XL6009 Adjustable Boost Converter
Step a 3.7 V Li-ion cell up to 12 V at half an amp with a single switching regulator — the foundation of every portable gadget that needs more voltage than its battery offers.
Difficulty: Through-hole. Estimated build time: about 40 minutes. Estimated parts cost: about US$7.20. 6-line bill of materials. Compare supplier offers when available. A boost converter — a switching regulator whose output voltage is HIGHER than its input — built around the XL6009 IC.
Identify the pins
On the bare XL6009 in TO-263-5 the pins are: SW (switch node), GND, FB (feedback), EN (enable, tie high or to Vin), Vin. On the popular green PCB module, all of this is hidden — you only see Vin+, Vin-, Vout+, Vout-, and a small adjustment trimmer. We will assume the module for clarity.
Connect the input
Wire your 5 V source (USB or a single 18650) to Vin+/Vin-. Add a 100 µF electrolytic across the input pads if your source is a battery — it eats the inrush current spike when the converter starts up.
Set the output voltage
Before connecting any load: hook a multimeter across Vout+ and Vout-, power the input on, and turn the on-board trimmer counterclockwise while watching the meter. Adjust until you read your target voltage (12 V for this build). The trimmer changes R1 in the feedback divider; nothing electrical about the pot setting persists in NVRAM, so re-check after a power cycle.
Add a load and verify
Connect a load — for first tests, a string of three white LEDs in series with a 100 Ω resistor draws ~30 mA at 12 V. The converter's output should hold steady; ripple is around 50 mV and is invisible to most loads. Probe Vout with a scope set to 20 mV/div and you can see the 400 kHz switching ripple as a small triangular wave.
Watch what happens at the limit
Increase the load until the output starts to droop. The XL6009 is rated 4 A peak inductor current; that translates to about 0.5–1 A at 12 V from a 5 V input (the input current is voltage-ratio higher than the output current, conservation of power). At the limit the converter clips into discontinuous mode and the output ripple grows. This is the moment to learn that switching regulators are not magic — they trade input current for output voltage on a strict P-in = P-out (minus efficiency) basis.