Power Reduction Techniques: Minimizing 555 Current Drain

The Problem: High Current Draw of 555 Timers

The 555 timer integrated circuit is one of the most ubiquitous chips used by hobbyists and engineers alike for generating pulses, timing events, and creating oscillators. However its functionality comes at a cost – the 555 can draw over 3 mA when operating continuously resulting in high current consumption especially for battery powered devices.

With the proliferation of IoT sensors and portable electronics needing maximized battery life, the quiescent and operating supply current of the 555 becomes highly problematic. We examine multiple techniques hardware designers can employ to reduce the 555 IC’s current drain by orders of magnitude while maintaining required timing accuracy and oscillator functionality.

Key Causes of High 555 Current Consumption

Internal Transistor Junction Leakage

Inside the 555 timer are over 20 bipolar and MOS transistors configured as comparators, flip flops, and discharge switches. The base-emitter, base-collector, and gate junctions of these transistors exhibit leakage currents that can sum to over 500 nA in total. As supply voltage is applied, the bipolar transistors’ base-emitter junction leakage occurs continuously leading to constant power drain.

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Continuous Oscillation/Switching

In oscillator mode, the 555’s internal circuitry switches the output pin on and off regularly to generate a clock signal. This rapid switching combined with capacitive loading can draw over 3 mA per the datasheet. The device is never idle and runs continuously with a strong dependence on supply current frequency.

Techniques to Minimize 555 Current

Use Low Power Variants (e.g. TLC551 Timer)

Instead of the standard NE555, consider its low power counterparts such as the TLC551 which feature vastly improved quiescent current specifications. The TLC551 enhances internal circuitry to reduce transistor leakage, utilizing CMOS transistors in the comparator and improved manufacturing processes. This enables a typical supply current under 1 uA – a great starting point for low power applications.

Enable Output Disconnect in Timers

Certain 555 variants contain output disconnect functionality where the output driver stage can be disabled internally, preventing any current draw through the output pin. This reduces power dissipated while oscillating but maintains supply current draw through internal leakage. Check device datasheets for availability and operating specifics of this feature.

Add External Sleep Circuitry

To selectively disable the 555 timer and halt all supply current draw, additional control circuitry can cut off voltage to the device dynamically. This puts the 555 into a high impedance sleep mode, eliminating any transistor leakage or switching current while timing operations are not needed.

Example Schematic with BJT Sleep Transistor

A PNP bipolar junction transistor (BJT) can serve as the enabling switch for the 555 timer, cutting off the collector from receiving base drive current to turn off the timer IC. A microcontroller or external logic gate activates the BJT to then re-enable the 555 device only when required. Typical sleep mode performance sees < 10 nA current draw using this technique.

Reduce Supply Voltage

Since power consumption scales directly with voltage from P=IV, reducing the 555 timer’s supply voltage from 5V to 3.3V or lower decreases its internal current proportionally. Ensure voltage remains high enough for correct timing functionality. This technique also lightens drain on the external power source e.g. coin cell battery.

Lower Frequency of Operation

When configured as a relaxation oscillator, the 555’s current consumption rises linearly with increasing output frequency due to higher switching rates internally. By designing the RC timing components for lower frequency targets e.g. 1 Hz, less capacitive charging/discharging cycles occur cutting supply current over 25x from the kHz range down to 1 mA or below.

Use External Gates Instead of Built-in Oscillator

Rather than leverage the 555’s internal flip flops and analog comparators for waveform generation, use external CMOS logic gates in an oscillator configuration which consumes mere nanoamps. This disables unused internal circuits in the 555 dramatically cutting quiescent and active current system-wide, at the expense of increased part count and design complexity.

Achieving Under 1uA Sleep Current

Case Study with CMOS 555 Variant

As a case study, the ICM7555 CMOS low power 555 timer can achieve under 1 A total supply current draw when enabled into shutdown mode. By tying the shutdown pin low, all internal bipolar transistors are turned off and the timing comparator disconnects power draw through the collector. Leakage settles around 300-800 nA during shutdown resulting in maximal power savings compared to the militia 555’s baseline current consumption.

Conclusion and Recommendations

With a host of current reducing techniques now understood, hardware designers can best leverage the venerable 555 timer IC in low power applications by:

Selecting micropower variants with < 1A quiescent current
Adding external circuitry to selectively disable the 555 device between timing operations
Operating the 555 at lower voltages and oscillation frequencies
Utilizing external CMOS logic gates for timing functions rather than the 555’s internal oscillators

Combining these methods allows radically cutting 555 timer current down to the microamp and even nanoamp region from an initial 3 mA baseline. This enables the continuous use of the 555 for timing and oscillations even in stringent low power applications.