The Universal Serial Bus has become one of the most widespread and convenient ways to connect electronic devices to the PC. Countless modern portable products with built in USB connectors readily use the USB data bus to transmit and receive data to and from PCs, but many of these battery powered units still use a separate power supply for battery charging
The voltage drop between the bus supply, VBUS, and the charger circuit should be carefully considered in designing a USB battery charger. The circuit s transistor, Q1 (D45H8), and diode, D1 (MBRS130L), were chosen for their low-dropout properties so that the circuit could charge the battery even under low input voltage conditions. With these preferred components, the voltage drop between the LM3525 input and the battery cathode must be 530 mV (typ.) or greater for the circuit to deliver the full 400 mA charge current to the battery. For an optimal charge time, the maximum charge current should be delivered to the battery until the battery reaches its full-charge voltage. With a 4.2V Li-Ion battery, this would require the voltage at the input to this charging circuit to be above 4.7V (typ.). The USB specification sets the minimum output supply voltage at 4.75V, but then provides for resistive drops in the USB cable and connectors up to 350 mV. Thus, in the absolute worst case, the voltage at the input to the charge circuit could be as low as 4.4V and still be within USB specifications, limiting the effectiveness of the charging circuit.
When the input voltage of the circuit falls to 4.6V and the battery nears its fully charged voltage of 4.2V, there is not sufficient voltage across Q1 and D1 to deliver the full charge current. With only 400 mV across Q1 and D1, these components are squeezed so they cannot pass more than 200 mA to the battery. The available charge current has been reduced by 50% as a result of the low input voltage, extending the time it takes to top off the battery with constant-voltage charging.
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