MC33064P-5G Undervoltage Sensing Circuit: Design and Application Guide
The reliable operation of electronic systems is critically dependent on a stable and adequate power supply. Voltage drops, whether from brownout conditions, excessive load, or a failing source, can cause microprocessors and other critical ICs to behave unpredictably, leading to system lock-ups, data corruption, or even hardware damage. To safeguard against these issues, an undervoltage protection (UVP) circuit is essential. The MC33064P-5G is a precision undervoltage sensing IC designed specifically for this vital supervisory role.
This article provides a comprehensive guide to the design and application of the MC33064P-5G, a robust and straightforward solution for monitoring power rail integrity.
Understanding the MC33064P-5G
The MC33064P-5G is a three-terminal undervoltage sensing circuit housed in a TO-92 package. Its primary function is to monitor a positive DC supply voltage (`VCC`) and provide a clean, controlled output signal when the voltage falls below a predefined threshold. The "-5G" suffix denotes a fixed 5.0V undervoltage detection threshold, making it ideal for standard 5V logic systems. Its key features include high accuracy, low operating current, and a built-in hysteresis function to prevent output oscillation as the supply voltage hovers near the trip point.
Internal Architecture and Key Specifications
The IC comprises a precision bandgap reference circuit, which provides a stable voltage benchmark that is independent of temperature and supply variations. This reference is compared to a scaled-down version of the `VCC` input. The output of this comparator drives an open-collector output transistor.
Undervoltage Threshold: 5.0V (typical)
Hysteresis: 80mV (typical) – This ensures a clean transition between output states.
Operating Current: 170µA (typical) – Minimizes the power burden on the system it is protecting.
Output Type: Open-collector – Provides flexibility to interface with various logic levels by using a pull-up resistor to the desired logic voltage rail.
Basic Circuit Design and Operation
The application circuit for the MC33064P-5G is remarkably simple, requiring only two external components: a pull-up resistor on the output and a small bypass capacitor on the `VCC` pin.
1. Normal Operation: When the `VCC` supply voltage is above the 5.0V threshold (plus the hysteresis band), the internal comparator switches the output transistor OFF. The Output pin is therefore pulled high to the voltage of the pull-up resistor (`V_PULLUP`), signaling a "power good" condition.
2. Undervoltage Condition: When `VCC` drops below the 5.0V threshold, the comparator switches the output transistor ON. This pulls the Output pin to ground (low), providing a clear reset signal or warning to the downstream microcontroller or logic system.
3. Hysteresis in Action: Once tripped, the `VCC` must rise back to approximately 5.08V (5.0V + 80mV hysteresis) before the output transistor turns off again. This prevents rapid toggling of the output if the supply voltage is noisy or slowly recovering.
Key Application Circuits
1. Microprocessor Reset Controller: The most common application is generating a reliable reset signal for a µP or µC. The output of the MC33064P-5G can be connected directly to the active-low reset (`/RESET`) pin of the processor. During power-up, it holds the processor in reset until `VCC` stabilizes above the safe operating level. During operation, it will assert reset immediately if a brownout occurs, ensuring a controlled shutdown and reboot.
2. Power Supply Supervisor in SMPS: In switch-mode power supplies, the MC33064P-5G can monitor the auxiliary bias supply. If this voltage sags, it can be used to disable the PWM controller IC, protecting the main power switches from operating in a suboptimal and potentially dangerous state.
3. Battery-Powered System Warning: In equipment powered by batteries, the IC can monitor the battery voltage. Once the battery voltage droops below the effective threshold for the 5V regulator's input, the output can activate a "low battery" LED warning indicator, giving the user time to save data or replace batteries.
Design Considerations for Robust Performance
Noise Immunity: Always include a 0.1µF ceramic bypass capacitor between the `VCC` and GND pins, placed as close to the IC as possible. This decouples high-frequency noise on the supply line that could otherwise cause false triggering.
Pull-Up Resistor Selection: The value of the output pull-up resistor (typically 10kΩ to 100kΩ) is a trade-off between power consumption and switching speed. A lower value provides faster rise times but draws more current when the output is low.
Interfacing with Different Logic Levels: The open-collector output allows the signal to be pulled up to a voltage different from the monitored `VCC`. For example, it can monitor a 5V rail but provide an output signal to a 3.3V logic device by using a pull-up resistor to a 3.3V source.
The MC33064P-5G stands out as an exceptionally simple, cost-effective, and reliable solution for power supply monitoring. Its fixed threshold eliminates the need for external resistors, reducing component count and design complexity. For any 5V system where data integrity and operational reliability are paramount, integrating this undervoltage sensing circuit is a fundamental and highly recommended design practice. It provides a critical layer of protection that ensures system stability in an unpredictable power environment.
Keywords:
Undervoltage Protection
Power Supply Monitoring
Reset Circuit
Voltage Supervisor
Hysteresis
