Adding a multi-cell battery system to single-cell designs: Page 2 of 5

March 22, 2013 //By Daniel Acevedo
Adding a multi-cell battery system to single-cell designs
Daniel Acevedo, applications engineer, Texas Instruments explains how a front-end power management unit enables system designers to continue using their single-cell-based designs, while increasing efficiency of subsystems needing higher input voltage.
output to input voltage increases in a boost converter and as more parallel WLED strings are needed. For a tablet, with a screen size two or three times that of a smartphone, the number of WLEDs increases respectively; typically at least six WLEDs in series are used and often with multiple strings in parallel. The efficiency then drops since the output voltage must now increase to 18 V to 24 V, plus the extra voltage to account current drive or sink voltage headroom, the number of strings increase, and the output to input voltage ratio increase. To overcome this toll on battery life due to the increased number of WLEDs, batteries often are added in parallel. This increases the current output capability and capacity, but does not improve efficiency.

The smartphone single-cell design has been through a number of revisions. It has been thoroughly tested and is in mass-market production. Preserving this known-good design reduces risk involved in redesign that normally would delay time-to-market. In a two or three cell Li-Ion stacked system, the voltage is nominally 7.2 V or 10.8 V. The higher voltage means a smaller difference between the battery stack output voltage and the voltage required to drive the forward voltage of the WLEDs, thus reducing losses in the boost converter.

This multi-cell battery stack configuration has trade-offs. Analogous to the way boosts are less efficient at high output-to-input voltage ratios, buck converters are less efficient at higher input-to-output voltage ratios and efficiency of converters connected to the processor, memory and IO may all suffer. Additionally, this higher voltage cannot be connected directly to most of the other circuitry in the system.

For example, the inter-integrated circuit (I 2C) bus used to communicate from integrated circuit (IC) to IC inside the cell phone typically operates at a maximum of 5 V. The single-cell power management unit likely has a 6 V input voltage limit The low

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