Versatile Usage of LM2931 Low Dropout Voltage Regulators
The LM2931 series of low dropout voltage regulators, typically employed for powering microcontroller systems, possesses an unexpected application — they can serve as low-frequency practically random noise generators. This random noise output finds utility in various scenarios within a system. It could be employed to manipulate an ADC (analog to digital converter) to generate random numbers intentionally or to cause a digital port to behave unpredictably based on the applied noise level. Clever programmers might discover additional inventive applications tailored to their specific needs.
Exploring Output Voltage and Noise Generation with LM2931 Series
The LM2931A-5.0 boasts an output voltage ranging between 5.19 V and 4.81 V, while the LM2931-5.0 specifies a range of 4.75 V to 5.25 V. Both ICs come with the ‘Z’ suffix, indicating their TO-92 package configuration. Additionally, both ICs yield an approximate 0.5 mVrms of output noise when observed across a 100 µF capacitor within the frequency span of 10 Hz to 100 kHz. When this noise is amplified over 200 times, it can yield a typical noise voltage of about 100 mV, adequate for inducing random bit toggling in a 10-bit ADC with a least significant bit (LSB) equivalent to 0.5 mV. These toggled bits can be utilized individually or collectively to construct larger random numbers.
Circuit Schematic and Power Supply Details
The circuit design features a power supply offering an output voltage in the range of 4.5 V to 5.5 V with a maximum output current of 80 to 100 mA. Diode D2 raises the output voltage of the LM2931Z-5.0 IC by approximately 0.6 to 0.7 V, while Diode D3 effectively eliminates the voltage lift introduced by D2. To safeguard the regulator from reverse voltage, Diode D1 is employed.
Noise Amplification and Output Signal Characteristics
The regulator IC’s output noise is directed through C7 and R2 and then amplified more than 200 times using T1 and T2. The output resistance of the amplifier is relatively low, enabling it to directly drive the input of ADCs. The gain of the transistorized amplifier can be adjusted primarily through resistor R3. Components D3, C5, and C6 are included to mitigate the load’s impact, which is typically a microcontroller system, on the noise encountered by the amplifier input.
Selecting Load Capacitance and Considerations with Alternative Regulators
Jumpers J1 and J2 play a role in choosing the minimum load capacitance between D3 and the regulator output, striking a balance between regulator stability and maximum noise output voltage. These jumpers are configured during the final testing phase. It’s worth noting that while the circuit can also operate with other voltage regulators like the 78L05, lower noise levels may necessitate significantly higher amplifier gains.