5585

Compact Lipo charger & booster in one tiny PCB, USB-C input, 5V output, high efficiency, and open-source design!

Batteries power our world, but managing them efficiently requires smart circuits. In this project, I designed a compact lithium battery charger with a 5V boost converter, combining a TI BQ24075 charger IC and a Diodes Inc. PAM2401 boost IC into a tiny 24mm × 34mm PCB.

But the journey wasn’t smooth! A hidden schematic mistake turned my board into a toaster before I fixed it. In this Instructable, I’ll walk you through:

✔ Circuit design & PCB layout
✔ Assembly & soldering tips
✔ The debugging nightmare (and how I fixed it!)
✔ Final testing & performance

Let’s dive in!

Supplies


Electronics Components
  1. IC1:BQ24075 (LiPo Charger)
  2. IC2:PAM2401 (Boost Converter)
  3. L1: 2.2µH Inductor (2A saturation current)
  4. D1: Schottky Diode (SS34)
  5. USB-C Connector (SMD, 16-pin)
  6. JST 2.0mm Connectors (Battery + Output)
  7. LEDs (Power/Charge Status)
Tools & Materials
  1. PCB Fabrication:JLCPCB (Purple solder mask!)
  2. Soldering:
    1. Stencil
    2. Solder Paste (Low-Temp)
    3. Hotplate
    4. Precision Tweezers
  3. Debugging:
    1. Oscilloscope
    2. Multimeter
    3. Microscope
  4. Design Software: Altium Designer
Circuit Design


The Two Key ICs
  1. BQ24075 Charger IC
  2. Why? Handles all lithium battery charging logic
  3. Key Feature: Built-in thermal regulation (no cooked batteries!)
  4. Setup: Identical to my previous project (USB-C in, 500mA default)
  5. PAM2401 Boost Converter
  6. Why? Converts low battery voltage 0.9V–4.2V battery voltage to a stable 5V/500mA output
  7. Key Specs: 1MHz switching, needs 2.2µH inductor (I used a 2A-rated one)
  8. Gotcha: LX pin requires careful PCB routing
elektor
Schematic Tips
  1. Isolate power paths! My mistake: Accidentally connecting charger/booster outputs (more in Step 4).
  2. Add reverse protection (Schottky diode on USB input).
Critical Design Choices
  1. Size: 24mm × 34mm (fits behind a battery)
  2. Ground Plane: Full copper pour under power components
  3. Component Placement:Inductor <5mm from PAM2401 (reduces noise)
  4. USB-C port on left edge for easy access
Design Tools Used
  1. Schematic Capture: Altium Designer
  2. Component Sourcing: Selected JLCPCB-compatible parts for easy assembly
 Pro Tip: Run a Design Rule Check (DRC) before ordering PCBs!

 Design Files:
  1. Full Schematic PDF
  2. Altium 365 Project
Assembly


After getting the Circuit very well designed, I generated its related GERBER files and uploaded them to JLCPCB website to order these Purple color PCBs.
elektor
Soldering Steps

Tools Needed:
  1. Stencil + solder paste (I used Chip Quik TS391LT)
  2. Hotplate (or reflow oven)
  3. Precision tweezers (for 0402 components)
1. Apply solder paste using a stencil.
elektor

2. Place components with the aid of tweezers in this order:

  1. Start with the small parts (Capacitors, resistors)
  2. PAM2401/BQ24075 ICs
  3. Inductor and passives
  4. USB-C connector (check alignment!)
    elektor

3. Reflow on a hotplate (220°C for 90 sec).
elektor

Check: Inspect joints under a microscope, look for bridged pins or cold solder.

Debugging the "Voltage War"


After getting the circuit very well assembled, I moved to do some tests targeting a 5V output voltage from a Lithium 3.7V battery so the needed result should be a boosted stable voltage coming from this circuit:
elektor
The Problem
  1. Booster output stuck at 3V instead of 5V
  2. Inductor overheating
  3. LX pin showing glitchy pulses (scope capture below)
elektor
Symptoms & Fixes

Inductor overheating ---> Output short ---> Check LX pin with scope
3V output (not 5V) ---> Booster disabled ---> Verify EN pin voltage
Erratic LX pulses Feedback ---> loop broken ---> Measure FB pin (0.6V?)

  1. Root Cause:
  2. My schematic accidentally connected the charger and booster outputs!
The Fix
  1. Cut the conflicting trace between ICs.
  2. Verified clean 5V output afterward.
  3. Check the output Pulses signal from the booster LX pin to the inductor
elektor
I then updated the Design files and fixed the GERBER as well :)

Performance Metrics
  1. Efficiency: 92% at 500mA load
  2. Output Stability: ±50mV ripple
  3. Charging Current: 500mA (adjustable via resistor)

Success! Now it charges batteries and delivers 5V reliably.
elektor