3.3V lithium-cell supply requires one inductor

-August 05, 1999

Because of the growing popularity of lithium-ion (Li-ion) batteries and 3.3V power supplies, portable-equipment designers must often create a 3.3V supply that a single Li-ion cell can power. The fact that the output of a Li-ion battery ranges above and below 3.3V during its discharge cycle complicates the design.

This situation calls for a buck/boost converter, which can perform both step-up and step-down conversions. This requirement is not new; for years, portable-equipment designers have faced the similar problem of deriving 5V from the output of four NiCd cells.

Using a flyback converter is tempting, but the size and expense of a transformer and the extra noise that this converter type creates prompt the search for an alternative. For example, the single-ended primary-inductance converter (SEPIC) is quieter, but this converter's buck/boost circuit has a maximum limited efficiency of 85%. This converter also requires either a transformer or two inductors in place of the single inductor that most dc/dc converters require.

You might overlook an alternative approach because the converter uses a linear regulator and takes an efficiency hit when you fully charge the Li-ion battery to about 4.2V (Figure 1). Nevertheless, this approach offers a longer battery life than the other two buck/boost circuits . For a large portion of the Li-ion battery's discharge cycle, battery voltage is within a range that allows the converter to exhibit excellent efficiency.

The operation of the circuit is straightforward. When the input voltage is above 3.3V, the IC stops switching. A linear regulator comprising Q1, R1, R2, R3, and an op amp internal to the IC step down the input voltage to 3.3V. When the input is below 3.3V, the IC operates as a step-up switching regulator and boosts the output to 3.3V. For this condition, the MOSFET is fully on, offering a virtual short from drain to source.

Efficiency is a function of the input voltage and the output current. As expected, the efficiency is a minimum of approximately 78.5% for a peak battery voltage of 4.2V. However, with a 3.6V input and an output current of less than 500 mA, the efficiency is above 89%. This behavior is significant because the output of a Li-ion cell is nearly 3.6V for most of its discharge cycle. For inputs of 3.3 to 3.6V and the same output-current conditions, the efficiency is even higher. Efficiency is also in the same range when the IC operates as a step-up switching converter, which it does for battery voltages below 3.3V. The efficiency gradually decreases as the output current exceeds 500 mA.(DI #2390)

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