App Note 140 Part 1/3: Basic Concepts of Linear Regulators and Switch-Mode Power Supplies
Today’s Electronic system designs require more and more supply rails and power supply solutions, with loads ranging from a few mA for backup power to over 100A for ASIC regulators. Selecting the right solution for the target application and meeting the specified performance requirements is critical, such as high efficiency, tight printed circuit board (PCB) space, accurate output voltage regulation, fast transient response, low solution cost, and more. Power management design work is becoming more frequent and more challenging for many system designers who may not have a strong background in power technology.
Power converters generate output voltage and current for a load from a given input power source. It needs to meet load voltage or current regulation requirements under steady state and transient conditions. The load and system must also be protected in the event of component failure. Depending on the application, designers can choose between linear regulator (LR) or switch-mode power supply (SMPS) solutions. To better select a solution, designers must be familiar with the advantages, disadvantages, and design considerations of each approach.
This article focuses on non-isolated power applications and introduces their operation and design basics.
How Linear Regulators Work
Let’s take a simple example first. In embedded systems, the front-end power supply provides a 12V bus supply rail. On the system board, the op amp requires a 3.3V supply voltage. The easiest way to generate 3.3V is to use a resistive divider for the 12V bus, as shown in Figure 1. Does it work well? The answer is usually no. The op amp’s VCC pin current may vary under different operating conditions. If a fixed resistor divider is used, the IC VCC voltage will vary with the load. Also, the 12V bus input may be poorly regulated. There may be multiple other loads in the same system sharing the 12V rail. The 12V bus voltage varies with bus load conditions due to bus impedance. Therefore, the resistive divider cannot provide the regulated 3.3V to the op amp for proper operation. Therefore, a dedicated voltage regulation loop is required. As shown in Figure 2, the feedback loop needs to adjust the value of the top resistor R1 to dynamically adjust 3.3V on VCC.
Figure 1. Resistive divider generates 3.3VDC from 12V bus input
Figure 2. Feedback Loop Adjusts Series Resistor R1 Value to Regulate 3.3V
This variable resistor can be implemented using a linear regulator, as shown in Figure 3. Linear regulators operate bipolar or field effect power transistors (FETs) in linear mode. Therefore, the transistor acts as a variable resistor in series with the output load. To establish the feedback loop, conceptually, the error amplifier senses the DC output voltage through a network of sampling resistors RA and RB, and then compares the feedback voltage VFB with the reference voltage VREF. The error amplifier output voltage drives the base of the series power transistor through the current amplifier. When the input VBUS voltage decreases or the load current increases, the VCC output voltage drops. The feedback voltage VFB also drops. Therefore, the feedback error amplifier and current amplifier generate more current to feed the base of transistor Q1. This reduces the voltage drop, VCE, and restores the VCC output voltage, making VFB equal to VREF. On the other hand, if the VCC output voltage increases, the negative feedback circuit also increases VCE, ensuring accurate regulation of the 3.3V output. All in all, any change in VO will be absorbed by the VCE voltage of the linear regulator transistor. Therefore, the output voltage VCC is always constant and well regulated.
Figure 3. Linear Regulator Implements Variable Resistor to Regulate Output Voltage
Why use a linear regulator?
Linear regulators have been widely used in industry for a long time. Linear regulators have been a fundamental component of the power industry since the introduction of switch-mode power supplies in the 1960s. Even today, linear regulators are used in a wide variety of applications.
In addition to simplicity of use, linear regulators offer other performance advantages. Power management vendors have developed many integrated linear regulators. A typical integrated linear regulator requires only VIN, VOUT, FB, and an optional GND pin. Figure 4 shows the LT1083, a typical 3-pin linear regulator developed by Analog Devices over 20 years ago. Only 1 input capacitor, 1 output capacitor, and 2 feedback resistors are required to set the output voltage. Almost any electrical engineer can use these simple linear regulators to design power supplies.
Figure 4. Integrated Linear Regulator Example: 7.5A Linear Regulator with Only 3 Pins
One downside – linear regulators are very power hungry
A major disadvantage of using a linear regulator is the excessive power dissipation of its series transistor Q1 operating in linear mode. As mentioned earlier, the linear regulator transistor is conceptually a variable resistor. Since all load current must pass through the series transistor, its power dissipation is PLoss = (VIN – VO) •IO. In this case, the efficiency of the linear regulator can be quickly estimated by the following formula:
Therefore, in the example in Figure 1, when the input is 12V and the output is 3.3V, the linear regulator is only 27.5% efficient. In this example, 72.5% of the input power is wasted and heat is generated in the regulator. This means that the transistors must have heat dissipation capabilities to handle power dissipation and heat dissipation at worst-case conditions at maximum VIN and full load. Therefore, the size of the linear regulator and its heat sink can be large, especially when VO is much smaller than VIN. Figure 5 shows that the maximum efficiency of a linear regulator is proportional to the VO/VIN ratio.
Figure 5. Maximum Linear Regulator Efficiency vs. VO/VIN Ratio
On the other hand, if VO is close to VIN, the efficiency of the linear regulator is high. However, a linear regulator (LR) has a limit, which is the minimum voltage difference between VIN and VO. The transistors in LR must work in linear mode. Therefore, some degree of minimum voltage drop is required from the collector to the emitter of a bipolar transistor or the drain to source of a FET. If VO is too close to VIN, LR may not be able to regulate the output voltage. Linear regulators that can operate with low headroom (VIN – VO) are called low dropout regulators (LDOs).
Obviously, linear regulators or LDOs can only provide step-down DC/DC conversion. Linear regulators obviously don’t work in applications that require a higher VO voltage than VIN, or a negative VO voltage from a positive VIN voltage.
Current-Sharing Linear Regulators for High Power
For applications requiring more power, the regulator must be mounted separately on a heat sink for heat dissipation. In an all-surface-mount system, this is not feasible, so power constraints (eg, 1W) limit the output current. Unfortunately, it is not easy to directly parallel parallel linear regulators to dissipate the generated heat.
Replacing the reference voltage shown in Figure 3 with a precision current source allows the linear regulators to be directly paralleled to spread the current load and thus the heat dissipated on the IC. This enables the use of linear regulators in high output current, all surface mount applications where only a limited amount of heat can be dissipated at any one point on the board.
Analog Devices’ LT3080 is the first adjustable linear regulator that can be used in parallel to increase current. As shown in Figure 6, its precision zero-TC 10µA internal current source is connected to the non-inverting input of the op amp. The output voltage of the linear regulator can be adjusted from 0V to (VIN – VDROPOUT) by using an external single voltage setting resistor, RSET.
Figure 6. Single Resistor Setup LDO LT3080 with Precision Current Source Reference
Figure 7 shows how simple it is to parallelize LT3080s to achieve current sharing. Simply connect the SET pins of the LT3080 together and the reference voltage for both regulators is the same. Because the op amp is precisely trimmed, the offset voltage between the trim pin and the output is less than 2mV. In this case, only 10mΩ ballast resistors (the sum of the small external resistors and the PCB trace resistance) are needed to balance the load currents with over 80% current sharing. Still need more power? Parallel connection of 5 to 10 devices is also reasonable.
Figure 7. Paralleling Two LT3080 Linear Regulators to Increase Output Current
More suitable for applications using linear regulators
Linear regulators or LDOs provide an excellent switching power supply solution in many applications, including:
1. Simple/Low Cost Solutions: Linear regulator or LDO solutions are simple and easy to use, especially for low power applications with low output current where thermal stress is less critical. No external power Inductor is required.
2. Low noise/low ripple applications: For noise sensitive applications such as communications and RF devices, it is important to minimize power supply noise. The output voltage ripple of a linear regulator is low because the components are not switched frequently, but the bandwidth is high. Therefore, there are almost no EMI problems. Some special LDOs, such as the ADI LT1761 LDO family, have noise voltages as low as 20µVRMS at the output. SMPS can hardly achieve this low noise level. Even with very low ESR capacitors, SMPS typically have 1mV output ripple.
3. Fast transient applications: Linear regulator feedback loops are usually internal, so no external compensation is required. In general, linear regulators have wider control loop bandwidth and faster transient response than SMPS.
4. Low dropout applications: For applications where the output voltage is close to the input voltage, an LDO may be more efficient than an SMPS. There are also very low dropout LDOs (VLDOs), such as the ADI LTC1844, LT3020, and LTC3025, with dropout voltages of 20mV to 90mV and currents up to 150mA. The minimum input voltage can be as low as 0.9V. Since there are no AC switching losses in the LR, the light load efficiency of an LR or LDO is similar to its full load efficiency. SMPS typically have lower light load efficiency due to AC switching losses. In battery powered applications where light load efficiency is equally important, LDOs provide a better solution than SMPS.
To sum up, designers use linear regulators or LDOs because of their simplicity, low noise, low cost, ease of use, and fast transient response. If VO is close to VIN, LDO may be more efficient than SMPS.
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