With the increasing popularity of portable devices, demands on rechargeable batteries have driven the growth of its surrounding electronics, such as battery protection IC and battery charger IC. Battery capacity below 500 mAh is a popular category for small form factor products. In such applications, LDO (low drop out) regulator based architecture is often used to design to recharge batteries.
One popular question often received from system designers is that the charge current during fast charge stage is insufficient and results in longer charge times. It may also trigger built-in timers and experiences premature termination. When investigated, two common scenarios were found – improper package selection or lack of thermal design. In order to fit in space limited portable systems, the smallest packages are selected, such as the 5-pin SOT-23 or small DFNs. Using a Lithium-Ion Polymer battery as an example, the fast charge voltage threshold is at 3V (typical) and 5V for the power supply of a portable device. The power dissipation can be calculated as following for LDO based battery charger:
Pdisipation = (Vin – Vbattery) x Iout
Ignoring the internal supply current, the worst case value will be 2V x 500 mA = 1 Watt
Typical thermal resistance of a SOT-23 package can be 230 C/W. For one watt dissipation, it will heat up to be 230C above room temp and so is practically not possible to deliver that much charge current.
If delivering less than 200 mA can’t fulfill a designer’s need, he may go for a thermally enhanced package, such as a 2mm x 3mm DFN package where typical thermal resistance value is about 76 C/W.
It is feasible for such a package to handle the current, but why does it still sometimes fails to deliver that much current?
The answer is the enclosed environment. So the PCB layout and thermal design of the product becomes even more important. Two tips to improve the thermal performance is to allow enough copper pour and apply copper to the exposed pad with vias under the DFN package. This will help to draw the heat away from package .