Power Management Unit (PMU) PCB Design Guide

The Power Management Unit (PMU) is the core of electronic products such as smartphones and industrial equipment. It converts electrical energy into different voltages and currents required by the system, and handles power distribution and protection. Its performance directly determines the device’s battery life, stability, and safety.

A well-designed PMU PCB achieves efficient conversion, precise voltage regulation, noise suppression, and reliable heat dissipation. Poor layout can cause excessive ripple, EMI issues, chip overheating, or even damage.

This guide starts from the core functions of the PMU and systematically explains layout, routing, heat dissipation, and common mistakes. It helps engineers master practical methods for designing robust and efficient power systems.

1. Core Functions and Composition of the PMU

PMU PCB

1.1 Core Functions of the PMU

As the “brain” and “heart” of the power system, the PMU integrates multiple power management and control functions:

Power Management: Provides accurate and appropriate voltage and current to different functional modules in the system, such as CPU, memory, RF circuits, and sensors.

Power Path Switching: Achieves seamless switching between battery power and external adapter power, preventing device restart or data loss during power source changes.

Battery Management: Monitors battery level, voltage, and temperature in real time. Implements smart charging strategies (such as trickle, constant current, constant voltage charging) to prevent overcharge and over-discharge, thus extending battery life.

Power Optimization: Dynamically adjusts power supply status based on system load. Enters low-power mode during standby or light load, and provides sufficient current when high performance is needed, optimizing overall energy efficiency.

Hardware Protection: Continuously monitors input/output voltage, current, and chip temperature. When abnormalities (such as overvoltage, overcurrent, overtemperature) are detected, it triggers protection mechanisms (such as reducing power, shutting down output, or disconnecting the power supply) to ensure system safety.

1.2 Typical Components of a PMU

A complete PMU typically consists of internal integrated modules and necessary external components:

DC-DC ConverterModule: Usually a switching power supply (Buck, Boost, or Buck-Boost). Used for high-efficiency voltage conversion to power main functional modules.

LDO (Low Dropout) LinearRegulator: Provides stable voltage with low noise and high Power Supply Rejection Ratio (PSRR). Very suitable for sensitive circuits such as analog circuits, RF circuits, and phase-locked loops.

Control Circuit: Handles power sequencing, voltage/current monitoring, status register management, and communication with the main chip (such as via I²C or SPI interface).

Protection Circuit: Integrates Over-Voltage Protection (OVP), Over-Current Protection (OCP), Over-Temperature Protection (OTP), and Under-Voltage Lockout (UVLO) to improve system robustness.

External Filtering Network: Includes ceramic capacitors, electrolytic capacitors, inductors, and ferrite beads at input and output. Used to filter switching ripple and noise.

Auxiliary Function Modules: May include battery fuel gauge, charging control circuit, LED driver, etc.

 

2. PMU PCB Layout Guidelines

PMU PCB

Good layout is the foundation of successful PCB design. For PMUs, the goals of layout are to optimize power loops, reduce noise coupling, and ensure good heat dissipation. The recommended layout sequence is: handle the DC-DC section first, then place LDOs, and finally place control circuits.

2.1 Layout Key Points for DC-DC Converters

DC-DC switching power supplies are the main source of noise in a PMU. Their layout is critical:

Place First: First determine the positions of the DC-DC converter chip and its key external components such as inductors and input/output capacitors.

Inductor Layout: The inductor is the core energy storage component. Ensure the connection between the inductor and the chip’s switch pin (SW/LX) is as short and wide as possible to reduce parasitic inductance and resistance.

Orthogonal Placement of Adjacent Inductors: If the PMU contains multiple DC-DC converters and their inductors are placed side by side, rotate adjacent inductors 90 degrees (orthogonal) to reduce magnetic field coupling and crosstalk.

Keep-Out Area Under Inductor: Do not place any critical signal lines or sensitive devices directly under the inductor. The inductor’s magnetic field can interfere with nearby signals. Also, avoid routing sensitive traces near the inductor’s pads.

Input High-Frequency Filtering Capacitor: Identify the “high di/dt loop” (hot loop) of the DC-DC converter. The high-frequency decoupling capacitor in this loop (typically 0.1µF~10µF X5R/X7R ceramic capacitor) must be placed very closeto the chip’s input pin (VIN) and ground pin (PGND). Connect them with short, wide traces or inner layer planes, avoiding vias in between.

2.2 Handling the Switch Node (SW) and High-dV/dt Nodes

The voltage at the switch node switches rapidly between the input voltage and ground. The voltage change rate (dV/dt) is extremely high, making it a primary source of EMI radiation.

Minimize Node Area: Without affecting heat dissipation, minimize the copper area of the switch node (the connection connecting the chip’s SW pin, one end of the inductor, the bootstrap circuit, etc.) to reduce the antenna effect.

Shielding Layer: In multi-layer board designs, it is recommended to place a complete ground plane on the layer directly below the switch node to provide additional isolation and suppress noise propagation.

2.3 Layout Key Points for LDOs

The layout for LDOs is simpler than for DC-DC converters, but there are still considerations:

Placement Order: Place LDOs after completing the DC-DC section layout.

Output Capacitor: The LDO’s output capacitor (especially low-ESR ceramic types) should be placed very closeto the LDO’s output pin and ground pin to ensure loop stability.

Heat Dissipation Consideration: For higher-power LDOs, the bottom thermal pad needs vias connected to the ground plane and possibly a copper area for heat spreading.

Backside Placement: When space is limited, smaller capacitors can be placed on the backside of the PCB, connected to the LDO’s pins through vias.

2.4 Overall Layout Optimization

Functional Partitioning: Physically separate the power section (DC-DC converters, inductors, large capacitors) from the small-signal control section (feedback resistors, compensation networks) to prevent power noise from coupling into the control circuits.

Maintain Adequate Spacing: Leave sufficient space between components for later routing, heat dissipation, and debugging. In particular, reserve space around thermal pads for thermal vias.

Final Adjustment: After placing all major components, perform overall fine-tuning and optimization. Check space utilization and critical paths.

 

3. PMU PCB Routing Guidelines

PMU PCB

Routing must follow three major principles: sufficient current carrying capacity, shortest return path, and minimal interference. It is recommended to fan out the DC-DC power section first.

3.1 Power Trace Routing

Trace Width Calculation: Power traces (input/output main paths) carry high currents. Insufficient width causes temperature rise and voltage drop. An empirical formula can be used: Trace width (mil) ≈ Current (A) × (10~20) / (Copper weight (OZ)). For example, for 5A current on 1OZ copper, a trace width of at least 120 mil is recommended.

Short and Wide: Traces from the DC-DC output pin to the inductor, and from the inductor to the output capacitor, should be short, wide, and straight to minimize parasitic resistance and inductance.

Via Placement: Add vias to connect to power planes only after the last output filter capacitor. The number of power vias should match the number of ground vias to ensure balanced current return paths.

3.2 Input and Output Loop Routing

Input Loop: The input current path should be as short and wide as possible. The GND terminal of the input capacitor should connect to the chip’s PGND (power ground) by the shortest path.

High-Frequency Decoupling Capacitor: The routing for the input high-frequency decoupling capacitor is critical. Its power and ground terminals should connect directly to the chip’s VIN and PGND pins via short, wide traces or inner layer areas. If vias are necessary, place them very close to the capacitor pads.

3.3 Feedback (FB) and Control Signal Routing

Close to Chip: The feedback resistor divider network and related compensation network components must be placed very closeto the chip’s feedback pin (FB).

Away from Noise Sources: The feedback trace from the divider resistor output to the chip’s FB pin must be routed away from inductors and switch nodes, and should not run parallel to high-current power traces. A thin trace (~10 mil) with a complete ground plane as reference is recommended.

Avoid Interference: Analog ground (AGND) and power ground (PGND) typically need to be separated. They should be connected at a single point, usually under the chip’s thermal pad or at a specific low-impedance point.

3.4 Via Usage

Thermal Vias: Add multiple vias denselyon the chip’s exposed thermal pad. Connect them to the ground plane or a dedicated copper area for heat spreading. Vias can be tented or filled with copper to enhance thermal conductivity.

Fan Out All Netted Pads: All pads that have nets should be fanned out to ensure signal integrity.

Quantity Matching: The number and position of power vias should be coordinated with the ground return vias to avoid creating large current loops.

3.5 Routing for Multiple Power Supplies

When multiple power supplies share one input source and operate asynchronously, separate their input power traces to prevent common-mode noise from propagating through the input paths and ground.