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Chapter 3: Power Fundamentals and Voltage Regulators

Between your battery and your microcontroller or SoC lies a power conversion stage. The efficiency of this stage can mean the difference between a device that lasts days and one that lasts months. This chapter covers the fundamentals of voltage regulation — the most common power loss point in embedded and portable systems.

3.1 Why Voltage Regulation Matters

Most microcontrollers operate at 3.3 V or lower, while common battery configurations deliver 3.7 V (single Li-Ion) or 5 V (USB). The voltage regulator steps the battery voltage down to the supply voltage the device needs.

The energy that disappears in this conversion becomes heat. For a regulator at 60% efficiency powering a 100 mA load, 40 mA-equivalent of current is wasted as heat per cycle. Over a year of continuous operation this is significant. Over a low-duty-cycle device’s sleep period — where the device may draw only 10–100 µA — a regulator with high quiescent current can consume more power than the load itself.

3.2 Linear Regulators (LDOs)

A low-dropout (LDO) linear regulator drops excess voltage as heat. The output voltage is fixed; the dropout voltage is the minimum difference between input and output at which regulation is maintained.

Efficiency: η = Vout / Vin × 100%

A 3.3 V LDO powered from a 4.2 V Li-Ion cell operates at about 79% efficiency. The remaining 21% of power is lost as heat regardless of load current.

When to use LDOs:

  • When the input-output voltage difference is small (the efficiency penalty is minor)
  • When low noise is essential (switching regulators introduce ripple)
  • When the load current is very low and simplicity matters

Key spec: quiescent current (Iq). This is the current the regulator draws when the load is zero. A cheap LDO may have Iq = 5–10 mA — more than many microcontrollers draw in deep sleep. For battery applications, choose an LDO with Iq < 10 µA. The MCP1703 (2 µA), XC6206 (1 µA), and TPS7A02 (0.5 µA) are good examples.

3.3 Switching Regulators (Buck Converters)

A buck converter switches a transistor at high frequency (typically 100 kHz–3 MHz) and uses an inductor and capacitor to average the switched output into a stable DC voltage. Efficiency is typically 85–95% across a wide load range.

Efficiency curve: Switching regulators are most efficient at moderate loads. At very low loads, the switching losses (constant per cycle) become dominant and efficiency drops. This is the sleep-mode trap: a high-frequency buck regulator optimized for 500 mA loads may be less efficient than an LDO when powering a 50 µA sleeping microcontroller.

Solutions for low-load efficiency:

  • Pulse-skipping mode (PSM): The regulator skips switching cycles when the output capacitor has enough charge. This trades output ripple for lower quiescent current. Many modern buck converters (TPS62840, MAX17222) enter PSM automatically at light loads.
  • Burst mode: Similar to PSM but with a defined burst pattern.

Key specs for embedded use:

  • Quiescent current: look for Iq < 30 µA for always-on applications
  • Load regulation: how much Vout changes with load current
  • Efficiency at your expected load range (not just peak efficiency)

3.4 Choosing Between LDO and Buck

Criterion LDO Buck Converter
Efficiency at moderate load Poor (Vdrop × I) Excellent (85–95%)
Efficiency at microamp load Good (if low-Iq LDO) Variable
Noise / ripple Very low Higher (needs filtering)
Component count 1 IC + 2 caps IC + inductor + 2 caps
Cost Low Moderate
Design complexity Minimal Moderate

For most battery-powered embedded projects:

  • Use a low-Iq LDO if Vin − Vout < 1 V (e.g., 3.7 V Li-Ion → 3.3 V logic)
  • Use a buck converter if Vin is significantly higher than Vout (e.g., 5 V USB → 3.3 V, or 12 V lead-acid → 3.3 V)
  • Use a buck-boost if Vout must be stable across a wide Vin range (e.g., 2× AA alkaline, which drop from 3 V fresh to 1.8 V discharged)

3.5 Decoupling and Power Rail Integrity

Even a well-chosen regulator cannot compensate for a noisy power rail caused by poor decoupling. Microcontrollers and RF modules have current demands that change in nanoseconds — far faster than a regulator can respond.

Decoupling capacitors placed close to each IC’s power pins smooth these transients locally. A common approach: 10 µF bulk capacitor per power domain + 100 nF ceramic capacitor per IC power pin, placed within 1–2 mm of the pin.

Poor decoupling causes:

  • Microcontroller resets under RF transmit load
  • ADC readings corrupted by switching noise
  • Increased EMI emissions

For battery-powered ESP32 projects in particular: the ESP32 datasheet recommends at least 100 µF on the 3.3 V rail to absorb the ~260 mA WiFi transmit burst without excessive voltage droop.

3.6 Power Sequencing and Domain Switching

On more complex boards, different subsystems operate at different voltages and may need to be powered down entirely during sleep. A GPIO-controlled load switch (e.g., a P-channel MOSFET or a dedicated load switch IC like the TPS22860) can cut power to a sensor, display, or radio completely — achieving true zero draw instead of the standby current of a “turned off” component.

This is especially effective for sensors that draw 100–500 µA in standby even when not sampling. Multiplied across months of operation, eliminating standby current via a load switch can double or triple battery life.

← Chapter 2: Measuring Power Consumption Table of Contents Chapter 4: ESP32 Deep Sleep and Wake Sources →

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