Applied Data Systems Bitsy Plus, Руководство пользователя

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Power and Power Management
ADS document # 110114-1001A Page 37
5.3.5 Supercapacitors (factory option)
Supercapacitors (sometimes known as "ultracapacitors") are energy storage devices that combine
the quick charge/discharge characteristics of capacitors with the higher energy density of batteries.
"Supercaps," as they are called, are useful for maintaining power when changing batteries or for
riding out power failures.
Supercaps are a factory option for the Bitsy Plus and must be requested when you place your
order. Supercaps are the two large cylinders mounted between the PCMCIA header and 50-pin
headers J9 and J10.
Charging
There are two important factors related to the charging phase of the capacitors. The first is how
long it takes to fully charge the capacitors. The second is the added load on the power supply it
takes to charge the capacitors. Both are important to effectively use the supercaps.
Charge time for capacitive circuits is typically measured in "time constants," the product of the
charging resistance, R and the capacitance, C. It takes three time constants (3RC) to charge fully-
discharged supercaps to 95% of their target voltage. For example, a system with 44 ohm charging
impedance and 1.65 F supercaps has a time constant of 73 seconds. Allow at least five minutes to
recharge the capacitors after the board has been disconnected from power. Charge time is shorter
if the supercaps aren't completely discharged.
The charging current for the supercaps starts out high and diminishes exponentially as the
capacitors reach full charge. Make sure to account for this current in your power budget (section
5.4.1). The charge current is calculated as
i(t) =
t/RC - i t
e
R
V - V
where
V
t
is the charging/final voltage of the capacitor (assume 5.0 V unless the capacitor
is not charged completely),
V
i
is the initial charge voltage of the capacitor, and
t is the time in seconds
Use the maximum current (time=0, V
i
=0, i(0)=i
max
=V
t
/R) in your power budget.
Discharging
When power fails on a system equipped with supercaps, the operating system shuts off all power
to the board and puts the system to sleep. When power is restored, the system remains asleep until
either the system is awakened (J3.45) or is reset (J10.45).
The amount of time that a system can remain asleep using only supercap power depends primarily
on how quickly external power drops off and how quickly the operating system can put the board
to sleep.
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 Bitsy Plus power fail circuit
(section 6.2.1). When possible, put the system to sleep with software if a power failure condition
is expected (e.g. changing a battery).