Don’t Crash Your Bus!

With ever-improving lithium battery technologies supporting lower and lower discharge voltages, smartphone system designers must consider if their systems operate as reliably from such low input voltages. Whereas the first Lithium Polymer batteries were typically discharged down to 3V, now newer Si-anode batteries might go as low as 2.7V. In the future, newer battery chemistries are said to go down as low as 2.5V and still have significant energy left to extend a portable device’s run time. With this lower bus voltage comes the risk that downstream regulators must be redesigned to operate as well as they did with a higher bus voltage. This is especially true during high peak current transients, such as GSM and 4G transmission. Such transients put a significant strain on the battery, dropping its voltage across its internal impedance and IR drops in the system.

As an example of these regulators that must cope with a lower input bus voltage, consider the typical power supply block diagram for a PMIC in a smartphone. Previously, with a higher battery voltage, the battery would be directly connected to the step-down converters and linear regulators for the PMIC’s functions.  

But some of these linear regulators are making 2.95V and 2.6V from the battery voltage. It is impossible to keep this same architecture with a battery voltage that goes below the desired output voltage of a linear regulator. Even as the battery voltage gets close to the output voltage, the linear regulator may lose some of its noise rejection performance and give a lower quality output voltage. There is only one choice for the system designer, if he or she wants to keep this same architecture—increase the voltage applied to the linear regulator.

A boost converter could then be placed between the battery and these linear regulators. But this would actually decrease the battery run time since now a higher input voltage is applied to the linear regulators—this decreases their efficiency and wastes power. As well, the boost converter has its own losses which waste power. What is really desired is a boost converter when the battery voltage gets low and a pass through of the battery voltage when it is high enough. Hmm, what device could do this?

Enter the new TPS61280—a boost converter with bypass operation. With very low Rdson switches that enable high efficiency, it can be configured to either boost the battery voltage to a programmable level (optimized for any system) or to pass through the battery voltage with minimal voltage drop and power loss. Packaged in a tiny 1.7mm x 1.7mm chipscale package and occupying only 20 mm2 of board space, it is a simple and high performance solution to the power challenge posed by lower battery voltages.

How can a boost + bypass device enhance your system’s operation?

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