Charger Ic: Preparation, Placement and Process Control

A switching charger is a small converter with a battery on its output, and its layout follows the same rules as any switching supply. Charger ic board layout decides the efficiency, the temperature and the electromagnetic behaviour of a part that runs whenever the product is plugged in.

What the Charger Does

The device converts the adapter voltage into a controlled current and voltage for the battery. It manages the transition from a small trickle current to a constant current and then to a constant voltage.

Because it operates at a fixed switching frequency, its behaviour is predictable, and the passive components around it are chosen to suit that frequency rather than to suit a particular current.

The Converter Topology

A step down converter with the switch and the synchronous rectifier integrated is the usual arrangement. A typical device may switch at one and a half megahertz and deliver two amperes of charge current.

The high frequency allows a small inductor and small capacitors, which is what makes the charger fit into a thin product. It also means the switch node area has to be small, because a high frequency field radiates from a small area efficiently.

Setting the Charge Current

The charge current is programmed by an external resistor, so the value of that resistor sets the thermal load of the whole charger. Its tolerance and its temperature coefficient belong to the accuracy of the charging.

The programming resistor is placed next to its pin and routed with a short trace, because the reference it sets is a small signal. A long trace can pick up noise from the switch node and shift the current.

Input Capacitor

The input capacitor absorbs the ripple current of the converter and holds the input voltage during the switching cycle. Its ripple current rating is the important parameter, and in the worst case it approaches half of the charge current.

The capacitor is placed as close to the input pin as the layout allows, and it is not optional even when the adapter is close by. Its capacitance matters less than its placement and its ripple rating.

Battery Capacitor

The capacitor at the battery pin stabilises the output while the battery is disconnected or while its internal resistance changes with age. It is placed close to the battery pin rather than near the connector.

Where the battery is connected through a long cable, a capacitor at the connector is added as well. Long leads add inductance, and the charger cannot control a voltage it cannot see.

The Inductor

The inductor value follows from the switching frequency, the input voltage and the ripple that is acceptable. Values around four point seven microhenries are typical for a device of this class.

A shielded, moulded inductor is preferred because its field is contained and its resistance is low. Its saturation current has to exceed the peak switch current, which is higher than the charge current.

Thermal Pad and Power Ground

The exposed pad under the device is the power ground connection as well as the thermal path. It has to be soldered to a copper area with vias into the ground plane, and it must not be left unconnected.

The area under the device carries the switch current as well as the heat, so the connection is designed rather than left as a by product of the footprint. A pad that is soldered but poorly connected shows up as a hot device at full current.

Switch Node Area

The switch node is the connection between the device, the inductor and the input capacitor. It carries a voltage that changes rapidly, so its area determines the radiated field.

Keep the node as small as the current carrying requirement allows, keep it on one layer, and keep other traces away from it. Every square millimetre added to that node adds coupling to whatever is near it.

Snubber Design

A series resistor and capacitor from the switch node to ground damps the ringing that the parasitic inductance and capacitance produce. A resistor of about ten ohms with a capacitor of about two point two nanofarads is a common starting point.

The snubber is a trade: it reduces the ringing at the cost of a small amount of efficiency. It is fitted where the electromagnetic measurement requires it and tuned by measurement rather than by calculation alone.

Charge Profile

The charger moves through trickle, constant current and constant voltage phases, and the transitions are set by the battery specification. The timer that terminates the charge is part of the same profile.

Those parameters are configured in firmware or by resistors, and the battery data sheet is the source. A profile that is shorter than the battery requires leaves it partly charged, and one that is longer stresses it.

Thermal Behaviour

The charger loses power as heat whenever the input voltage exceeds the battery voltage by more than the converter needs. At the start of a charge, with a flat battery, that loss is at its maximum.

Thermal regulation inside the device reduces the charge current when the junction temperature reaches its limit. That behaviour is normal, but it means the measured charge time depends on the layout and on the copper area around the die.

Charging and System Load Together

Many products operate while they charge, so the charger supplies the system load as well as the battery. The adapter has to be sized for the sum, and the loop between the input, the system and the battery has to be laid out accordingly.

Where the load exceeds the adapter capability, a power path arrangement shares the current between the adapter and the battery. The sense points for that function are part of the layout, not of the device alone.

Bench Measurement

Measure the efficiency at several currents and input voltages, and measure the switch waveform with a short ground lead. The ringing amplitude and the case temperature are the two numbers that show whether the layout is correct.

Measure the radiated emission with the charger at full current as well. Our notes on converter layout describe the same measurements for a general supply.

Design Checklist

Confirm the input capacitor placement and rating, the battery capacitor position, the inductor saturation current, the thermal pad connection and the switch node area. Add the snubber only where it is needed.

Then confirm the charge profile against the battery specification and the thermal behaviour at full current, and check the sense resistor value against the current that was actually measured on the bench. The general review criteria are described in our design quality and current capacity notes.

FAQ

Can the input capacitor be omitted? No. It carries the ripple current of the converter and holds the input during the switching cycle, and the part cannot do that work itself.

Why does the charger get hot at the start of a charge? Because the voltage difference between the input and the battery is largest then. The loss is converted into heat in the device.

Is a snubber always needed? Only where the ringing or the measured emission requires it. It costs a small amount of efficiency, so it is added deliberately rather than by default.