Digi JACKRABBIT BL1800 - Manual (Page 84)

80 Jackrabbit (BL1800)
The linear voltage regulator is simply a fixed-voltage regulator with a ±5% voltage output
tolerance as the temperature changes. The regulator has a small heat sink, which increases
the maximum external input voltage. Higher external input voltages increase the voltage
dropped by the regulator. The Vcc coming out of the regulator is always 5 V.
The power necessarily dissipated by the regulator can be calculated if both the external
input voltage and the current drawn by the Jackrabbit board and daughterboards connected
to the Jackrabbit board are known. The current provided by the high-power output drivers
does not have to be included if a separate power supply is connected to K so that power
does not come from Vcc.
The linear regulator maintains its output voltage to within ±5% as long as the heat sink is
dissipating less than 2 W. The regulator will operate outside its specifications when the
heat sink is dissipating 2 W to 3.3 W. Thermal shutdown turns the regulator off above
3.3 W. Figure C-2 shows the power operating curves.
Figure C-2. 7805 Linear Regulator Power Operating Curve
The Jackrabbit operating at 14.74 MHz with no loading at the outputs typically consumes
105 mA when the programming cable is connected, and 95 mA when the programming
cable is not connected. This means that DCIN can safely be from 7.5 V to 25 V. An addi-
tional 50 mA is available for a daughterboard, but the voltage regulation would suffer
slightly.
5 10 15 20 25 30 35
350
300
250
200
150
100
50
Current (mA)
DCIN (V)
3.3 W __________
(DCIN – 5 V)
2 W __________
(DCIN – 5 V)
Regulated
within
specifications
Regulated
outside
specifications
Undervolt
age dropout area
Thermal
shutdown