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Loads on Main Supply
The main power supply (DCIN_POS or VBATT_POS) is loaded by the 5 V and 3.3 V supplies as
indicated in the diagram of section 5.3.1. Assume 85% efficiency for external loads that cascade
through the 5 V supply. Consider these loads when creating your power budget.
5.3.2
Power Loads During Sleep
When designing systems for minimal power consumption during Sleep mode, make sure to
consider DC losses to external connections. The following are a few of the ways your system may
"leak" when asleep:
•
PCMCIA and CF cards
Cards in place when the system is asleep can drain power through the Card Detect and
Voltage Sense lines. Assume that all four lines ground the BitsyXb PCMCIA pull-ups
while the card is inserted.
•
Digital I/Os
Review digital I/O connections for potential voltage differences from external
connections when the BitsyXb is asleep.
•
USB
Depending on how USB devices are powered and how the operating system handles
USB, USB devices may draw power during Sleep.
5.4
Power Supply Examples
The following are basic examples of how to configure power supplies for the BitsyXb.
5.4.1
Basic DC Supply
The simplest way to power the BitsyXb is to supply DC power to the VBatt_Pos input, as shown
below.
VBatt_Pos
DC power
System ground
Ground
BATPOS
GND
+
This diagram also illustrates how to connect a coin cell to the RTC circuit for systems that require
the RTC to be maintained under all power conditions.
