October 9, 1997

Simple circuit "feeds" CMOS devices

Luis H Loss, Iochpe-Maxion, Porto Alegre, RS, Brazil

To retain the data of CMOS devices, such as real-time clocks or setup RAM, you need to provide power to these devices while the rest of the circuit is off. The conventional solution uses a CMOS switch, such as the CD4066 or equivalent, to deliver 5V to the positive terminal and simultaneously keep the device's RD, WR, and CS pins at logic one (inactive) to guarantee data integrity and low current consumption (Figure 1).

An alternative circuit does away with the CMOS switch by floating the V_SS pin instead of V_DD using a transistor, a resistor, a diode, and the battery itself (Figure 2a). The RD, WR, and CS lines follow the V_DD level. V_DD goes to 0V at power-off but is still a logic one with respect to V_SS (Figure 2b). Changing the value of the resistor adjusts the 3.6V NiCd battery's charging current.

The application created for this circuit keeps data in 128k bytes of RAM valid for a year, turning on and off every day and never losing data. (DI #2099)

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