# Buck/Boost circuit not working

I'm a co-op student working on a hardware design for an industry client, and I'm trying to use a TPS63000 to get a Vout of 3.45V. Vin is 5V. When I power this circuit up, I read 5V on Vin but only 0.7V from Vout.

Here's the PCB design:

I'm not sure what approach to take in troubleshooting this. Any help would be appreciated.

EDIT: Unfortunately I don't have easy access to an oscilloscope, but when I do, I will definitely try to confirm that there's a pulse across the inductor. Yesterday I assembled another PCB, and I'm getting a very similar result. Vout actually stabilizes at 322mV, and does so with a high degree to accuracy. During startup, I do see Vout shift drastically, but always settles around 322mV. Sorry that that's not very much to go on, an oscilloscope will definitely help give a better picture.

So it seems to me possible causes are: A. lack of R3/C3 on the Vin pin, B. narrow traces on the PCB, or C. bad chip. Thanks for all the responses so far.

• Is it switching at all? Probe the EN pin too. – winny May 25 '17 at 17:40
• Connect oscilloscope probes to both pins of the inductor. – Todor Simeonov May 25 '17 at 17:42
• Leadless parts (DFN,QFN) are difficult to manually solder, can be a bad solder joint too – sstobbe May 26 '17 at 18:03

You have no delay on the VINA, EN and PS/SYNC pins. It is unclear what mode the thing will go into when connected like that.

The device specs shows a suitable delay circuit.

Regardless of the following, I fail to see why your circuit does not work. $$\R_1\$$ and $$\R_2\$$ are correct for 3.45V. $$\C_1\$$, $$\C_2\$$ and $$\L_1\$$ are within parameters. Why your circuit does not show some life is strange. It is scope time! Calcs have been confirmed.

Bad chip (is always lame)? Try connecting up a load during power up. Try connecting up $$\W_1\$$, when your source has stabilized.

I do not think the addition of $$\R_3\$$ and $$\C_3\$$ is the answer. The app-note does not even mention them. But it is certainly worth a try.

From TPS6300 datasheet:

As for all switching power supplies, the layout is an important step in the design, especially at high peak currents and high switching frequencies. If the layout is not carefully done, the regulator could show stability problems as well as EMI problems. Therefore, use wide and short traces for the main current path and for the power ground tracks. The input capacitor, output capacitor, and the inductor should be placed as close as possible to the IC. Use a common ground node for power ground and a different one for control ground to minimize the effects of ground noise. Connect these ground nodes at any place close to one of the ground pins of the IC.

The feedback divider should be placed as close as possible to the control ground pin of the IC. To lay out the control ground, TI recommends to use short traces as well, separated from the power ground traces. This avoids ground shift problems, which can occur due to superimposition of power ground current and control ground current.

Irregardless, you have skinny and long, as opposed to short and really, really fat, so you will definitely have production problems.

• Thank you, I should definitely re-do the PCB design to reflect what's in the datasheet. – ebrau May 26 '17 at 17:47

A few guesses/pointers: -

• Check R1 isn't 1.8 kohm (or a wrong value)
• Is exposed thermal pad properly earthed?
• Have you got too much output load?
• Check that oscillations are running about 1350 kHz
• Is your meter working correctly?

Good luck.

My guess is that you don't have a proper power supply bypassing. You have omitted the separate bypass cap C3 from the typical application schematics, where R3 & C3 form a low-pass filter to separate the power supply of the power part from the controller part of the IC. You might be able to succeed without R3 (but you shouldn't), but you definitely need C3, and the controller side power supply should be separated enough (with at least a thin wire) so that the highly fluctuating current between C1 & the power side of the IC (pin 5) does not affect the controller side power supply of the IC (pin 8).

You should also look at page 16 in the data sheet and read the layout recommendations (note that the layout recommendation picture shows a differing schematics, with a different C3).

I know this question is old, but Google brought me here on this exact topic and I see no concrete answer, I solved the issue myself, the error is on the feedback, the TPS63000 is a 3.3V (fixed regulator) and no set point is required, only direct feedback, however the circuit and pcb layout shown can accommodate both TPS63000 (fixed) and TPS63001 (adjust)

Solution for your circuit using a TPS63000 option, requires a zero ohm resistor for R1, so feedback is taken directly from the output, and omit R2.