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Key points
TP4056 current-programming limits
Formula from data sheet:
\[
I{CHG}[{\rm mA}] = \frac{1000}{R{PROG}\,[{\rm k\Omega}]}\;(\text{approximated})
\]
• RPROG = 1.2 kΩ → ≈ 0.83 A (common “1 A” module).
• To push toward 1.2 A, RPROG would have to drop to ≈ 0.83 kΩ (820 Ω E24).
However:
• Absolute-maximum charge current in the official data sheet is 1 A.
• Internal power transistor and thermal shutdown point (~125 °C) are sized for 1 W dissipation.
With 5 V in, 4 V battery, 1.2 A would dissipate
\[
P_D = (5-4)\times1.2 \approx 1.2\;{\rm W},
\]
already above the guaranteed 1 W capability and on most low-cost modules the tiny copper area cannot remove this heat.
• The IC will therefore enter thermal-regulation mode and clamp the current back down, or eventually fail.
Parallel-module alternative (experimental)
• Connect two identical TP4056 boards to the same cell, RPROG ≈ 2 kΩ on each (≈600 mA).
• Add 0.1–0.2 Ω ballast resistors in series with each BAT+ to reduce current hogging.
• Drawbacks: unsynchronised termination (one stops at C/10 before the other), LED indications inconsistent, extra parasitic resistances, doubled board area and heat.
• Still linear, so total heat ≈1.2 W shared by two packages – manageable with good airflow.
Preferred engineering solution
• Select a buck-type single-cell charger:
– TI BQ25895/BQ24195 (2–3 A, power-path, I²C control)
– CN3791 (1.8–3 A, simple resistor-prog, often sold as “2 A solar/li-ion charger module”)
– LTC4002/LTC4055 families, or Microchip MCP73871 (1.8 A)
• Advantages: ≥90 % efficiency, dramatically lower temperature rise, accurate termination, safety timers, NTC input, USB BC1.2 / QC negotiation on some parts.
Battery and supply considerations
• Verify cell spec: standard 2 000 mAh 18650 ≈1 C max rapid charge → 2 A is OK; smaller 1 000 mAh pouch may be limited to 0.5–1 A.
• Power source must deliver at least 2 A at 5 V with minimal sag; many phone chargers labelled 2 A still droop to 4.6 V and cause TP4056 dropout.
• Wiring and PCB traces ≥1 mm width (1 oz Cu) or use stitched copper pours.
Online sources (Elektroda, Arduino Forum, StackExchange 2023-24) strongly discourage exceeding 1 A per TP4056 and favour switching chargers. USB Type-C PD sinks that negotiate 5 V / 2–3 A paired with modern buck chargers are now common on hobby modules (<3 USD, 2024 prices). Integration of fuel-gauge + charger (e.g. MAX17320) is also trending.
• Why linear chargers run hot: all excess voltage is burned as heat in the internal pass transistor; efficiency \( \eta \approx V{BAT}/V{IN} \).
• Buck topology uses inductor + FET switching, so dissipation is \( I^2 \times R_{DS(ON)} + \text{switch losses} \) → typically <0.3 W at 1.2 A.
• Analogy: driving a car in first gear (linear) vs. using a proper gearbox (switch-mode).
• Lithium-ion safety: over-temperature, over-charge, and thermal runaway can cause fire. Operating out-of-spec charger voids conformity (CE/FCC) and product liability.
• Follow IEC 62133 / UL 2054 if designing commercial equipment.
• Dispose of failed cells according to local hazardous-waste regulations.
Implementation steps if you still want to experiment with one TP4056:
Best practice (recommended):
Potential challenges
• Heat management in tight enclosures.
• Ensuring accurate termination when paralleled.
• USB-C identification for >1.5 A current.
• Evaluate combo PMICs (e.g. TI BQ25792) that support both charging and boost for USB-OTG.
• Investigate gallium-nitride (GaN) USB-PD supplies for compact, efficient power.
• Thermal simulation tools (e.g. TI WEBENCH, ADIsimPower) to predict hot-spot temperature before PCB fab.
The TP4056 is a solid 1 A linear charger but cannot be relied on for 1.2 A without exceeding its ratings and courting thermal problems. For a robust design, either share the load across two TP4056 modules (with the caveats listed) or—preferably—switch to a modern switching charger IC/module explicitly specified for currents ≥1.2 A. This path delivers higher efficiency, lower temperature, and better safety compliance.
