September 2006

Rev 2

1/14

AN2407

Application note

800mA standalone linear Li-Ion

battery charger with thermal regulation

Introduction

One way to minimize the size and complexity of a battery charger is to use a linear-type

charger. The linear charger drops the AC adapter voltage down to the battery voltage. The

number of external components is low: linear chargers require input and output bypass

capacitors, and sometimes need an external pass transistor, and resistors for setting voltage

and current limits.

The main pitfall of a linear charger is power dissipation. The charger simply drops the AC

adapter voltage down to the battery voltage.

In the case of an 800mA charger, a 5V±10% regulated AC adapter voltage, and battery

voltage that varies between 4.2V and 2.5V, the power dissipation can range from 0.6W to

2.0W.

This type of charger is simpler than the switch-mode type, mainly because the passive LC

filter is not required. It dissipates the most power when the battery voltage is at its minimum,

since the difference between the fixed input voltage and the battery voltage is greatest

during this condition.

Application diagram

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