  • \$\begingroup\$ @JohnD I have 2 boards acting this way. I can solder up another if needed for confirmation, but that's the last board I have. FB is sitting at 0.13V. I don't have an easy way to try increasing the load current as I don't have any kind of adjustable load and the converter is rated for a few amps, so I'd imagine I need at least an amp to get it out of diode emulation mode. If that's the next troubleshooting step I can try to rig something up. \$\endgroup\$
    – Stephen D.
    Commented Nov 21, 2023 at 23:06

1 Answer 1

Reset to default
2
\$\begingroup\$

C15 isn't connected properly. It should go between the BST (pin-6) and SW (pin-5) pins. This is the bootstrap capacitor which provides the level-sifted voltage necessary to drive the high-side transistor.

\$\endgroup\$
3
  • \$\begingroup\$ Great catch, thank you. I'm going to fix that either tonight or tomorrow and will accept this answer if it works. \$\endgroup\$
    – Stephen D.
    Commented Nov 21, 2023 at 23:50
  • \$\begingroup\$ Worked great, and I didn't even need a bodge wire due to the layout of the board! Thanks again! \$\endgroup\$
    – Stephen D.
    Commented Nov 22, 2023 at 2:28
  • \$\begingroup\$ Yes, good catch, can't believe I didn't notice that either :) \$\endgroup\$
    – John D
    Commented Nov 22, 2023 at 16:46

Not the answer you're looking for? Browse other questions tagged or ask your own question.