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PT0324S-1 12/21/10
7
PT8A2803
500mA Li-ion/Polymer Battery Charger
Charge Current Limit
During the constant-current (CC) charging mode, the charging current is primarily determined by I
CHG
as calculated in previous
formula. However, the actual charge current is the CC mode could also be limited by other factors as described by below Figure 5.
When the input (VIN) and output (VBAT) voltage are too close to each other, the on-resistance of the internal pass element may
limit the amount of current that passes through it. For example, the solid curve describes a typical case in which the battery
voltage is 4.0V and the charge current (I
CHG
) is set at 350mA. When the input voltage is sufficiently higher than the battery
voltage but has not increased the die temperature over the thermal limit yet, the charging current is accurately regulated at 350mA.
Figure 5: PT8A2803 Charge Current Limits In CC Mode
When the input voltage is reduced (or the battery voltage is increases towards the input voltage), the charge current is limited by
the on-resistance of the pass element. Therefore, it is recommended to employ sufficiently high input voltage for applications that
require constant charging current over the entire charging period. But for applications that needs to minimize the heat dissipation,
a current-limiting adapter maybe applied to maintain constant charging current at whole charging phase.
In addition, if the input voltage increases, the charge current may also be reduced due to the thermal foldback function. The high
voltage drop across the pass element increases the power dissipation therein and thus causing the die temperature to increase
significantly.
Layout Guidance
The PT8A2803 employs thermally-enhanced DFN package, which have an exposed thermal pad at its bottom side. It is
recommended to connect as much copper as possible between the exposed pad and PCB to make it effective in dissipating the heat
away from the die. For applications requiring high charging current, the thermal impedance should be further reduced by
employing more layers of copper to connect with the exposed pad through thermal via.
Input Power Sources
PT8A2803 works with different types of AC/DC adapter or USB port (any type) with no special requirements. For PT8A2803, it
works with input voltage ranges from 4.3V to 5.5V for normal operation but the maximum input voltage is 7V.
