The output voltage can be varied by using different values for R1 and R2. 33V is about as much as you can get out of the converter (I actually measured 32.6V with R1 and R2 as shown). See the LT1613 datasheet for the formula to calculate the values for R1 and R2 for lower voltages.
R3 in the schema above is optional. It is not needed for the LT1613 (input voltages of less than 10V). There's a jumper in the diagram below (just above the 39K resistor) in place of R3.
C2 is optional too. It helps reduce output ripple. I simply soldered a small ceramic 2.2nF cap over the resistor.
Layout is the most crucial design aspect for obtaining low noise (and to actually make it work - because of the high switching frequency it won't run as a bread-board prototype). Use big area trace to minimize impedance. The LT1613 GND pin (center left pin in the diagram below) carries high speed, switched current; its path to the circuit's power exit should be direct and highly conductive at all frequencies. R2's return current (bottom left pin), to the extend possible, should not mix with pin 2's large dynamic currents (center left pin). C1 and C3 should be located close to pin 5 (top right) and D1 respectively. Their grounded ends should tie directly to the ground plane. Pin 1 (top left) has a small area, minimizing radiation. (Source: Analog Circuit Design Volume 2: Immersion in the Black Art of Analog Design, Volume 2 and LinearTechnology LT1613 data sheet.)
The following picture shows a possible component layout on a small PCB using both sides for the components. The components drawn in black are visible (front side), the components drawn in gray are located on the back side. The 5-pin component in the center is the LT1613.
Component selection is also important. I used the following components:
- Inductor: Coiltronics DR73-100, 10uH
- Capacitors (C1, C3): Kemet T495D226K035ATE300, Tantalum 22uF 35V
- Schottky-Diode: ST 1N5819RL, 40V
Creating the PCB
This PDF (editable with Adobe Illustrator) shows the mirrored trace layout in actual size. Use the PDF to create your own PCB. I did it as follows:
- Print the layout file on transparent laser printer foil using a laser printer
- Put a flat board on your work desk
- Put a piece of circuit board (one sided 35um copper layer) on the board. Copper up. Make the printout slightly smaller than your circuit board
- Put your printout on top, Toner against the copper.
- Use Scotch Magic tape to tape your printout to the copper around the edges.
- Put a thin piece of linen cloth on top.
- Fire up your household iron. Set it to high (linen or similar)
- Iron the toner onto the copper using a lot of pressure and, in my case, one to two minutes of time. You may have to experiment a little at this point to get perfect results.
- Peel the foil of the circuit board. All toner should stay on the copper.
- Cut your board to size
- Put the board in a bath of natrium persulfate (a little more than hand-warm) for an hour or two (you'll see when it's done). Don't put your hand in the solution. I actually used to baths: a bucket with hot water and a small tupperware container with the persulfate solution - the tupperware container swimming in the bigger bucket.
- Here's a site that provides much more info on this process: https://www.smallbearelec.com/HowTos/DirectPCBoards/DirectPCBoards.htm
And here some pictures for illustration
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| A laser printer printout ironed onto copper and then peeled off. |
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| The PCB in the natrium persulfate bath |
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| Not pretty but effective: thick short straight connections from the IC to the caps. |
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| I first used a conductor with through-hole leads. I then switched to a different surface mount inductor which required improvised solder pads. |
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| Ready for a test drive. |
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| The DC/DC converter in my RealTube-One pedal. L2 and C4 are for additional noise filtering. |
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| Without additional noise filtering. |
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| With additional LC noise filter (L2 and C4) |
