The charging IC is the gatekeeper of a laptop's power. It decides whether the adapter is acceptable, connects it to the board, produces the system rail, charges the battery, switches to the battery when the adapter is removed, and reports all of this to the embedded controller. Most "no power" and "not charging" faults pass through it. Knowing its pins and what each one should do turns the charger page of a schematic from a tangle of lines into a short checklist.
What the charging IC does
| Job | How |
|---|---|
| Detect a valid adapter | Compares the adapter voltage, through a divider, with an internal threshold |
| Connect the adapter | Drives the input MOSFETs that pass adapter power to the board |
| Make the system rail | Runs a switching converter from the adapter |
| Charge the battery | Regulates charge current, then charge voltage |
| Switch to battery | Turns on the battery MOSFET when the adapter is gone |
| Limit adapter current | Reduces charging when the system load plus charging would overload the adapter |
| Report and obey | Talks to the EC over SMBus or I²C |
The general idea is in the glossary under charger ic.
Adapter detection
The adapter voltage reaches the charger through a resistor divider into a detection pin, often named ACDET, ACIN or similar. When the divided voltage crosses the internal threshold, the charger declares the adapter valid and raises an output — often named ACOK, ACPRN or ACIN (inverted on some designs) — to tell the EC. The signal is described under acok.
Faults here are common and easy to find:
- A wrong divider resistor makes a perfectly good adapter look too low or too high.
- Corrosion on the divider pulls the detection voltage down.
- A missing adapter-detect signal leaves the EC thinking no adapter is present, so it never allows charging.
Measure the detection pin with the adapter connected and compare it with the threshold in the charger IC's datasheet.
Input MOSFETs
Between the adapter and the system rail sit one or two MOSFETs, driven by the charger. They block current from flowing back into the adapter jack and disconnect an adapter that is out of range. Many designs use a back-to-back pair. These are covered in the glossary under input protection mosfets.
If the adapter rail is present before the MOSFETs but missing after them, check the gate drive from the charger. A charger that has not detected a valid adapter will not turn them on.
The system rail: NVDC
Most current laptops use a "narrow voltage DC" (NVDC) architecture. The charger runs a buck converter (or a buck-boost converter on USB-C designs with wide input ranges) that produces a system rail close to the battery voltage. The battery connects to the system rail through the battery MOSFET:
- Adapter present, battery charging — the converter supplies the system and charges the battery through the battery MOSFET, regulating charge current.
- Adapter present, battery full — the converter supplies the system; the battery MOSFET may be off or in a regulated state.
- Adapter absent — the battery MOSFET turns fully on and the battery supplies the system directly.
Because the system rail is near the battery voltage, the converters downstream can be designed for a narrower input range. Older designs fed the adapter voltage straight to the system rail and used the charger only for charging.
Charging phases
Lithium-ion charging has distinct phases, all controlled by the charger IC with settings from the EC:
- Pre-charge — a small current for a deeply discharged pack, until its voltage is safe.
- Constant current — the main charging phase at the programmed current.
- Constant voltage — once the pack reaches its target voltage, the current tapers off.
- Termination — charging stops when the current falls below a threshold.
The EC writes the charge voltage and current into the charger's registers over SMBus, based on what the battery's fuel gauge requests. If the EC never writes them, many chargers do not charge at all. The bus is described under smbus.
Current sensing
The charger measures two currents with two low-value sense resistors:
- Input current — between the adapter and the converter, to limit adapter load.
- Charge current — between the system rail and the battery, to regulate charging.
The sense pins (often ACP/ACN and SRP/SRN, names vary) read the tiny voltage across each resistor. A cracked or wrong-value sense resistor makes the charger misjudge current: charging stops, runs too slowly, or the adapter is limited far below its rating. These resistors and how to check them are covered in Current Sense Resistors in Laptop Charging Circuits.
Typical pins and what to measure
| Pin (names vary) | What to expect |
|---|---|
| Input supply (VCC, VIN) | Adapter or battery voltage |
| Internal regulator output (REGN, LDO) | A fixed low voltage when the charger is active |
| Adapter detect (ACDET) | Above threshold with a valid adapter |
| Adapter OK output (ACOK) | Active with a valid adapter |
| High-side and low-side gate drivers | Switching when the converter runs |
| Phase / switch node | Switching node between MOSFETs |
| Sense pins (ACP/ACN, SRP/SRN) | Nearly equal pairs; small difference proportional to current |
| Battery MOSFET drive (BATDRV) | Changes when the adapter is removed |
| SMBus (SDA/SCL) | Pull-up voltage when idle |
The internal regulator output is an especially useful check: if it is missing while the input supply is present, the charger is not running at all — damaged, or disabled by a pin.
Diagnosing with this knowledge
- No system rail, adapter present → check adapter detection, the input MOSFETs and the charger's internal regulator.
- System rail present, battery not charging → check that the EC writes charge settings (SMBus), the sense resistors, the battery MOSFET and the battery itself.
- Charging very slowly → check the input current sense resistor and the adapter ID.
- Laptop dies when the adapter is removed → check the battery MOSFET drive.
The full troubleshooting routine is in Laptop Not Charging: Charging Circuit Diagnosis.
Example: no charging after a repair
After a DC jack replacement, a laptop runs from the adapter but never charges. The adapter-detect pin is above its threshold and the adapter-OK output is active, so the charger accepts the adapter. The internal regulator output is present. With the battery fitted, the battery MOSFET drive is in the charging state, but no charge current flows. The charge-current sense pins read exactly equal, and with power removed, the charge sense resistor reads open — it was cracked by the heat of the jack replacement next to it. A new resistor of the same value restores charging.
FAQ
What does ACOK mean?
It is the charger's signal that a valid adapter is connected. The EC uses it to decide whether to allow charging.
Why will a laptop not charge if the EC is faulty?
The EC usually programs the charge voltage and current. Without those settings, many chargers do not charge.
What is NVDC?
A charging architecture where the system rail stays close to battery voltage, with the battery connected through a controlled MOSFET.
Can a wrong sense resistor value cause problems?
Yes. The charger measures current from the voltage across it; a different value makes it charge too fast, too slowly or not at all.
Why does the charger IC have its own regulator output?
It powers the IC's internal circuits and gate drivers. Its presence confirms the charger is alive.
Bottom line
The charging IC detects the adapter, connects it, makes the system rail, charges the battery and switches to it. Check adapter detection and ACOK, the input MOSFET drive, the internal regulator output, the sense resistors and the SMBus link to the EC. With those five checks, most charger-related faults reveal themselves.