User`s guide

6-2 Intel
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StrongARM
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SA-1110 Microprocessor Development Board User’s Guide
System Power Management
A second battery threshold circuit, a MAX812 chip set to 3.08 V is used to force the SA-1110
system into sleep mode. This signal drives the VDD_FAULT input on the SA-1110. When this
interrupt occurs there is no time for user warnings or a user-friendly shutdown. The system is
forced into sleep mode.
The state of charge on Li-ion cells may be measured with reasonable accuracy simply by
measuring the battery terminal voltage and temperature while imposing a consistent load. When
measuring the terminal voltage the software must ensure that the same resources are powered up to
provide a consistent load that provides a more accurate state of charge indicator. For example the
terminal voltage should not be measured with the radio on at one point and the radio off at the next
point. Software gas gauging may use a table lookup of the battery terminal voltage and battery
temperature to indicate charge condition in 10% increments. The table should be calibrated for one
Li-ion cell. A one time per system software calibration step may be required to obtain accurate
results with the UCB1300 analog to digital converters. Battery temperature should also be
measured and included in the charge state calculation. Battery temperature has a significant effect
on usable charge. Cold batteries have substantially reduced capacity and the remaining charge
varies with ambient temperature.
The charge state measurement is a rolling average over several minutes that eliminates noise and
short term voltage dips due to varying loads. When the battery terminal voltage approaches the
10% charge remaining point the user should be warned by a software implementation of the
approaching bottom of charge condition.
The batteries must not be allowed to run down below the 10% point. The bottom 10% of the battery
charge is needed to maintain the SDRAM memory contents during sleep. The 10% reserve battery
scheme eliminates the need for a backup battery. A super cap is used to preserve the SDRAM
contents for over 2 minutes when the main batteries are changed.
A built in semi-smart Li-ion battery charger chip a MAX846 controls by the SA-1110 software.
The battery terminal voltage and temperature should be monitored during charging to determine
end of charge and overcharge conditions. The presence of live input power to the charger may be
sensed via the UCB1300 AD0. The charger circuit is controlled via the SA-1110 Development
Board Control Register (BCR).
The MAX846 is calibrated for single cell Li-ion charging and, if left to itself, will correctly manage
the charge cycle. However, the MAX846 does not monitor battery temperature.
Battery temperature is monitored during battery charge cycles with the temperature-sensor
component (Analog Devices, TMP37), which is connected to the UCB1300 analog input AD2. For
more information on the battery temperature, see Section 6.1.2.
Warning: Battery over temperature is a serious and dangerous condition with Li-ion cells. Personal injury or
fire may result if the charge cycle is not terminated and if the batteries are not allowed to cool.
A software implementation should monitor and indicate the following conditions:
• Battery at high temperature, which is over 40 °C, should indicate the immediate termination of
the charge cycle.
• Battery over temperature, which is over 50 °C, should indicate the immediate termination of
the charge cycle and the sounding of an audio alarm and flashing LED indicators.