Low-Power Hardware Design
Low-power design is not just choosing a low-power MCU. It is a system budget across regulators, sensors, pull resistors, memories, radios, clocks, leakage paths, firmware states, and wake sources.
Learning Objectives
By the end of this lesson, you should be able to build a power-mode budget, estimate battery life, identify hidden leakage, select regulators, and design load switching for sleep modes.
Power Modes
Define every mode:
- ship or storage mode;
- deep sleep;
- periodic measurement;
- active compute;
- radio transmit;
- fault or alarm mode;
- firmware update mode.
For each mode, list which rails and loads are on.
Battery-Life Estimate
Average current for repeating modes:
$$
I_{avg} = \frac{\sum I_i t_i}{\sum t_i}
$$
Battery life estimate:
$$
Life = \frac{Capacity}{I_{avg}}
$$
If a device sleeps at 20 uA for 59 s, measures at 5 mA for 0.5 s, and transmits at 80 mA for 0.5 s:
$$
I_{avg} = \frac{20uA \times 59 + 5mA \times 0.5 + 80mA \times 0.5}{60}
$$
$$
I_{avg} \approx 0.728 mA
$$
A 2000 mAh battery would ideally last about 2747 h, or 114 days, before derating.
Regulator Choice
Linear regulators are simple and quiet, but efficiency is:
$$
\eta_{LDO} \approx \frac{V_{OUT}}{V_{IN}}
$$
Switching regulators are more efficient at high current and large voltage drop, but they add ripple, EMI, quiescent current, and layout sensitivity.
For sleep-dominated products, regulator quiescent current can dominate. A buck regulator with 50 uA quiescent current may be poor for a product that otherwise sleeps at 5 uA.
Leakage Sources
Hidden current often comes from:
- pull-up and pull-down resistors;
- voltage dividers left enabled;
- level shifters;
- sensors with standby current;
- LEDs and power indicators;
- debug probes or USB bridges;
- ESD leakage at high temperature;
- back-power through IO pins.
Pull resistor current:
$$
I = \frac{V}{R}
$$
A 10 kOhm pull-up to 3.3 V consumes 330 uA when held low. That is unacceptable in many sleep designs.
Load Switching
Turn off blocks that are not needed:
- high-side load switch for sensors and radios;
- P-channel MOSFET or dedicated load-switch IC;
- enable pin on regulators;
- analog switch for dividers;
- GPIO-controlled sensor power.
Check inrush current, discharge paths, startup time, and whether IO pins back-power an unpowered device.
Wake Sources
Wake circuits must be powered while the rest of the system sleeps. Examples:
- RTC alarm;
- button interrupt;
- accelerometer motion interrupt;
- comparator threshold;
- radio wake signal;
- charger or external power detect.
The wake source current belongs in the sleep budget.
Measuring Low Power
Use appropriate tools:
- DMM for steady sleep current;
- source meter for precise low currents;
- oscilloscope with current probe for bursts;
- power analyzer for dynamic profiles.
Measure across the full cycle. A DMM may hide radio transmit pulses and inrush events.
Practical Checks
- Record current in every mode.
- Remove or gate indicator LEDs.
- Increase pull resistor values when timing allows.
- Disable dividers except during measurement.
- Confirm unpowered devices are not back-powered through IO.
- Check regulator quiescent current at temperature.
- Test with real batteries, not only bench supplies.
Common Mistakes
- Calculating battery life from active current only.
- Leaving a voltage divider permanently connected.
- Choosing a regulator for efficiency but ignoring quiescent current.
- Back-powering a sensor through I2C pull-ups.
- Measuring average current with the wrong instrument.
- Forgetting self-discharge and battery capacity derating.
Summary
Low-power hardware requires a mode-by-mode current budget and a design that actually turns off unused loads. Regulator quiescent current, pull resistors, dividers, sensors, IO back-power, and measurement method often decide real battery life.
Further Reading
- Nordic Semiconductor, low-power design guides.
- Texas Instruments, "IQ: What it is, what it isn't, and how to use it."
- Microchip, low-power MCU application notes.
- Battery manufacturer datasheets for capacity versus load and temperature.