What is an LDO and Will You Need One in Your Design?By ZM Peterson • Apr 5, 2020
Whether you’re building a custom power supply or you’re designing a power regulation strategy for your next board, you’ll need to consider how you’ll regulate the power sent to your components. Power distribution is an important idea in high speed digital systems, but many important systems run at DC and require stable power output.
If you’re taking power from a line voltage, or you’re taking power from a battery, you’ll need to regulate the input power down to the level you need for your system. An LDO regulator is one of those less-understood DC-DC converters that is critical for working with battery-powered systems. So what is an LDO, and does your design need this type of regulator? Let’s look at this question in more detail to see when you should add an LDO regulator to your power supply.
What is an LDO?
The acronym "LDO" stands for "low dropout" and refers to the range of input voltages this type of regulator can use to produce a stable output. An LDO regulator is a step-down DC-DC converter that can accept input voltages that are only slightly larger than the desired output voltage. This minimum difference between the desired output and the input voltage is called the dropout voltage. An LDO is just a linear regulator; it has low efficiency, but it can accommodate a dropout voltage as low as ~1 V.
The important thing to remember is that an LDO is not a switching regulator; it is only a linear regulator that allows a low dropout voltage. The actual definition of "low" depends on which manufacturer or engineer you ask. The key portion of an LDO is the use of an operational amplifier (called an error amplifier, or EA in the circuit diagram) as part of a feedback loop in the LDO circuit. The EA drives the gate in the input FET, thereby forcing the FET into saturation. The FET can only remain saturated when the source voltage is sufficiently large, and it is this saturation that ensures the output voltage remains stable.
The circuit diagram shown above is a simple LDO stage that nicely illustrates the key components of the circuit. Here, the output from the regulator and the gain in the circuit are tuned by adjusting the values of the resistors in the feedback loop. The output stabilizes at its final value once the voltage at the non-inverting input matches the internal reference voltage. Some LDOs will be constructed with an input for an external reference voltage from a band gap regulator, rather than using the op-amp’s internal reference.
Simple circuit diagram for an LDO
When to Use an LDO
In short, an LDO should be used whenever there is a wide input voltage range, but you need stable voltage output from your power supply. These components are less efficient than switching regulators, but they don’t require any feedback to the switching element, so there is no need to adjust a PWM signal in a feedback loop with a microcontroller. This means an LDO requires a smaller number of components and no programming.
The ability to regulate a range of voltages down to a specific value is extremely important. In mobile devices and other low power electronics, the output from a large battery can drop by several volts, and the LDO’s job is to provide constant output power despite the input voltage drop. In a system with a larger battery pack, such as a recent submersible project we worked on, the input DC voltage could vary by 100-200 V! Accommodating this huge range of voltages required building a custom switching regulator circuit with current-sensing feedback to ensure constant output power.
Although an LDO is not a switching regulator, it can be used on the output of a switching regulator. This is a useful strategy for regulating down the output from a switching regulator to the desired output voltage. An example flowchart for this type of regulation circuit is shown below.
In the above example, the LDO is the most inefficient portion of the power supply, but it only drops a small amount of voltage to produce a stable 8 VDC output. As a result, it can generate less heat than the switching regulator section. In a high power application, you may need to build the regulator sections from discrete components; regulator ICs are normally designed for high voltage or high current, but not for both as the die will quickly heat up and fail. Larger discrete FETs are normally used in each regulator section, including in the LDO on the output section.
More Advanced LDO Circuits
As was mentioned above, more advanced LDOs will use another input to provide a user-selectable reference voltage, which makes the LDO programmable. An LDO will normally include a shunt capacitor across the output to bypass high frequencies to ground. Most circuit designers normally only focus on filtering the output from a multi-stage regulator, but the input should also be filtered.
If you want your new design to be EMC/CISPR compliant, some simple EMI filters (e.g., Pi filter) can be used to remove noise from your power supply input. This is quite important when working with switching regulators, as the regulator can generate higher order harmonics from noise signals. If you can remove noise before it passes to successive regulator stages, you’ll suppress conducted and radiated EMI from your power supply.
There are some other important aspects of power distribution in your next system that need to be considered, both for digital and analog systems. Take a look at the articles below to learn more about power distribution:
- PCB Design, Simulation, and Analysis: Which Tools to Use
- Causes of Ripple Voltage: Power Supply Noise and Transients in Your PDN
- PDN Design and Power Rail Ripple Suppression in Your PCB
At NWES, we understand what is an LDO, and we have experience designing power electronics and other high voltage/high current circuits for different applications. We’ll also create a high quality, fully manufacturable PCB layout for your system. We're here to help electronics companies design modern PCBs and create cutting-edge technology. We've also partnered directly with EDA companies and several advanced PCB manufacturers, and we'll make sure your next layout is fully manufacturable at scale. Contact NWES for a consultation.