Personal Computer User Manual

Intel® PXA27x Processor Family Power Requirements
Application Note 15
The system schematic in Figure 2 shows one recommended configuration for connecting the
PXA27x processor directly to the backup battery. In such a configuration, the regulated main
battery powers VCC_BATT through regulator U7, and the backup battery powers VCC_BATT
when the main battery discharges. Regulator U7 also charges the backup battery and its output
voltage must be chosen to ensure that VCC_BATT remains between 2.25 V and 3.75 V when
VCC_IO is disabled and within 200 mV of VCC_IO when VCC_IO is enabled. D1 protects
regulator U7 from back current when the backup battery drives VCC_BATT to a higher potential
than the output of U7. D3 and R2 are chosen to limit intentional charging current to the backup
battery. D2 and R1 prevent the PXA27x processor from driving unintended charging current into
the backup battery if an input signal on the VCC_REG domain is driven above the backup battery
voltage while the processor is powering the VCC_REG domain from VCC_BATT.
Signals from the PMIC to the processor on the VCC_REG domain must be powered from the
VCC_BATT supply voltage when SYS_EN de-asserts in deep-sleep mode. Doing so prevents
forward-biasing the PXA27x processor input protection diodes.
2.2.3 Battery Chargers and Main Power
The PMIC includes as an option a way of charging the main battery when the system is plugged
into an AC power outlet or through the USB port. An external power brick is often used to convert
the main voltage (90 VAC to 240 VAC) to a low DC voltage suitable for powering the regulators
and charging the batteries.
The PMIC must have an input (voltage detect) that can sense when AC power is supplied to the
system to manage main power. An output from the PMIC must make this information available to a
PXA27x processor GPIO at a suitable voltage (normally, 3.3 V CMOS logic levels). For GPIO<0>
or GPIO<1> to generate deep sleep wake-up events, the PMIC must make the input (voltage
detect) information available as an output to one of these GPIO signals.
Table 6. Possible Backup Battery Configurations
Backup Battery
Connection
Description
VCC_BATT and PMIC The backup battery connects to both the VCC_BATT input and PMIC charging
regulator (driven from the main battery or AC adaptor supply). Powering VCC_BATT
from a battery directly eliminates the inefficiency of an external regulator in the
PMIC, maximizing the battery life in sleep and deep sleep. In such a configuration,
ensure that the requirements for limiting current to the backup battery are observed,
regardless of whether it is a rechargeable or non-rechargeable type. Information on
battery current limits is available from the battery manufacturer. Series resistors and
diodes might be needed to limit intentional charging current, to prevent the backup
battery from being drained by a discharged main battery, and to prevent
unintentional backup battery charging by the PXA27x processor
. These components
may be internal or external to the PMIC.
PMIC only There is more flexibility in the number of cells and allowable charging voltage when
the backup battery is connected only to the PMIC and the PMIC drives VCC_BATT.
In this configuration, the PMIC must ensure that requirements for limiting current into
the backup battery are observed, regardless of whether it is a rechargeable or non-
rechargeable type.