BitsyXb - User Manual
48 110118-0001A
If the operating system can shut down the system before the power supply drops below the
supercap charge level, you will get significantly longer sleep time from your supercaps. You can
calculate how much time the system will have to go to sleep based on (1) how quickly your power
supply will fall when main power fails and (2) the trip point of the BitsyXb power fail circuit
(section 6.3.1). When possible, put the system to sleep with software if a power failure condition
is expected (e.g. changing a battery).
The supercaps discharge linearly from the constant-current drain during sleep according to the
equation
t =
Eff * C * 0.2)
- V - (V
fi
sleep
I
where
t is the duration the system can sleep, in seconds;
V
i
is the initial charge voltage of the capacitor;
V
f
is the supercap voltage at which onboard systems will fail (assume 3.2 V);
C is the capacitance of the supercaps, in Farads;
Eff is the efficiency of power delivery; 85-90% is a safe value to use;
I
sleep
is the amount of current the system consumes while in sleep mode.
For example, a system with fully charged 1.65 F supercaps and a sleep current of 4 mA can expect
to run for up to ten minutes after a power failure. Consult the electrical specifications of section
6.3.4 and power consumption specifications of 6.3.5 for the values to use in your calculations.
5.2.6 Backlight Power
The BitsyXb provides software control of Backlight Intensity and On/Off using the ADSmartIO
controller. Power for the backlight is not routed through the board and must be supplied to the
backlight separately from the BitsyXb. This provides greater flexibility when selecting backlight
inverters for an application. See section 4.6.6 for further details about backlight control.
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