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5.2.7
Battery Trickle Charger (volume production option)
The BitsyXb includes a trickle charger for basic charging of external batteries. The following
diagram illustrates the charging circuit and a standard means of combining it with DC line power.
BitsyXb
VBatt_Pos
Ground
VBatt_Neg
DCIn_Pos
DC "Line"
power
Charge
Ground
The Charge signal comes from the ADSmartIO controller (port PB0) and is also available on J3
pin 22 for external use. Writing a logic "1" to PB0 turns on the trickle charging. It is up to the
application to manage battery charging.
The diode on the negative terminal of the battery allows the battery to power the BitsyXb 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 6.3.4).
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 6.3.4 for the values populated on standard systems.
