Adding a multi-cell battery system to single-cell designs

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.

Early tablet designs were often dominated by their predecessors: the smartphone. This meant that design teams continued to use a single-cell Lithium-Ion (Li-Ion) or Lithium-Ion Polymer (Li-Polymer) battery stack and simply added cells in parallel to achieve longer run times.

The drawback, also common to smartphone designs, was that backlight efficiency suffered. The tablet form factor also suffered due to the higher current draw of more white light-emitting diodes (WLEDs). This one-series multiple-parallel battery configuration requires a longer charge-time, thicker board traces, and higher-current connectors. Hence, a number of designers now are considering multi-cell stacks to improve backlight efficiency and reduce current levels. This creates another problem, however, how to use a smartphone design validated with components compatible with single-cell?

This article takes a different approach: using a front-end power management unit (PMU) that stands between the multi-cell battery stack of future systems with the single-cell-based designs of past systems. The PMU enables system designers to continue using their single-cell-based designs, while increasing efficiency of subsystems needing higher input voltage.

Improving efficiency
With the emergence of tablets, design teams must overcome the challenge of improving run-time with the burden of the larger screen, while minimizing time-to-market and keeping charge time reasonable.

Common smartphone systems use a single Li-Ion cell that operates at a voltage between 2.5 volts (V) and 4.35 V with 3.6 V to 3.7 V nominal. This battery needs to drive approximately eight WLEDs, depending on screen size, configured with two strings of four WLEDs. With a typical WLED forward voltage drop of 3.0 V to 4.0 V, the eight WLEDs require a forward voltage of 12 V to 16 V with additional voltage to allow for headroom drop across the current source or sink and other losses. This means boosting the Li-Ion battery voltage to meet the forward voltage drop of the WLED string.

A good rule of thumb is that efficiency drops as the ratio of

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