Designing a power supply circuit is a crucial step when you create a device. Especially portable ones: you have to think about the global consumption of the device, which battery suits the best for a not-ridiculous autonomy, how to charge it and how to protect the circuit; to make sound, not smoke.
This estimate was done early in the design process to size the LDO regulator correctly. The audio amp (MAX98357A) is fed directly from the battery output rather than through the AP2112K, which would never survive a 500 mA peak on top of everything else.
| Component | Typical current |
|---|---|
| ESP32-S3 | ~240 mA peak; ~80 mA average |
| SX1278 | ~120 mA (TX) |
| SH1106 | ~20 mA |
| Vibration motor | ~100 mA peak; ~60 - 80 mA average |
| LEDs | ~20 mA |
| Audio amp | ~500 mA peak; not connected on regulator |
| Buttons | ~0 mA |
| Switch | ~0 mA |
| Total peak | ~500 mA (excluding audio amp) |
| Total peak | ~1000 mA (including audio amp) |
The AP2112K-3.3 is a low-dropout linear regulator that converts the battery voltage down to a stable 3.3 V for the logic side of the circuit. It has a dropout voltage of ~300 mV, which means it needs at least 3.6 V on its input to guarantee a stable 3.3 V output. The audio amp (MAX98357A) bypasses it entirely and runs directly off the battery output, keeping the LDO within its limits. It can also provide up to 600 mA, which is perfect for our application.
The TP4056 is a single-cell Li-Ion charge controller. It handles the full charging
cycle and signals its state via the CHRG pin.
We configured the charge current to 580 mA via RPROG (R8); other values are available in the
datasheet.
There is also a STDBY pin, used to get the charge termination status. We won't use it in this project.
| Terminal | Output | Voltage | Usage |
|---|---|---|---|
B+ / B- |
Raw battery voltage | 3.7 V |
Physical battery connection |
OUT+ / OUT- |
Protected battery voltage | 3.7 V |
Rest of the circuit |
The DW01A is a Li-Ion protection IC. It monitors the battery and cuts the circuit in three situations:
| Fault | Threshold | Action |
|---|---|---|
| Overcharge (OC) | > 4.25 V |
Cuts charge MOSFET |
| Overdischarge (OD) | < 2.5 – 2.7 V |
Cuts discharge MOSFET |
| Overcurrent / short | Current spike | Cuts immediately |
The DW01A doesn't cut current directly; it drives the FS8205A, a dual back-to-back MOSFET in a single package. Two MOSFETs are needed because a single one can't block current in both directions due to its body diode.
One more thing to keep in mind: the AP2112K LDO drops out at VIN < 3.6 V, while the
DW01A only cuts at ~2.6 V. There's a grey zone between ~3.0 V and ~3.6 V where the
battery is still connected but the 3.3 V rail is no longer guaranteed; the firmware
should monitor battery voltage via ADC and warn the user before reaching this zone. (todo!)
The TP4056 exposes a CHRG open-drain pin that reflects the charging state. We use it
both for a visual LED indicator and for a GPIO read by the ESP32-S3 to show it on the screen.
| State | CHRG pin | LED | ESP32-S3 reads |
|---|---|---|---|
| Charging | LOW (active) | ON | LOW |
| Charge complete | Floating (pull-up) | OFF | HIGH |
| No battery / error | Blinking | Blinking | Alternating |
The GPIO is pulled up to 3.3 V via 10 kΩ when CHRG is floating, and pulled to GND via
47 kΩ when active. The ESP32-S3 displays the corresponding icon on the OLED display.