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LTC3455 Datasheet(PDF) 11 Page - Linear Technology |
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LTC3455 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 28 page  LTC3455/LTC3455-1 11 3455fc APPLICATIONS INFORMATION Operation When No Battery Is Present As long as USB or wall power is available, the LTC3455/ LTC3455-1 will operate with no battery present, a crucial requirement for systems with a removable battery. Keep in mind, however, that if the LTC3455/LTC3455-1 are USB powered and the battery is not present, absence of the battery means that there is no reservoir if the system needs more power than the USB port can supply. This is espe- cially a problem when starting up the LTC3455/LTC3455-1 in USB low power mode with no battery present, which is discussed in greater detail on the next page, in the section entitled “Startup Issues in USB Low Power Mode When No Battery is Present”. Similarly, if external power is available, the LTC3455/LTC3455-1 will operate even if the battery is bad or in deep-discharge. The LTC3455/LTC3455-1 are also a good choice for systems that are always powered by a USB supply or wall adapter. The charger can then be used to charge a large capacitor or backup battery, which can briefly provide power to the system after the external power has been removed. This gives the microcontroller enough time to follow proper shutdown procedures even after the main power source is abruptly removed. If USB powered, the large capacitor or backup battery will also be used to provide additional current if the system briefly needs more power than the USB bus can provide. Concerns When Wall Adapter Powered Always choose a wall adapter that can provide power for all load and battery charging requirements. Choosing a wall adapter with a power rating that is too small will result in very long charge times and erratic system operation. If the total current needed (load and battery charging) exceeds what the adapter can provide, the voltage on the VMAX pin will begin to droop. If it droops close enough to the bat- tery voltage (the VBAT pin), the charge current decreases and eventually reduces to zero. If the load current is still too much for the wall adapter to provide, the wall adapter will provide what it can and the battery will provide the rest. When wall powered, this operation is intended only for surviving fault conditions and should not be a normal mode of operation. Concerns When USB Powered The popularity of USB (Universal Serial Bus) makes it an attractive choice for transferring data in a variety of portable devices. Therefore, utilizing the USB port to power these portable devices while charging their battery is very desirable, but it is not necessarily an easy task. As the performance of digital cameras, handheld computers, and MP3 players increases, the power needed to operate them also increases. The power available from a single USB port (maximum 2.5W) is barely enough to support the peak power needed by many full-featured portable devices, even without the power needed to quickly charge their batteries. To further complicate matters, a USB port is not the ideal power source. Each device can draw a maximum of 500mA (in high power mode), but the voltage provided to the por- table device can vary quite significantly. Although a USB power supply has a 5V nominal rating, when you include normal supply variations, cable losses, and transient conditions, the USB voltage showing up at the portable device is typically much lower—often falling to only 4V. Since the USB port has a strict current limit of 500mA, this means the amount of power available to the portable device can be as low as 2W. The reduced USB voltage also presents problems when trying to fully charge a single-cell Lithium-Ion battery when the USB voltage may itself be below or near the float voltage. The LTC3455/LTC3455-1 are specifically designed to al- leviate these problems and make the most of the power the USB port has to offer. See the sections entitled ”Large Transient Loads when USB powered” and ”Special Charger Features when USB powered” for more detailed discussions of the LTC3455/LTC3455-1’s special USB features. |
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Similar Description - LTC3455_15 |
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