LTC4053

The USB is fast becoming the most popular way to attach peripherals to personal computers. An important feature of the USB standard is that it allows for power transmission over the USB cable, making it possible for a PDA or other handheld device to charge its battery while it is connected to a host PC.

The USB specification defines load current limits that USB peripherals can draw from the USB. Three operating modes are allowed:
• Suspend mode, where the USB peripheral draws less than 500mA from the USB cable
• Low power mode, where the current consumption can be up to 100mA
• High power mode, where the current is allowed to be up to 500mA

The USB specification of LTC4053 also states that USB peripherals are not allowed to backfeed into the USB cable. Figure 1 shows a minimum component solution to the USB battery charger problem. In this case, only five components including the LTC®4053, are required to achieve a fully compliant USB charger.

The charger is based on the LTC4053-a standalone timer-terminated constant current/constant voltage linear charger. The LTC4053 features a thermal loop which maximizes the charging rate under all conditions by regulating the maximum die temperature. Less sophisticated linear chargers must run lower charging rates or risk overheating, thus increasing charge times.

Operation of the LTC4053 is simple. The charge current is set by a resistor on the PROG pin, the termination timer period is set by a capacitor on the timer pin and float voltage is preset at 4.2V.