0
\$\begingroup\$

I am trying to incorporate a Buck-Boost DC-DC converter into my design using the LTC3534 and I cannot seem to get it working.

Below I will post images of the datasheet recommended design, my implementation and also my PCB design files (top and bottom layer) although it is pretty hard to read.

Testing the implemented design:

  1. Input voltage = 4V;
  2. Maximum current = 0.1A;
  3. No load.

The result is that the device wants to draw as much current as it can (I have tested it up to 400mA and it is the same). The voltage supply drops to 2.3V because of the current limiting, the output goes to 1.33V. The feedback pin is at 0.4V. (which seems logical as (R1 / R1 + R2)*1.33V = 0.4V, but it should be 1V at an output of 3.3V).

Important things to note: PWM pin is directly hooked to Vin, R6 shorted and C22 removed.

I have prior tested the board without the output capacitor C22 mounted on the board. (I forgot to solder it) Could have this damaged the IC?

Lastly, is it possible that my inductor is saturating, as it is rated at 2.4A@100kHz and the circuit operates at 1MHz (In my opinion this does not matter as I used no load so the current through it is a minimum)? This is the inductor used.

I have also tried another board with no current limiting and it drew a lot of current until one of the pins connected to the inductor became shorted to ground and the IC became toast.

Datasheet recommended design: enter image description here

My implementation: enter image description here

PCB Top:

enter image description here

PCB Bottom:

enter image description here

\$\endgroup\$
  • \$\begingroup\$ Measure your peak inductor current and/or inrush voltage-time area to find out if it's saturating. \$\endgroup\$ – winny Apr 24 '18 at 11:09
  • \$\begingroup\$ But does the inductor current matter if I am testing it with no load? \$\endgroup\$ – Cezar Chirila Apr 24 '18 at 11:33
  • \$\begingroup\$ You can still saturate it during charging of the output capacitor. \$\endgroup\$ – winny Apr 24 '18 at 12:44
  • \$\begingroup\$ Which part gets hot - the ic or the inductor or both? Also, of the many times I've suspected a dc dc converter ic of being damaged, 90% of the time the problem ends up being something else (e.g a solder short) and the ic is fine. LT parts are generally pretty tough. It's certainly possible, but in the meantime do a close inspection of your circuit for solder issues. \$\endgroup\$ – Selvek Apr 24 '18 at 13:19
  • \$\begingroup\$ At 0.1A neither gets hot. Although I did manage to toast one for giving it no current limit (one Inductor pin shorted to ground). \$\endgroup\$ – Cezar Chirila Apr 24 '18 at 17:36
2
\$\begingroup\$

I have prior tested the board without the output capacitor C22 mounted on the board. (I forgot to solder it) Could have this damaged the IC?

Yes, this could damage the device because the inductor current/energy has nowhere to "dump" and it will generate a fairly sizable peak voltage. Difficult to be exact.

With no load, the device MUST operate in burst mode control, You have PWM mode set to prevent burst mode and this is a likely problem. If load currents are less than 1 mA you must run in burst mode - see the top 2 graphs on page 4 of the data sheet.

However, I think the main issue is running it without the output capacitor.

\$\endgroup\$
  • \$\begingroup\$ Thank you! I am going to wait for a new IC. Should I also put a 3.3kOhm resistor across the output capacitor to provide 1mA of load? \$\endgroup\$ – Cezar Chirila Apr 24 '18 at 11:35
  • \$\begingroup\$ @CezarChirila This should be a lesson - never order 1 of anything, in case you break it. Always at least 2, preferably 5 if it's cheap enough. \$\endgroup\$ – user253751 Apr 24 '18 at 11:43
  • 1
    \$\begingroup\$ @immibis I actually ordered 2, but because I was cocky I did not current limit it and it went toast. Thank god Arrow ships overseas for free. As you said, a lesson. \$\endgroup\$ – Cezar Chirila Apr 24 '18 at 11:51
  • \$\begingroup\$ LT dc-dc controllers typically are smart enough that they don't need a minimum load, but i haven't checked this exact one. \$\endgroup\$ – Selvek Apr 24 '18 at 13:07
  • \$\begingroup\$ @Selvek yes, I agree but I would only say that a significant number are capable of doing this. I did scan the DS but found the graph to be the only indicator that there may be an off-load problem. \$\endgroup\$ – Andy aka Apr 24 '18 at 13:16

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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