4.63V, Active Low Precision Reset Generator Circuit# Technical Documentation: FM809LS3X Power-On Reset IC
## 1. Application Scenarios
### 1.1 Typical Use Cases
The FM809LS3X is a microprocessor (μP) supervisory circuit designed to monitor power supplies in digital systems. Its primary function is to assert a reset signal during power-up, power-down, and brownout conditions, ensuring controlled initialization of microprocessors, microcontrollers, DSPs, or ASICs.
Key operational scenarios include:
- Power-Up Sequencing : Provides a minimum 140ms (typical) reset pulse width when VCC rises above the threshold voltage, allowing the μP to stabilize before code execution.
- Brownout Protection : Monitors VCC continuously; asserts reset if supply voltage falls below the preset threshold (typically 4.63V for 5V systems).
- Manual Reset Control : Incorporates a manual reset input (MR) for system debugging, testing, or user-initiated resets.
- Battery-Powered Systems : Low quiescent current (typically 15μA) makes it suitable for portable and battery-operated equipment.
### 1.2 Industry Applications
- Industrial Control Systems : PLCs, motor controllers, and automation equipment requiring reliable startup sequencing.
- Consumer Electronics : Set-top boxes, routers, smart home devices, and gaming consoles.
- Telecommunications : Network switches, base stations, and communication infrastructure.
- Automotive Electronics : Non-safety-critical modules like infotainment systems (operating within specified temperature ranges).
- Medical Devices : Patient monitoring equipment and diagnostic instruments (where stable initialization is critical).
### 1.3 Practical Advantages and Limitations
Advantages:
- High Accuracy : Reset threshold accuracy of ±2.5% ensures reliable operation across temperature variations.
- Low Power Consumption : 15μA typical supply current extends battery life in portable applications.
- Small Form Factor : Available in SOT-23-5 package (2.9mm × 1.6mm) for space-constrained designs.
- No External Components : Requires no external timing capacitors for reset delay generation.
- Wide Temperature Range : Operates from -40°C to +85°C (industrial grade).
Limitations:
- Fixed Threshold Options : Available in standard threshold voltages (e.g., 4.63V, 4.38V, 3.08V, 2.93V); custom thresholds require alternative parts.
- Limited Output Configuration : Push-pull RESET output only; open-drain versions require different variants.
- No Watchdog Timer : Basic reset functionality without additional monitoring features found in more advanced supervisors.
- Maximum VCC Rating : 6V absolute maximum; not suitable for higher voltage systems without regulation.
## 2. Design Considerations
### 2.1 Common Design Pitfalls and Solutions
Pitfall 1: Insufficient Reset Pulse Width
- Problem : Microprocessor fails to initialize properly if reset pulse is too short.
- Solution : FM809LS3X provides fixed 140ms minimum reset pulse width, adequate for most μPs. Verify processor datasheet reset timing requirements.
Pitfall 2: Reset Glitching During Power-Up
- Problem : Noisy power supply causes multiple reset assertions during startup.
- Solution : Implement proper power supply decoupling (see Section 2.3). The FM809LS3X includes hysteresis (typically 150mV) to prevent chatter.
Pitfall 3: Incorrect Threshold Selection
- Problem : Reset occurs too early or too late relative to processor's operational voltage range.
- Solution : Select appropriate threshold variant based on processor specifications:
- FM809LS3X-4.63: For 5V systems with 4.5V minimum operating voltage
- FM809
