There are a number of advantages to offering USB and high input voltage power and battery-charging capability in handheld devices such as GPS navigators, PDAs, digital still cameras, photoviewers and MP3 players.
For instance, the charging and operation from USB offers the obvious convenience of not requiring a travel adapter. High voltage sources, such as Firewire and 12 V to 24 V adapters are even better, since they provide faster charging than USB and allow charging in more places, such as in the car. Nevertheless, there is an important design consideration with high voltage power sources: the voltage difference between the high voltage source and the battery in the handheld device is very large. Since a linear charger cannot handle the power dissipation, a switching charger is required.
The LTC4089 and LTC4089-5 (Figure 1) conveniently integrates a high voltage and wide input range (6 V to 36 V, with 40 V absolute maximum) monolithic 1,2 A buck switching regulator and a USB power manager/charger into a compact thermally enhanced 3 x 6 mm DFN package.
The LTC4089's buck regulator output voltage tracks the battery voltage to within 300 mV. Linear Technology's 'Bat-Track' feature minimises overall power dissipation. The LTC4089-5 has a fixed 5 V at OUT when power is applied at HVIN. When power is supplied from the USB port, the power manager maximises the available power to the system load; up to the full USB available power of 2,5 W. It automatically adjusts the Li-Ion battery charge current with respect to the system load current to maintain the total input current compliance within the USB limits.
The total solution size is less than 2 cm² with all components on one side of the PCB.
Adaptive buck minimises total power loss
The LTC4089's buck converter output voltage VOUT tracks the battery voltage VBAT. It is always 0,3 V higher than VBAT, so that the battery can be charged quickly while minimising overall power dissipation. Figure 2 shows the overall efficiency at various input voltages, where the total power dissipation is less than 1,1 W. Furthermore, if the battery is excessively discharged and VBAT falls too low, the minimum VOUT is 3,6 V to ensure continuous system operation.
Power available to the system is maximised
In a traditional dual input device, the input charges the battery and the system's power is directly taken from the battery. This creates a number of problems. One of these is that the system's available power is reduced by the low-battery voltage when there is USB power present. For example, when VBAT = 3,3 V, the available power to the system is only 1,65 W while the USB itself supplies 2,5 W. The balance is dissipated as heat. The LTC4089 successfully solves this problem by providing an intermediate voltage VOUT to power the system load. This VOUT is independent of the battery voltage and equal to the USB voltage, thus the full USB power is available to the system load. Table 1 shows the advantages of the LTC4089 power manager over the traditional dual input configuration.
Summary
The LTC4089 integrates a high voltage wide input monolithic switching regulator, USB power manager and Li-Ion battery charger into a 3 x 6 mm DFN package and improves the functionality of USB-based and multiple power input portable devices. With all the necessary components on the same side of a PCB, the total solution size is less than 2 cm² (1,1 x 1,8 cm).
© Technews Publishing (Pty) Ltd | All Rights Reserved
printer friendly version