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Add multi-cell battery system to single-cell designs

Posted: 01 Apr 2013 ?? ?Print Version ?Bookmark and Share

Keywords:single-cell? Lithium-Ion? Li-Polymer?

Early tablet designs were often heavily influenced 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 sub-systems needing higher input voltage.

Figure 1: Tablet design with multi-cell battery stack simplified block diagram.

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 minimising time-to-market and keeping charge time reasonable.

Common smartphone systems use a single Li-Ion cell that operates at a voltage between 2.5V (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 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.

Figure 2: Functional block diagram of a frontend PMU.

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.

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