Digi BL1800 Battery Backup Circuit, Figure C-5shows the Jackrabbit battery backup circuitry

C.2.1 Battery Backup Circuit

Figure C-5shows the Jackrabbit battery backup circuitry.

		R12	VRAM
		0 W
		MMBT5088LT1
		Q1
	VBAT		R10
			4.3 MW
	R13	R14
3 V
	1.3 kW	R15
		D80	R80
			0 W
		D81	R81
			0 W
		D82	R82
			0 W

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. VBE 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 VBE. 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.

User’s Manual 83