Jameco Electronics 3000, 2000 manual Battery-BackedClock Current Consumption

16.9 Battery-Backed Clock Current Consumption

When using the suggested tiny logic oscillator, the oscillator and clock consume current as shown in Figure 16-12below. Normally a resistor is placed in the battery circuit to limit the current to about 3 µA, which results in a voltage setpoint of about 1.7 V. When operat- ing at 3.3 V in sleepy mode, the current of the oscillator and the real-time clock—about 12 µA—must be added.

Using the suggested tiny logic oscillator circuit, the external 32.768 kHz oscillator con- sumes the following current in µA, where V is the operating voltage.

Iosc (µA) = 0.35 × V2 + 0.31 × V

Generally the oscillator will not start unless the voltage is about 1.4 V. However, the oscil- lator will continue to run until the voltage drops to about 0.8 V. If the oscillator stops, the current draw is very much lower than when it is running. Below about 1.4 V most of the current draw is used to charge and discharge the capacitive load.

The current consumed by the battery-backed portion of the Rabbit 3000, which is driven by the 32.768 kHz oscillator, is given by

Irab (µA) = 0.91 × V2 - 1.04 × V (V > 1.14 V)

The current is negligible for V < 1.14 V.

Total Battery Backed

Rabbit 3000 Real-Time Clock

Tiny Logic 32 kHz Osc

	12.00
	10.00
(µA)	8.00
Current	6.00
	4.00
	2.00
	0.00
	1	.2	.4	.6	.8	2	.2	.4	.6	.8	3	.2
		1	1	1	1		2	2	2	2		3

Battery-Backup Voltage (V)

Figure 16-12. Current Consumption—Real-Time Clock and 32 kHz Oscillator Circuit

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