Power Supply Design and Layout Guidelines for LDO and DCDC

Power supply design is not simply about choosing the right regulator and adding enough copper. The PCB layout directly affects thermal performance, noise, EMI, stability, and overall power integrity.

LDOs and DCDC converters have different layout priorities: LDOs require careful attention to heat dissipation, capacitor placement, and power loss, while DCDC converters are more sensitive to high-frequency current loops, feedback routing, and grounding. When both are used in the same system, power-stage isolation and decoupling become equally important.

In this guide, we’ll break down the key power supply layout considerations and common mistakes for LDOs, DCDC converters, and mixed power architectures.

1. LDO (Low-Dropout Regulator): Quiet but Sensitive to Heat

（LDO regulator）

Advantages: Low noise, no switching interference, and simple peripheral circuitry

Disadvantages: Efficiency depends on the input-to-output voltage difference, and large voltage drops or high load currents can generate significant heat.

Three Common LDO PCB Layout Mistakes

Mistake 1: Poor Thermal Management → Excessive Temperature Rise

Incorrect: Leaving the exposed pad floating without sufficient copper area or thermal vias.

Consequence: Higher thermal resistance can increase junction temperature, potentially triggering thermal protection or affecting long-term reliability.

Correct:

• Connect the exposed pad to a sufficiently large copper area.
• Add thermal vias according to the package and datasheet recommendations, connecting them to internal or bottom-layer copper planes.
• Avoid routing signal traces underneath the thermal pad.
• Evaluate the required copper area and thermal performance based on actual power dissipation.

Mistake 2: Input/Output Capacitors Too Far from the Pins → Reduced Stability and Transient Performance

Incorrect: Placing the input and output capacitors far away from the LDO pins.

Consequence: Increased parasitic impedance can affect stability, transient response, and high-frequency noise performance.

Correct:

• Place Cin and Cout as close as possible to the corresponding LDO pins.
• Select capacitor values, quantities, and ESR according to the device datasheet.
• Prefer low-ESR capacitors recommended by the manufacturer.
• Keep the feedback path short and avoid unnecessary routing and noise coupling.

Mistake 3: Ignoring Voltage Drop and Power Dissipation → Efficiency and Thermal Issues

Incorrect: Using an LDO for high input voltage and high current without evaluating power dissipation.

Consequence: Higher LDO power dissipation increases junction temperature and may trigger thermal protection or reduce system reliability.

Correct:

The main power dissipation of an LDO can be estimated using: P ≈ (Vin − Vout) × Iout

• The larger the voltage drop and output current, the higher the LDO power dissipation.
• For high-power applications, evaluate efficiency, junction temperature, and PCB thermal performance together.
• When the voltage drop or load current is high, consider using a DCDC converter first and an LDO for downstream regulation and noise reduction.

2. DCDC (Switching Power Supply): Efficient but Noisy

（dc-dc converter）

Advantages: High conversion efficiency, higher output current capability, and suitability for a wide range of input and output voltages

Disadvantages: Switching nodes and high di/dt currents can generate noise, making DCDC PCB layout critical to EMI, stability, and overall power performance.

Five Common DCDC PCB Layout Mistakes

Mistake 1: Long Power Loops → Increased Ringing and EMI

Incorrect: Placing the input capacitor, power switch, inductor, and other key components too far apart, resulting in a large high-frequency current loop.

Consequence: Increased parasitic inductance can produce more pronounced voltage spikes and ringing under high di/dt conditions, increasing EMI and component stress.

Correct:

• Minimize the area of critical high-frequency power loops.
• Place input bypass capacitors, power devices, and inductors as close together as possible.
• Use sufficiently wide power traces. The required width should be determined by current, copper thickness, and allowable temperature rise.
• Avoid unnecessarily long high di/dt return paths.

Mistake 2: Noisy Feedback Path → Output Voltage Fluctuation

Incorrect: Routing the FB trace close to or parallel with the SW node, inductor, or other noisy areas.

Consequence: Switching noise can couple into the feedback network, affecting output voltage regulation and system stability.

Correct:

• Keep the FB trace short and away from the SW node and high di/dt current paths.
• Place the feedback resistors close to the IC's FB pin.
• Sense the output voltage from a stable, low-noise point.
• For high-precision applications, consider Kelvin sensing or other methods recommended by the device manufacturer to reduce voltage drop and noise.

Mistake 3: Improper Ground Plane Design → Increased EMI and Noise

Incorrect: Improper AGND, PGND, or high-current return-path routing causes sensitive control circuitry to share impedance with power return currents.

Consequence: Ground bounce and common-impedance coupling can affect control signals, feedback, and output voltage.

Correct:

• Follow the DCDC IC manufacturer's recommended method for connecting AGND and PGND.
• Provide a low-impedance return path for PGND.
• Keep sensitive analog ground areas away from high-current power return paths whenever possible.
• Prevent high di/dt currents from passing through sensitive signal areas.

Mistake 4: Improper Input Capacitor Placement → Increased Input Ripple

Incorrect: Relying on a single bulk capacitor while placing it far from the VIN and GND pins.

Consequence: A larger high-frequency current loop can increase input ripple and noise.

Correct:

• Configure input capacitors according to the DCDC converter datasheet.
• Use capacitors with different values when necessary to address different frequency ranges.
• Place input capacitors as close as possible to the VIN and GND pins.
• Keep the input capacitor loop short and compact.

Mistake 5: Ignoring the Freewheeling Path → Reduced Efficiency and Poorer EMI Performance

Incorrect: Placing the freewheeling diode, synchronous MOSFET, or related circuitry too far apart.

Consequence: Increased parasitic parameters can lead to additional losses, ringing, and EMI.

Correct:

• Optimize the freewheeling and synchronous rectification paths according to the specific converter topology.
• Minimize high-current return paths.
• For multiphase DCDC converters, keep the power paths of each phase well organized and reasonably symmetrical.

3. LDO + DCDC Mixed Power Supply: Balancing Efficiency and Low Noise

（power supply pcb）

In practical systems, DCDC converters and LDOs are often used together rather than as alternatives.

For example:

DCDC: 12V → 3.3V for efficient voltage conversion

LDO: 3.3V → 1.8V for noise-sensitive circuits such as ADCs, PLLs, and clocks

This approach combines the efficiency of a DCDC converter with the low-noise characteristics of an LDO, making it useful for applications that require both power efficiency and clean power rails.

Three Common Mixed Power Supply Layout Mistakes

Mistake 1: Insufficient Filtering at the LDO Input → Upstream Noise May Reach the Downstream Rail

Incorrect: Connecting the DCDC output directly to an LDO-powered sensitive load without evaluating the DCDC switching noise and the LDO's PSRR.

Consequence: Switching ripple from the upstream converter may couple into downstream sensitive circuits through the power path, affecting ADC, PLL, and other noise-sensitive devices.

Correct:

• Evaluate whether additional filtering is required based on the DCDC switching frequency, LDO PSRR, and load noise requirements.
• When necessary, add a ferrite bead, LC filter, or π filter between the two power stages.
• Place filtering components close to the corresponding power input and keep the filter loop area under control.
• Avoid adding filtering components blindly; select them based on the actual noise frequency range.

Mistake 2: Poor Power Region Layout → Noise Coupling

Incorrect: Placing the high-noise DCDC switching area too close to sensitive LDO power areas or creating unnecessary coupling between different power regions.

Consequence: Switching noise can couple into sensitive power rails through parasitic capacitance, electromagnetic coupling, or shared impedance.

Correct:

• Properly separate the DCDC power stage, LDO area, and sensitive load areas.
• Keep high-noise SW nodes away from sensitive power rails and signals.
• Maintain a solid reference ground plane beneath the LDO output power area whenever possible.
• Keep sensitive power traces away from high di/dt return paths of the DCDC converter.

Mistake 3: Poorly Designed Decoupling Network → Insufficient High-Frequency Response

Incorrect: Placing all capacitors at the DCDC output while providing insufficient local decoupling near sensitive loads.

Consequence: Parasitic impedance in the power path can reduce high-frequency decoupling effectiveness, preventing the load from receiving transient current quickly enough.

Correct:

• Configure DCDC output capacitors according to the converter requirements.
• Use the required input and output capacitors at the LDO according to the datasheet.
• Add local decoupling capacitors close to sensitive loads such as ADCs, PLLs, and MCUs.
• Design each power stage's decoupling network according to the actual load and frequency requirements rather than simply adding more capacitors.

Conclusion

Although LDOs and DCDC converters are both power management devices, their power supply layout priorities are different.

LDO layout guidelines focus more on thermal management, capacitor placement, and power dissipation. DCDC PCB layout focuses more on high-frequency power loops, feedback paths, grounding, and EMI. Mixed power supplies require additional attention to noise isolation and decoupling between different power stages.

A good power supply pcb design is not simply about adding more copper. It is about giving current the shortest and most appropriate path, keeping noise away from sensitive areas, and placing every critical component where it can perform properly.