Adding a multi-cell battery system to single-cell designs: Page 3 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.
dropout (LDO) regulators are most efficient when the input voltage is equal to the output plus the dropout voltage. Efficiency drastically decreases, if connected directly to a multi-cell battery stack input. While overall system efficiency with the multi-series combination may be better, modifying each system component to tolerate the higher input voltage may require substantial redesign. This leads to another approach in shifting from a single-cell to a multi-cell battery stack.

Shifting to a multicell battery stack and maintaining validated design
A solution that allows for improved efficiency of the backlight with the upgrade to a multi-cell battery stack and the reuse of the validated single-cell design is to use a front-end PMU. An integrated device such as the TPS65090, which has a high input voltage range (for example, 6 V to 17 V), a charger with power-path management, three step-down converters, two always-on LDOs, seven load switches and an I 2C interface, can stand between the multi-cell battery and the single-cell power system. This approach allows for the shift to a multi-cell battery stack solution by adding the front-end PMU and using an LED driver optimized for the higher voltage IC.

The front-end PMU can connect directly to the battery stack with the power-path and PMU charger functionality. This protects the system and controls charging the multi-cell battery stack using an AC adapter. The power-path, or system voltage, connects to the WLED driver and the frontend PMU switching converters input and LDOs, with outputs of 5 V, 3.3 V and a converter with adjustable output from 1.0 V to 3.3 V. Since these voltages are typical for single-cell battery systems, and the switching converters are capable of delivering a single-cell battery-like 4A continuous current, these front-end PMU outputs drive the downstream ICs in the existing single-cell design. Moreover, the TPS65090 has seven load switches, capable of reducing power consumption of disabled peripheral ICs down to 1 uA.

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