AN2370

Board layout considerations

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Board layout considerations

Because of the small size of the ThinSOT package, it is very important to use a good

thermal PC board layout to maximize the available charge current. The thermal path for the

heat generated by the IC is from the die to the copper lead frame, through the package

leads, (especially the ground lead) to the PC board copper. The PC board copper is the heat

sink. The footprint copper pads should be as wide as possible and expand out to larger

copper areas to spread and dissipate the heat to the surrounding ambient. Feed through

vias to inner or backside copper layers are also useful in improving the overall thermal

performance of the charger. Other heat sources on the board, not related to the charger,

must also be considered when designing a PC board layout because they will affect overall

temperature rise and the maximum charge current.

Table 1 lists thermal resistance for several different board sizes and copper areas.

Figure 3 shows the complete assembly board.

Table 1.

Measured thermal resistance (2-layer board)

Copper area

Board area

Junction-to-ambient

Thermal resistance

Topside

Downside

125 °C/W

125 °C/W

130 °C/W

135 °C/W

150 °C/W