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There are some ways to reduce power dissipation, please see it as below:
1. Use a Proper DC Input Source
A low voltage input reduces the power dissipation. In order to charge the single-cell Li+ battery, we need a well regulated 4.2V±1% or 4.1V±1% (depending on battery chemistry) output. The input voltage needs to be higher to cover the voltage drops between the battery positive terminal and the input DC source. Figure 1 shows these for a typical charger.
2. Optimize Charge Current and Power Dissipation
Figure 3 shows the circuit used for testing. It is a linear charger with a 500mA charge current and a 6 hours timer limit.
We can deliver a safe and reliable linear charger for a single-cell Li+ battery by optimizing the DC input source, the charge current, and the power dissipation with proper heatsinking method. Figure 4 shows an actual test result using the MAX1898 charger with a 4.2V, 900mA Li+ cell and a regulated 5V/1A MAX5021 AC adapter.
By Optoroute, please see more technical solutions from optoroute blog.