What are the five factors to consider when designing a linear regulator?

When looking for a linear regulator, with an infinite number of product models, using a parametric search tool can narrow down your choices to just a few, which seems pretty easy. What output voltage is required? What is the load current? What is the acceptable input voltage range? What dropout voltage does the regulator need to operate at? What is the maximum input voltage? Package and external component dimensions? Next comes the details. What if the load is very sensitive to power fluctuations? Very low output noise and high PSRR may be required. If the design is battery powered, the quiescent current requirements are also very strict.

When looking for a linear regulator, with an infinite number of product models, using a parametric search tool can narrow down your choices to just a few, which seems pretty easy. What output voltage is required? What is the load current? What is the acceptable input voltage range? What dropout voltage does the regulator need to operate at? What is the maximum input voltage? Package and external component dimensions? Next comes the details. What if the load is very sensitive to power fluctuations? Very low output noise and high PSRR may be required. If the design is battery powered, the quiescent current requirements are also very strict.

Now you’ve narrowed it down to those devices that meet your specific application. But it didn’t end there. Before making a final decision, the following 5 factors still need to be considered.

1. Start

Most regulators are equipped with an enable input that controls the power-up or shutdown of the regulator to save power. Regulators with an enable input usually also have a soft-start feature. Soft-start prevents overloading the input supply when the regulator is turned on. Soft start is usually done in one of two ways.

current soft start

The first method is current soft start. Most regulators have a current limit; current soft-start is ramping or stepping to that current limit. Since the output capacitor charge is much less than the maximum load current, soft-start causes the output voltage to rise slowly. The advantage of current soft-start is that the regulator input current rises steadily and does not pass load-start transient currents to the input.

When the load is enabled, you may notice a point where the output voltage slope suddenly changes direction. This is because the load circuit turns on and tries to start operation with the regulator in current limit. If the load current exceeds the soft-start current, the load itself will enter an undervoltage state, causing a reset. This cycle continues as the load current is turned on and off. Finally, the soft-start current reaches a high enough level to support the load power supply, release the reset, and the load circuit wakes up normally.

Voltage soft start

The second type of soft-start is to ramp up the output voltage. Ramping the output voltage produces a monotonic change in the output voltage without any voltage transients when the downstream circuit turns on. This also prevents the load from entering the reset state multiple times because the output voltage crosses the load undervoltage threshold only once.

The inrush current during voltage soft-start depends on the output voltage and the slope of the output voltage change, plus the current drawn by the load. Typically, the output voltage slope is set so that the inrush current is approximately 1% to 10% of the maximum rated output current (using the recommended minimum output capacitor). Setting the inrush current to less than 10% of the maximum load current provides headroom for the current required by the load and any additional output capacitance. The disadvantage is that the input current is related to the load change and cannot be directly controlled; the advantage is that it can avoid multiple resets of the system.

What are the five factors to consider when designing a linear regulator?
Figure 1 shows a comparison of current soft-start and voltage soft-start.

2. Quiescent current and voltage difference

If the system is battery powered, the supply current of the regulator is very important. The load circuit can work for a short time and then be in standby state for a long time to save power consumption. At this point, battery life is largely determined by the quiescent current of the regulator and load. If this is the case, consider choosing a low quiescent current linear regulator.

Assume that the voltage difference between the input and output reaches a small state as the battery power is depleted. Linear regulators at this time, even if the load current is very small, will force the FET to conduct, minimizing the voltage drop between the input and output. A potential problem with operating at the lowest dropout voltage is that the gate-drive circuitry driving the regulator’s output FETs will draw more current (Figure 2). Changes “standby mode” to “battery fast discharge mode”.

What are the five factors to consider when designing a linear regulator?
Figure 2. Increased quiescent current due to MG drive impedance at lowest dropout voltage.

Even with good IC design, it is not uncommon for quiescent current to increase at the lowest dropout conditions. It is common to increase supply current by a factor of 2 at small dropouts, and some designs even increase it by a factor of 10 or more. Some devices give voltage drop versus supply current in an EC meter or a typical operating characteristic curve of quiescent current versus input voltage. But more often, the data sheet gives what is called the supply current at high dropout.

For a specific application, if quiescent current under low dropout conditions is important, an LDO that provides this information should be selected, or an actual measurement should be made to determine that the performance meets the requirements.

3. Load transient response

During fast load changes, most regulators have some ability to keep the output within regulation. When the load changes, the output FET gate drive needs to change with it. The time it takes for the gate drive to reach a new level determines the transient undershoot or overshoot of the output voltage.

Fast transients at full load can cause worst-case transient undershoot. Before choosing a regulator, it is important to check the transient response. Starting from 10% full load usually gives better results than starting from 1% full load as the initial condition; because the output FET gate voltage is closer to its final value. It is difficult to obtain a better load transient response when the load changes from no load to full load.

Large load transients can be avoided to some extent by keeping the regulator output load at a minimum, but this is not an efficient solution. Output overshoot tends to occur when a regulator transitions from full load to light load. While the regulator is recovering from an overshoot condition, the device is in a more sensitive state — the output FET is completely unbiased at this point. In this state, if another load step occurs, the output undershoots more severely than the first time.

If there is any rapid turn-on, turn-off of the load, it is a good idea to examine the load transient response of each regulator under similar conditions. (Figure 3) shows the performance during a two-pulse load transient.

What are the five factors to consider when designing a linear regulator?
Figure 3. Output undershoot during double-pulse load transient.

4. Noise to Power Supply Rejection Ratio (PSRR)

Obviously, most regulators designed for low noise output also have excellent PSRR. Whatever the reason, loads are very sensitive to power supply ripple.

When using a switching regulator, PSRR is more of a problem than output noise. For example, the front end of a linear regulator uses the voltage produced by a buck regulator as an input, and the load at its output is very sensitive to noise. If the ripple of the buck regulator is 50mVP-P @ 100kHz, and the PSRR of the linear regulator is 60dB at 100kHz, the output ripple is 50uVP-P, and the equivalent output noise is about 15uVRMS. While the total output noise of the same linear regulator in the 10Hz to 100kHz bandwidth range may be less than 5uVRMS, due to PSRR and input voltage ripple, the noise generated by the output ripple is three times that of the regulator itself, as shown in (Figure 4 ) shown.

What are the five factors to consider when designing a linear regulator?
Figure 4. Variation in output noise specification is primarily a function of PSRR.

For higher output voltages, the output noise of the linear regulator can become the determining factor for PSRR. This is because the feedback input noise after voltage division increases. Suppose a linear regulator converts the noisy 17V output of a boost converter to a less noisy 16V supply with less than 100uV of ripple. With a PSRR of 60dB at the switching frequency, a 50mVP-P boost converter ripple will attenuate to 50uVP-P, or 15uVRMS of output noise. With a 5uVRMS low noise reference and feedback op amp input, let’s look at the problems that the feedback input creates. If the feedback input is adjusted at 1.25V and the resistive feedback network sets the output to 16V, then the output noise will increase to 5uVRMSx (16V/1.25V), or 64uVRMS, which can become the dominant noise source. (Figure 5) shows the degradation of output noise performance due to high voltage output.

When looking for a linear regulator, if powering a noise-sensitive load, you often need to consider both output noise and PSRR.

What are the five factors to consider when designing a linear regulator?
Figure 5. Noise performance degradation due to high voltage output.

5. Input Protection

The output pass transistors of linear regulators mostly contain a body diode, which prevents the output from going more than 0.7V above the input. Most of the time this diode is not a problem, but in two cases it can cause trouble.

reverse voltage protection

In some cases, the input voltage may be reversed, causing the polarity to be reversed, such as placing the two metal contacts of a 9V battery. Although the connector prevents the battery from being permanently reversed, there is a reverse voltage for a few seconds or more when the user replaces the battery.

Reverse voltage protection allows input pin voltages to go below ground without sinking significant current. To achieve this, the body diode of the output FET needs to be disconnected by a series switch. Most voltage regulators include diodes that prevent any pin voltage from going below ground, preventing electrostatic discharge, or ESD, from occurring at the pins. To achieve reverse voltage protection, it is also necessary to remove the effect of this diode and adopt different protection devices, see (Figure 6).

The MAX1725 is a reverse-voltage protected device that allows the input to be 12V below ground without sinking large currents.

What are the five factors to consider when designing a linear regulator?
Figure 6. Reverse Voltage Protection.

6. Reverse current protection

Reverse current protection of linear regulators can easily be confused with reverse voltage protection. Although the effect is similar, both blocking reverse current conduction from the body diode of the output FET, the control method is completely different. (Figure 7) shows the working principle of reverse current protection.

For higher capacitive loads, such as audio circuits with many distributed power debounce capacitors, a linear regulator is used. Also assume that the linear regulator is powered by a high-current buck converter that shorts its output to ground in the off-state. We would not be surprised to find that during the first turn-off event, the linear regulator can be damaged due to the simultaneous discharge of the load capacitor network through the linear regulator’s body diode.

Linear regulators with reverse current protection can avoid this problem by disconnecting the body diode when the input voltage drops below the output voltage. If the output voltage was previously within regulation, the output FET would conduct and a small amount of reverse current would flow before triggering the protection circuit. Note that reverse current protection only removes current from the output to the input, but does not block current when the input pin voltage is below ground, as reverse voltage protection does. The MAX8902 is a device with reverse current protection that blocks current backflow from the load capacitor when the input is shorted to ground.

What are the five factors to consider when designing a linear regulator?
Figure 7. Reverse current protection.

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