Applied Data Systems Bitsy Plus, User Manual, Page: 49 / 68

Power and Power Management
ADS document # 110114-1001A Page 39
The diode on the negative terminal of the battery allows the battery to power the Bitsy Plus at any
time, but prevents the battery from being charged by a DC voltage on the DCIN_Pos input. The
charge current is calculated as follows:
I
charge
=
R
V - 0.2) (2 - DCIN_Pos
batt
∗
where
Vbatt is the battery voltage (Vbatt_pos-Vbatt_neg), and
R is the charge resistor (section 6.2.4).
While standard systems include the charging circuit, the charge resistors must be selected for
effective charging of specific batteries to be used. Select a resistor value that will provide the
desired charge current but not exceed the power rating of the resistor. Since P=I
2
R,
I
charge, max
=
R / P
ch
where P
ch
is the maximum power the charge resistor can support. Consult the electrical
specifications in section 6.2.2 for the values populated on standard systems.
5.3.8 Power Supply Efficiency
The Bitsy Plus power supply achieves high efficiency through several means. First, it utilizes
high-efficiency switching regulators. These regulators use conventional step-down switchers
under operating load conditions, but are configured by the system for linear and "burst" mode
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operation during low-load conditions that occur during system sleep. Additionally, there is only
one level of cascaded regulation, reducing the losses that multiply through each stage.
5.4 Designing for Optimal Power Management
Designing a system for optimal power management requires careful attention to many details.
This section provides some guidelines and tips for best power management.
5.4.1 Create a Power Budget for Peripherals
Embedded system designers using the Bitsy Plus should have a clear understanding of how power
usage will be allocated in the system they design. Designers should create a power budget that
takes into account the types of devices that are expected to be used with the Bitsy Plus.
The following lists detail some of the typical external loads that can be placed the Bitsy Plus
power supplies. Baseline power consumption of the Bitsy Plus is listed in section 6.2.5.
3.3 V Loads
Typical external loads on the 3.3 V power supply include the following:
•
Display
•
Personality Board
•
CF and some PCMCIA cards
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"Burst mode" in this context is a registered trademark of Linear Technology Corporation