Even in the present day, many logic circuits still operate on 5 volts, making it tempting to power the circuit using a standard regulator from a rectangular 9-V battery. However, this approach has its drawbacks. The 9-V battery has relatively low capacity and a high price tag, even with the advancements in NiMH batteries which have increased capacities for penlight batteries. It would be more cost-effective if 5 volts could be obtained from a 6-volt source, for instance, which could be achieved with 4 regular cells or 5 NiMH cells. Additionally, using an ‘old-fashioned’ sealed lead-acid battery or two lithium cells would be suitable. With the LP2951, such a power supply can be easily implemented. The LP2951, a reliable component from National Semiconductor, has been utilized in numerous Elektor Electronics designs.
Versatile Voltage Regulator Options
This IC is capable of delivering a maximum current of 100 mA when the input voltage exceeds 5.4 V. Variants of this IC are available with 3.3 V and 3 V outputs, as well as an adjustable version. The design incorporates a battery indicator feature, ensuring not only an indication of the battery status but also protecting the battery from excessive discharge. If the IC detects too low an input voltage, the ERROR output signal goes low, triggering the regulator to shut down via IC2d. Manual restart is initiated with the RESET push button.
Battery Indicator and Discharge Protection
The battery voltage is divided using specific resistors and compared with the regulator IC’s reference voltage (1.23 V). To customize the indicator for different voltages, a simple adjustment involves changing the 100-k resistor. The comparator utilized here is the LP339, an energy-efficient version of the LM339. Although the LM339 can be used if available, the LP339 is a preferable choice due to its significantly lower current consumption (60 μA) and the ability to sink 30 mA at its output. In comparison, the LM339 consumes 14 times more current (less than 1 mA), making the LP339 the more efficient option.
Idle State and Output Current Considerations
In its idle state, the LP2951 consumes around 100 μA, with a slightly higher consumption depending on the output current to be delivered. This design, highlighted in the National Semiconductor application note (054011-1), emphasizes the importance of choosing an appropriate voltage regulator tailored to the specific requirements of the application.