For a hobby project, I have designed a board using the LTC3355 DC/DC regulator to work as a power supply. I am a software engineer by trade, and I am thus totally underequipped to properly design and troubleshoot this circuit. This is one of my first boards, and it is bound to contain nearly all beginner mistakes you can imagine. I have probably chosen a way too complicated circuit for my level of knowledge, but such is life :).
I can provide oscilloscope views of any node of my circuit if anybody can recommend what I need to measure.
This IC has three main functions:
- Buck regulator while input power is available
- Supercapacitor charger
- Boost regulator while input power is unavailable (sourcing current from the supercapacitor).
The datasheet is available here: http://www.linear.com/docs/44566.
I am having two (possibly related) problems with functions 2 (charging) and 3 (boost). The buck functionality works as intended and I get satisfactory efficiency and output stability.
This question intends to focus on the problematic boost functionality, but if the charging gets solved at the same time, all the better.
When I disconnect the input power, and there is low or zero load on the output, the output is regulated just fine (albeit somewhat more noisy than during buck operation) and the supercapacitor charge is used to keep the output voltage.
However, when the load is increased just slightly, to around 50mA, the output voltage drops to approximately 4V, and V_out and V_scap both appear very noisy. V_scap gets oscillations of around 2V peak-to-peak, at a 2V average voltage. The rated maximum current is 1A, so this should be no problem at all.
Secondly, when I connect the EN_CHG pin to +5V (and input power is available), the supercapacitor is not charged at all. No current is drawn from the input either.
- Input voltage: 7-20V
- Output voltage: 5V
- Supercapacitor: 100F, 2.7V
- No input current limit set resistor.
- Resistors and capacitors are of size 0805 to simplify hand soldering and to accommodate for routing between pads, keeping the board single sided.
- Both inductors are shielded and rated for use in switch-mode supplies well above 1Mhz as used by this circuit), and have plenty of current capability.
I have configured the IC very close to the reference design provided in the datasheet, with a few minor differences. The voltage dividers have been altered slightly to give a 5V output (within specs), and a max 2.7V supercapacitor voltage.
My inductors are of the same inductance as the reference design and rated well above the frequency of the regulator.
Here are a few possible issues that I am aware of, where I differ from the reference design or datasheet recommendations, which might adversely affect the performance of my circuit:
- Contrary to datasheet recommendations, good design practices, and better judgment, I have laid the circuit out on a single-sided PCB, due to the fact that I lack tools for drilling and manufacturing multi-layer PCB:s. However, I haven't run into thermal issues, which the datasheet hints at, but other design aspects are also adversely affected by this choice:
- Poor grounding (stemming from point 1) possibly causing all kinds of interference problems. I am guessing this is the most likely culprit.
- The feedback pin of the IC is on the opposite side of what it's measuring so I had to use an aerial wire to avoid routing it all around the board. This is suboptimal... A different layer is used for this purpose in the manufacturer's demo board.
- Datasheet reference image mentions a supercapacitor between 1 and 50F. I only have a 100F supercapacitor on hand but didn't think this would cause any problems.
- Poor soldering skills. The IC and inductors were soldered with hot-air (no exposed leads, and I might have damaged it by excessive heat. I do think all pads have contact and have probed that there are no shorts. The functionality that actually works suggests that they do have contact. The PFOB, RSTB and CPGOOD pins respond on power failure, output going out of regulation, and capacitor losing voltage.
- A few of my capacitors are generally of a larger package (all 0805) than the reference circuits, which (I've learned) alters the equivalent circuit, and might change the frequency response of my system.
What I've tried
I have tried adding extra ceramic capacitors between the supercapacitor and ground, as seen in the Linear demo board here (C10 and C11). These capacitors are not present in the datasheet example circuit:
This did not make any significant difference in keeping the boost regulator in regulation.
The demo board also lists several OPT (optional?) capacitors, that are not in the datasheet reference design. I have not tried putting anything in these places.
Circuit diagram and board layout
I am having problems getting DipTrace to present my board layout in a more readable fashion, for example by showing the components better. For that reason, I also provide a 3d rendering, which gives a complementary view.
Behold, the horror. PCB and its 3-D view:
I have tried to keep current paths as fat as possible, and relevant sensitive nodes as small as I can, but the single-sided layout is limiting... The nodes with blue lines are connected by short air wires. Pad 14 and 13 have pull-down resistors to ground to default to PWM-mode, and no charging (not visible in images). The red lines inside L1 are from an "unpoured" copper fill and are not actual copper on the real board.
The exposed bottom pad of the IC is used for ground, and I routed it out by the corners.
The input power is connected to the polygon under label C6 and the ground fill. The supercapacitor is connected to the polygon at label L2 and the ground fill
I am wondering if it is totally doomed to fail to have this circuit on a single-sided board, while the manufacturer's demo board uses 4 layers, or whether my attempt can be salvaged somehow. The datasheet mentions adjusting the compensation network at pin V_cbst if needed, but I have no idea how to go about doing that, except randomly trying other component values. I'm lacking a wide assortment of capacitor values and will order more if I get any plausible recommendations.
After getting rid of the alligator clip wires, as recommended by Michael Karcher, and tidying up one of the jumper wires, I got some improvements. Previously I could only reach a maximum load of ~130mA, while I can now reach to approximately 350mA. This is still far from the rated 5 A output from the boost converter. However, my goal is ~1 A, which would be at the same level as the buck regulator used when input power is available.
The supercapacitor is now soldered to the board, using about 1 cm of wire to each lead on the capacitor.
The yellow trace is Vout, and the blue trace is Vcap.
At about 0-5 mA of load, this is how the waveform looks, with the switching frequency of 1 Mhz visible. Boost mode, 5mA load:
However, at about 55-60mA the waveform abruptly changes appearance to this, with a frequency closer to 100 kHz. Here, the output voltage still is regulated at approximately 5V. Boost mode, 60mA load:
At the maximum load current I can achieve, at ~350mA, the output voltage has dropped significantly to 4.5V. Boost mode, 350mA load: