Nbatt_Fault; Block Diagram - Intel PXA27 Series Design Manual

Clocks and Power Interface
3.2.3.5

nBATT_FAULT

nBATT_FAULT indicates that the main battery is low or is being removed from the device,
conveying to the PXA27x processor that power will be depleted shortly. During this time, the
processor operates for a limited time from a Lithium/Lithium manganese "coin-cell" backup
battery, or from a "super cap" that supplies the processor only for a few cycles of full-run power.
In the event of nBATT_FAULT assertion, the PXA27x processor enters an emergency form of
sleep. In emergency sleep, the only handshaking is with external SDRAM memory (putting it into
self-refresh mode.) This communication between the PXA27x processor and the external SDRAM
memory ensures that memory contents are preserved, if possible. Obviously, the refresh currents
eventually depletes the cap or backup battery, but not as fast as the PXA27x processor in run mode.
Supporting these features must be understood at both the board level design and by the power
controller/regulator.
Note: The processor does not recognize a wake-up event while nBATT_FAULT is asserted.
3.3

Block Diagram

See the system schematic in
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. The
output voltage of U7 must ensure VCC_BATT remains:
•
Between 2.25 V and 3.75 V when VCC_IO is disabled
•
Within 200 mV of VCC_IO when VCC_IO is enabled
D1 protects regulator U7 from the back current when the backup battery drives VCC_BATT to a
higher potential than the output of U7. D3 and R2 limit over-charging current to the backup battery.
While the processor is powering up the VCC_REG domain from VCC_BATT, D2 and R1 prevent
the PXA27x processor from over-charging the backup battery. See
action from D2 and R1 occurs if an input signal on the VCC_REG domain is driven above the
backup battery voltage.
II:3-4
Figure 3-1 for illustration of one recommended configuration for
®
Intel PXA27x Processor Family Design Guide
Figure 3-1. This preventive