Digi JACKRABBIT BL1800 - Manual (Page 87)

User’s Manual 83
C.2.1 Battery Backup Circuit
Figure C-5 shows the Jackrabbit battery backup circuitry.
Figure C-5. Jackrabbit Battery Backup Circuit
Resistor R12, shown in Figure C-5, is typically not stuffed on the Jackrabbit board.
VRAM and Vcc are equal when power is supplied to the Jackrabbit. R13 prevents any cat-
astrophic failure of Q1 from allowing unlimited current to enter the soldered-in battery.
Resistors R14 and R15 make up a voltage divider between the battery voltage and the tem-
perature-compensation voltage at the anode of diode D80. This voltage divider biases the
base of Q1 to about 2.6 V. V
BE
on Q1 is about 0.55 V. Therefore, VRAM is about 2.05 V.
These voltages vary with temperature. VRAM varies the least because temperature-com-
pensation diodes D80–D82 will offset the variation with temperature of Q1 V
BE
. R80–
R82 may be stuffed instead of the corresponding D80–D82 to provide the optimum tem-
perature compensation.
Resistor R10 provides a minimum load to the regulator circuit.
MMBT5088LT1
Q1
R12
0 W
R15
R10
4.3 MW
3 V
VBAT
R13
1.3 kW
R14
D80
D81
D82
R80
0 W
R81
0 W
R82
0 W
VRAM