LM809M3-3.08/NOPB

The LM809M3-3.08/NOPB from Texas Instruments is a 3-pin microprocessor supervisory circuit that monitors a single power supply voltage and generates a reset signal when the voltage falls below the factory-programmed threshold. The 3.08 V threshold version is specifically designed for monitoring 3.3 V power supply rails, which are the most common voltage in modern digital systems.

The LM809 is one of the simplest and most cost-effective voltage supervisors available. With only 3 pins (VCC, GND, RESET), it requires no external components beyond decoupling capacitors. This simplicity makes it ideal for adding brownout detection and power-on reset functionality to microcontroller-based systems that might otherwise rely on simple RC reset circuits, which cannot provide accurate threshold detection.

The 3.08 V threshold is targeted at 3.3 V systems. When the 3.3 V supply drops below 3.08 V (approximately 6.7 percent below nominal), the RESET output asserts LOW, placing the microcontroller in a known reset state. This prevents the microcontroller from executing instructions with insufficient supply voltage, which could cause data corruption, peripheral misconfiguration, or unintended I/O states. The 240 ms timeout ensures the supply has fully stabilized before releasing the microcontroller from reset.

The push-pull (totem-pole) RESET output is a key feature that differentiates the LM809 from open-drain supervisors. A push-pull output can both source and sink current, actively driving the RESET line HIGH when the supply is good and LOW during a fault condition. This provides faster and more reliable reset transitions compared to open-drain outputs that rely on an external pull-up resistor. However, the push-pull output cannot be wire-ORed with other reset sources; if multiple reset sources must be combined, use an open-drain supervisor instead.

The LM809 is designated as a superior upgrade to the Maxim MAX809. Both devices have the same pinout and function, but the LM809 offers several improvements: wider operating voltage range (1.0 V to 5.5 V vs 1.0 V to 5.5 V for MAX809), wider temperature range (-40 to 105 degrees C vs -40 to 85 degrees C for some MAX809 variants), and a guaranteed RESET valid at VCC as low as 1.0 V, ensuring the microcontroller is held in reset even during a severe brownout.

The 15 uA typical supply current makes the LM809 suitable for battery-powered applications where quiescent current must be minimized. In a 3.3 V system, the 15 uA supply current translates to only 49.5 uW of power consumption, which is negligible compared to the microcontroller and other system components.

The power supply transient immunity feature ensures the LM809 does not generate false reset pulses due to brief voltage dips or spikes on the supply rail. The internal filtering rejects transients shorter than the minimum timeout period, preventing nuisance resets from switching noise or load transients. This is particularly important in systems with switching power supplies or motors that generate significant supply noise.

The available threshold options cover the most common supply voltages: 2.63 V for 3.0 V systems, 2.93 V for 3.0-3.3 V systems, 3.08 V for 3.3 V systems, 4.38 V for 5.0 V systems (with 12 percent margin), and 4.63 V for 5.0 V systems (with 7.4 percent margin). The 3.08 V and 4.63 V thresholds provide approximately 6-7 percent margin below the nominal voltage, which is the standard brownout detection threshold for most digital ICs.

At $0.26 per unit in volume (1000+), the LM809M3-3.08/NOPB costs less than a cup of coffee, yet provides critical protection against one of the most common causes of microcontroller malfunction: supply voltage dips. The cost-to-benefit ratio makes it a must-have component in any 3.3 V microcontroller design.

The LM809M3-3.08/NOPB operates as a precision voltage comparator with a built-in voltage reference, time delay generator, and output driver.

Voltage Monitoring: The LM809 continuously monitors the VCC supply voltage using an internal precision voltage reference and a resistor divider network. The internal reference is trimmed at the factory to set the reset threshold at 3.08 V (with a tolerance of plus or minus 5 percent, resulting in a threshold range of 3.04 V to 3.11 V over the full temperature range). The VCC voltage is compared to this reference by a high-gain comparator.

Reset Assertion: When the VCC voltage falls below the reset threshold (3.04 V minimum over temperature), the comparator output changes state and the RESET output is driven LOW by the push-pull output stage. The RESET output remains LOW as long as VCC is below the threshold. The push-pull output can sink at least 1.2 mA while pulling the RESET line LOW, which is sufficient to drive the RESET input of any standard CMOS microcontroller.

Reset Timeout: When VCC rises above the reset threshold, the LM809 does not immediately release the RESET output. Instead, an internal timer generates a reset timeout period of 240 ms typical (140 ms minimum). During this timeout, the RESET output remains LOW, ensuring the microcontroller stays in reset while the power supply stabilizes. The timeout period is generated by an internal oscillator and counter, requiring no external timing components. The 240 ms timeout covers the typical power supply settling time after a brownout or power-up event.

RESET Output at Low VCC: A critical feature of the LM809 is that the RESET output is guaranteed to be valid (LOW) for VCC at or above 1.0 V. This is important because during a power-down event, the VCC voltage drops through the threshold and continues falling toward 0 V. Without this feature, the RESET output might become undefined at low VCC voltages, allowing the microcontroller to execute random instructions. The LM809 ensures the RESET line is held LOW even when VCC is as low as 1.0 V, providing reliable reset coverage throughout the entire power-down sequence.

Push-Pull Output Stage: The push-pull (totem-pole) output stage consists of a P-channel MOSFET (pull-up) and an N-channel MOSFET (pull-down). When the output is HIGH (VCC above threshold after timeout), the P-channel MOSFET is on, actively driving the RESET line toward VCC. When the output is LOW (VCC below threshold or during timeout), the N-channel MOSFET is on, actively pulling the RESET line toward GND. This active drive in both directions provides faster edges than an open-drain output with a pull-up resistor. The push-pull output can source at least 0.4 mA and sink at least 1.2 mA.

Power Supply Transient Immunity: The LM809 includes internal filtering that rejects short-duration voltage transients on the VCC supply. A voltage dip shorter than the minimum timeout period (140 ms) will not cause a reset assertion, provided the dip does not fall below approximately 1.0 V. This prevents false resets from switching noise, load transients, or brief supply glitches. For very slow voltage declines (brownout), the LM809 will assert RESET as soon as VCC crosses the threshold.

Low Quiescent Current: The 15 uA typical supply current is achieved by using a low-power internal reference and comparator design. The LM809 draws this current continuously while monitoring the supply voltage, regardless of the RESET output state. In battery-powered applications, the 15 uA current represents a very small fraction of the total system current budget.