Hardware designers are familiar with the LDO, or low-dropout regulator. It is a compact device used to provide a stable voltage rail for electronic circuits. Although the regulator itself is responsible for controlling the output voltage, PCB layout has a direct effect on how well the circuit performs. Poor placement or routing can increase voltage drop, introduce noise, weaken immunity, and make the output less stable.
For this reason, LDO PCB design should be considered part of the power-supply design rather than an afterthought. The main goal is straightforward: keep the current path short, provide a low-resistance return path, and place the input and output filtering close to the regulator. The following sections cover the basic LDO function and the key placement and routing practices that support a stable local supply.
What Is an LDO?
LDO stands for low-dropout regulator. Its primary role is to step a higher DC voltage down to a lower voltage required by ICs and other circuits. In the description used here, the LDO consists of a DC voltage regulator and a current amplifier. A controlled voltage difference between the input and output allows the regulator to maintain the required output voltage while operating with limited headroom.
Four characteristics explain why LDOs are widely used for local power supplies:
- Low dropout: An LDO can regulate a lower output voltage from a higher input rail while maintaining a relatively small voltage difference between the input and output.
- Stable output: The regulator reduces the effect of input-voltage variation on the local supply and can provide useful rejection of noise and interference.
- Low power: Unlike a switching regulator, an LDO uses linear regulation rather than continuously switching the power path, which can make it suitable for applications where low power consumption is required.
- Fast response: The regulator can respond to changes in load conditions and adjust the output to help maintain the required supply voltage.
LDOs are commonly used as local regulators in phones, wearables, communications equipment, embedded systems, and other electronic products. In these applications, the regulator may be electrically small, but its surrounding PCB layout still has a significant influence on the quality of the power rail.
LDO PCB Placement: Two Points Matter Most
Placement establishes the physical current path before any traces are routed. If the regulator, capacitors, and vias are placed poorly, routing may require long or indirect paths that are difficult to correct later. The basic layout principle is therefore to keep the power loop as small as practical.
Use an In-Line or L-Shaped Placement
Arrange the LDO and its surrounding components according to the input-to-output power flow. An in-line, or straight, arrangement provides a direct path through the regulator and filtering components. An L-shaped arrangement can also be used when the available board area does not allow a straight placement.
Both approaches help keep the power loop short. A shorter loop reduces the length of the current path and limits the area through which unwanted interference can couple. It also helps reduce the resistance of the power path, which supports lower voltage drop between the source, regulator, and load.
The important point is not simply whether the components form a perfect straight line or an L shape. The placement should create a compact and logical current path without forcing power traces to travel around unrelated components.
Place Filter Capacitors Large Then Small
Place the input and output filter capacitors close to the LDO and arrange them from the larger capacitor to the smaller capacitor along the relevant power path. This arrangement supports filtering and helps keep the input and output voltages stable.
The capacitors should remain close enough to the regulator that the filtered power path does not become unnecessarily long. A capacitor placed far away may still be electrically connected, but the additional trace length increases the physical size of the current loop and reduces the benefit of compact placement.
The same principle applies to the output side. Keep the output filtering close to the regulator before routing the regulated rail toward the load. Avoid reversing the intended capacitor order when implementing this layout approach.
LDO PCB Routing: Four Rules
If placement defines the basic structure of the LDO circuit, routing determines how effectively current and return current move through that structure. Power traces, vias, and ground connections should be treated as part of the regulator circuit rather than as ordinary signal routing.
Size Power Traces and Vias for the Required Current
Power-trace width and via count should match the actual current carried by the LDO power path. A trace or via structure that is too small for the required current can increase resistance, causing additional voltage drop and unwanted heating.
The same consideration applies when power is transferred between PCB layers. A single via may not provide the desired current-carrying capability for a higher-current path, so the via structure should be selected according to the actual power requirement. The objective is to maintain a sufficiently low-resistance path from the input supply through the LDO and toward the load.
Place Input and Output Vias in the Correct Order
When vias are required to move the power path between PCB layers, their positions should preserve the intended filtering sequence. Place the input power via before the first input filter capacitor. On the output side, place the output power via after the last output filter capacitor.
This arrangement keeps the capacitors directly within the intended power path. The input supply reaches the filtering stage before entering the regulator, while the regulated output passes through the output filtering before continuing toward the load. Avoid placing a power via in a position that effectively bypasses the associated capacitor.
Keep GND Wide and Put GND Vias Next to the IC Pins
The ground path is just as important as the power path. Keep the GND connection as wide as practical to reduce resistance and maintain a stable return path. A narrow or unnecessarily long ground connection can add resistance to the return path and increase the size of the ground loop.
Place GND vias close to the LDO ground pins when a layer transition is required. Keeping the vias near the IC pins shortens the return path and reduces unnecessary loop area. Multiple ground vias can also be grouped close to the relevant ground connection when the board structure requires them.
The goal is to make the ground path as direct and continuous as the power path. Routing the supply carefully while treating ground as an afterthought can undermine the benefit of good power placement.
Connect Input Ground to Output Ground
Input ground and output ground must have a reliable connection. Do not leave the two ground regions electrically disconnected or force the return current to take an unnecessarily long route around the regulator.
A continuous ground return gives the circuit a defined path for current to return from the output side to the input side. A broken or poorly connected return path can increase noise and disturb the output voltage. The ground connection should therefore remain continuous while keeping its physical path short and low in resistance.
How Placement and Routing Work Together
The four routing rules are most effective when they are considered during component placement. For example, placing the LDO close to its capacitors but then routing the ground through a distant via can recreate a large loop. Similarly, using wide power traces does not compensate for a poorly arranged current path with unnecessary detours.
A practical LDO layout therefore follows the complete path from input power to output power and back through ground. The input path should reach the input filtering before entering the regulator. The regulated output should pass through the output filtering before continuing to the load. At the same time, the ground connection should provide a short and continuous return path between the two sides.
This approach also makes the layout easier to review. Instead of checking each trace independently, inspect the complete power loop and ask whether any component, via, or routing segment forces current to travel farther than necessary.
Common LDO PCB Layout Problems
Several common problems can be traced back to the same basic issue: the power loop has become unnecessarily large or resistive. Long traces between the LDO and its filter capacitors increase the physical loop area. Narrow power traces can increase voltage drop. Insufficient or poorly positioned vias can add resistance when power moves between layers.
Ground routing can create similar problems. A narrow ground connection, a distant GND via, or a disconnected return path can prevent the LDO from having the short return path intended by the placement. These issues are particularly important because the ground network is part of the complete current loop, not simply a reference connection.
Reviewing the placement and routing together can prevent these problems before fabrication. Check the input path, output path, capacitor positions, power vias, GND vias, and ground return as one connected system rather than as separate layout details.
Summary
LDO PCB design has many details, but the core logic is simple: shorten the loop, provide adequate current-carrying capacity, keep the ground path stable, and maintain the intended filtering sequence. An in-line or L-shaped placement creates a compact power path, while placing the capacitors in the specified large-then-small order keeps the filtering close to the regulator.
Routing should then preserve that compact structure. Power traces and vias need to support the required current, input and output vias should be positioned so the filter capacitors remain in the power path, GND should be wide and connected close to the IC pins, and input and output grounds should share a continuous return path. Together, these practices help the LDO provide a stable local supply and reduce common PCB power-layout problems.
