Ultra-Low Power: Microcontroller Standby Current to 130uA with OPEnS Power Control Board

At the OPEnS Lab, a key focus for our projects is achieving exceptionally long battery life. This was particularly true for the OPEnSampler, a device that presented a unique challenge due to its need for simultaneous operation at three different voltage levels (3.3V, 5V, and 12V). While existing circuits using MOSFETs could manipulate power sources and significantly reduce standby current, designing a solution that latched its state in both directions (on and off) and powered itself solely from the source battery proved to be a hurdle.

This blog post delves into the features and key considerations surrounding Release 1.0.0 of the OPEnS Power Control Board, a solution specifically developed to address this challenge.

The schematic

Mitch Nelke designed the circuit, which contains two essential integrated circuits. IC1 acts as the flip flop IC, responsible for latching its EN-bar output to either HIGH or LOW states following a HIGH pulse on its Clock Pulse input or a LOW pulse on its C-bar input, respectively. IC2 functions as the 12v-5v switching regulator, reducing the battery’s 12v supply to 5v for powering the microcontroller and other elements.

The Flip Flop

While the OPEnS Power Control Board boasts impressive low-power capabilities, understanding its magic requires a peek under the hood. This section dives into the details of how the board itself is powered and controlled.

Fueling the Flip-Flop:

The brain of the board, the flip-flop circuit, gets its power directly from the battery’s raw 12V supply. However, to ensure safe operation, a zener diode (D1) acts as a voltage regulator, dropping it down to a more manageable 4.3V.

Calculating the Right Current:

A resistor, R1, plays a crucial role in ensuring the flip-flop gets the current it needs to function. This value is carefully chosen based on two factors:

  1. Minimum Current Requirement: The switching regulator’s enable pin and the flip-flop itself have a minimal current draw. R1 is selected to provide at least 60uA to meet this combined need.
  2. Buffer: A small additional current allowance is factored in as a buffer for potential variations or future modifications.

Setting the Stage for Latched Power Control:

This 4.3V source also serves another purpose. It keeps the flip-flop’s D input permanently pulled HIGH. This acts as a default setting, influencing the flip-flop’s output based on other control signals.

Clock Pulse: The Power-On Signal:

The Clock Pulse (CP) input is typically pulled LOW by a resistor (R10k in this case). When a rising edge (voltage increase) is detected on this pin, it acts as a trigger for the flip-flop. Importantly, at this point, the flip-flop also reads the state of its D input (which is HIGH, as mentioned earlier). This combination sets the flip-flop’s output (Q) to HIGH as well.

In simpler terms, a brief voltage spike on the CP input acts like a “turn on” switch, and the HIGH D input ensures this always results in powering up the connected devices.

Overriding the On-State: The C-bar Input

The C-bar input has a different role. It has a weak pull-up resistor (R2) that keeps it at 4.3V most of the time. However, when a LOW signal is applied to this input, it overrides the CP input entirely. This LOW signal can come from two sources:

  1. RTC_INT: This input is typically connected to the interrupt pin of a Real-Time Clock (RTC) module. When the RTC triggers an interrupt, it pulls C-bar LOW, effectively shutting down the device.
  2. High Input from INT_2: This alternative allows a microcontroller’s output pin (INT_2) to control the power state. A HIGH signal on INT_2 sends a LOW signal to C-bar, triggering a power down.

C-bar’s Dominance and the Role of GPIO_READ:

Once C-bar is pulled LOW, it essentially ignores any subsequent clock pulses. The flip-flop’s output (Q) is forced LOW, turning off the power. This state persists until the next time C-bar is pulled HIGH.

It’s important to note that another pin, GPIO_READ, allows a microcontroller input pin to “peek” at the state of the RTC’s interrupt even when the device is powered down. This allows for coordinated power management when the microcontroller wakes up.

Connecting the Dots: Putting it All Together

By understanding how these internal circuits interact – the constant power supply to the flip-flop, the control signals on CP and C-bar, and the ability for the microcontroller to monitor the RTC – we gain a clearer picture of how the OPEnS Power Control Board achieves its remarkable ultra-low power standby current.

The Switching Regulator

The switching regulator effectively transforms the 12v input into a 5v output, a design borrowed largely from the recommended circuit in the LM2575 datasheet. Its EN-bar input functions as a low-true output-enable, directly linked to the flip flop’s Q output. The 5v output from the regulator passes through the MODE jumper. During normal operation, the first two pins of the MODE jumper should be connected, but when a USB is connected to the microcontroller (for programming purposes, for instance), they should be disconnected to avoid conflicting 5v supplies.

The Board

Sam Edwards used EAGLE CAD to create the board layout. I’ve included dimensions and disabled tStop and pour planes to enhance clarity.

Connections

The board is tailored specifically for use with Adafruit’s Feather M0 microcontroller series and requires a 12v battery connected to the 2.5×5.5mm barrel jack receptacle for power.

The +5V pin should be linked to either the M0’s USB power pin or the Arduino Uno’s Vin. The MODE setting should be closed with a jumper to activate the board’s 5v output.

The +12V output needs to be connected to any circuitry operating on 12v.

RTC_INT should be connected to the real-time clock’s interrupt output, or to any other independent circuitry providing a LOW-TRUE signal to power on the device.

GPIO_READ should be connected to an input pin on the microcontroller if the real-time clock’s alarm needs to be read by the microcontroller after power has been enabled.

GPIO_CP should be linked to the microcontroller’s digital output assigned as the shutoff pin.

INT2 should be connected to any external circuitry that activates the device with a HIGH-TRUE signal.

Testing

A current draw assessment was performed by gauging the current using a multimeter positioned between the positive lead of the battery and the positive input on the power control board. The power control board was linked to the OPEnSampler’s Microcontroller Breakout Board, which housed Adafruit’s Feather M0 Wifi microcontroller. The microcontroller was pre-programmed to illuminate its LED upon startup. One jumper cable was linked to the real-time clock’s 3v3 power pin, drawing power from the M0, and temporarily connected to the power control board’s GPIO_CP pin to deactivate the device. Another jumper cable was connected between ground and RTC_INT to activate the device.

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About The Author

Ibrar Ayyub

I am an experienced technical writer holding a Master's degree in computer science from BZU Multan, Pakistan University. With a background spanning various industries, particularly in home automation and engineering, I have honed my skills in crafting clear and concise content. Proficient in leveraging infographics and diagrams, I strive to simplify complex concepts for readers. My strength lies in thorough research and presenting information in a structured and logical format.

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