HEF4528BT Dual Monostable Multivibrator: Datasheet, Circuit Design, and Application Notes
The HEF4528BT is a versatile and widely-used integrated circuit from the 4000 series CMOS logic family. It contains two independent, retriggerable monostable multivibrators (one-shots) in a single 16-pin package. This device is engineered to produce a precise output pulse of a predetermined duration in response to an input trigger signal. Its ability to generate clean, stable pulses makes it a fundamental component in applications like timing control, pulse shaping, and noise elimination.
Datasheet Overview and Key Specifications
Understanding the HEF4528BT begins with its core electrical characteristics and pin configuration. The device operates over a wide voltage range, typically from 3V to 15V, making it suitable for both TTL-level and higher voltage interfaces. Key pins for each monostable include:
TR+ (Positive Trigger Input): A low-to-high transition on this pin triggers the output pulse.
TR- (Negative Trigger Input): A high-to-low transition on this pin triggers the output pulse.
Q and /Q (Outputs): Complementary outputs; Q goes high for the pulse duration, while /Q goes low.
Rx, Cx (External Timing Resistor and Capacitor Pins): These pins connect external components (Rext and Cext) that directly determine the output pulse width.
CD (Clear Direct Input): An active-low signal on this pin immediately terminates the output pulse, overriding the timing cycle.
The output pulse width (tw) is calculated using the approximate formula: tw ≈ Rext Cext. For accurate timing, it is crucial to use a stable, low-leakage capacitor and a resistor value within the datasheet's recommended range (typically 5kΩ to 1MΩ).
Circuit Design and Configuration
Designing with the HEF4528BT is straightforward. The most common configuration is the non-retriggerable mode, where a new trigger during an active output pulse is ignored. To implement this:
1. Connect the timing resistor (Rext) between the VDD and Rx pins.
2. Connect the timing capacitor (Cext) between the Cx and Rx pins.
3. Choose either the TR+ or TR- pin as the active trigger input, ensuring the unused trigger pin is tied to the appropriate logic level (e.g., ground TR+ if using TR-).
4. Connect the CD pin to VDD to disable the reset function if not used.
For retriggerable operation, a new trigger signal during an active output pulse will restart the timing cycle from zero, effectively extending the pulse. This is achieved by applying the trigger signal to the active input while the pulse is high.
Critical Application Notes
1. Power Supply Decoupling: Always use a 0.1µF decoupling capacitor close to the VDD and VSS pins to suppress noise and ensure stable operation.
2. Triggering: The trigger input pulse must be shorter than the desired output pulse. For noisy environments, using the unused trigger pin (e.g., tying TR- low when using TR+) can improve noise immunity.
3. Timing Accuracy: The timing formula is an approximation. For highly precise applications, factors like internal component tolerances and voltage supply variations must be considered. The pulse width can be adjusted by varying Rext or Cext.
4. Unused Inputs: Never leave any input pin floating. All unused inputs (including the second monostable's inputs) must be tied to either VDD or VSS to prevent erratic behavior caused by static charge.
5. Output Load: The output current sourcing and sinking capabilities are limited. For driving heavier loads like LEDs or relays, use a buffer transistor.
Common Applications
The HEF4528BT is employed in numerous scenarios:
Pulse Width Modulation (PWM): Generating fixed-width pulses for control systems.
Debouncing Switches: Eliminating contact bounce from mechanical switches to produce a single, clean digital signal.
Time Delay Generation: Creating precise delays between circuit events.
Frequency Division: When configured in a cascaded mode.
In summary, the HEF4528BT is a highly reliable and flexible dual monostable multivibrator IC. Its simplicity, wide operating voltage, and predictable performance, governed by external RC networks, make it an indispensable tool for digital designers. Whether for basic pulse generation or complex timing sequences, it remains a cornerstone of timing circuit design.
Keywords: Monostable Multivibrator, Pulse Width, HEF4528BT Datasheet, Retriggerable, Timing Circuit
