Low dropout voltage (LDO) regulators are reliable tools for powering noise-sensitive devices. In addition to providing a direct power rail, the LDO regulator can post-regulate other power supplies. Noise from switching converters can permeate many designs, often requiring a downstream LDO regulator to eliminate the noise. While LDO regulators are effective, their power consumption can negatively impact system efficiency. Specially designed voltage input-to-output control (VIOC) pins reduce power consumption and increase efficiency with a single connection. The VIOC introduces automatic control of the switching converter for optimal system efficiency. This article focuses on an ultra-low noise LDO regulator that outperforms VIOC-free LDO regulators.
People rely on sophisticated electronics in many areas of their daily lives. These devices can provide sophisticated medical diagnostics, quality control of final products, accurate measurement of chemical concentrations in water and air, and much more. The sophisticated hardware built into test equipment and instrumentation consists of noise-sensitive devices that require complex planning in design and test to reduce noise. A key aspect of reducing system noise is the power rails. The rails must be able to deliver voltages with minimal noise and ripple to provide excellent performance in noise-sensitive applications. Conversely, providing noisy power rails to the signal chain can lead to poor system performance. An LDO regulator is a device that provides a low-noise power supply.
LDO regulators reliably reduce and regulate the DC voltage with a simple resistor divider setup or a single resistor setting. LDO regulators provide clean, low-noise outputs, but have the disadvantage of lower efficiency compared to another regulated device, switch-mode power supplies (SMPS). Modern SMPS devices have efficiencies of more than 90%. However, due to the rapid switching of current across the inductor, the switching converter generates a current similar to a triangular waveform, resulting in a high noise output. The voltage of the inductor is proportional to the differential current of the current flowing through it. Figure 1 shows an example of a current waveform.
Figure 1The output current of the buck converter.
Switching converters also generate voltage spurs and higher frequency harmonics at their switching frequency. This can be shown in the spectral noise content of any switching converter. The voltage noise image is shown in Figure 2.
Figure 2Voltage noise of switching converters.
Filtering the output of the switching converter reduces noise. However, this requires a large capacitance and introduces parasitic effects such as equivalent series resistance (ESR). ESR increases the power consumption of the power supply and can lead to reduced efficiency. In addition to switching noise ripple, switching converters are susceptible to wideband noise, high-frequency spikes, and ringing.
Combining a switching converter with a post-regulated LDO regulator reduces noise. The use of an LDO regulator downstream of the switching converter allows for both the efficiency of the switching converter and the inherent power supply rejection ratio (PSRR) of the LDO regulator, which enables the purification of high-noise outputs. However, this approach is still inefficient due to the voltage drop across the LDO regulator.
Analog Devices' purpose-built VIOC technology addresses the conflicting requirements of low noise and high efficiency by reducing the voltage drop in downstream LDO regulators. The VIOC is an active control system that provides feedback from an LDO regulator to regulate the output voltage of a switching converter. LDO regulators with VIOC automatically optimize the output voltage of the switching converter. This article will discuss the technical details of VIOC functionality, provide experimental evidence of efficiency improvements, and consider other possible ways in which VIOC can be used for variable downstream power rails.
LDO regulator for rear regulation.
In Figure 3, the switching converter reduces the input voltage to power the LDO regulator. This output typically contains ripple, as shown in Figure 4.
Figure 3Block diagram of an LDO regulator in a post-regulated state.
Figure 4The output voltage of the switching converter.
The LDO regulator reduces the output voltage of the switching converter and regulates it to a programmed output voltage to produce the clean voltage signal required for a precision signal chain. The PSRR is the indicator that determines the noise reduction effect of the LDO regulator. psrr can be calculated using the formula: psrr = |20 log(∆vinput)/(∆voutput )|This measurement is typically made over a wide frequency spectrum of 10 Hz to 1 MHz. LDO regulators with high PSRR (e.g., 80 dB at 1 MHz) attenuate switching noise very well and are therefore ideal for purifying distorted output voltages. An example of an LDO regulator's output rail is shown in Figure 5.
Figure 5The output voltage of the LDO regulator.
While a re-regulated LDO regulator can effectively reduce the noise of the power rails, this solution is inefficient. In the system shown in Figure 3, the switching converter has an efficiency of 90%, while the LDO regulator has an efficiency of 66%, and the overall efficiency is about 59%.
Design challenges for post-regulated LDO regulators without VIOC.
The challenge for a rear-regulated LDO regulator is to design a system that is very efficient. The low efficiency in Figure 3 indicates that the LDO regulator has considerable power dissipation due to large input-to-output dropouts and load currents. Equation 1 shows how to calculate the power dissipation of an LDO regulator.
Using an ultra-low noise LDO regulator with VIOC from Analog Devices and pairing it with a switching converter improves system efficiency. The VIOC pin causes the switching converter to regulate its output voltage to an ideal level, improving its efficiency by reducing the voltage drop across the LDO regulator.
How VIOC works.
Figure 6 illustrates the connection of the LT3041 LDO regulator with VIOC functionality to an upstream switching converter. The connection between the VIOC and the feedback (FB) pin of the switching converter ensures that the voltage differential on the LDO regulator will be set to the regulated voltage on the FB pin of the switching converter. By selecting a switching converter with a low FB voltage (typically less than 1 V), the voltage difference across the LDO regulator can be minimized sufficiently, thereby improving the overall efficiency. In one example, the LT8648S is used as an upstream converter with an FB pin voltage of 600 mV and a constant 600 mV drop will be maintained on the LDO regulator. With this connection, the VIOC pin will affect the output of the switching converter, producing an input voltage signal that satisfies Equation 2.
Figure 6Typical application circuits.
By setting the voltage differential across the LDO regulator, the VIOC reduces the output voltage of the switching converter, making the LT3041 a reliable power-saving tool.
Advantages of VIOC:
Figure 7 shows a post-regulated LDO regulator solution used to experimentally demonstrate the VIOC effect. The evaluation kit for the LT3041 is located downstream of the evaluation kit for the LT8648S, an ADI Silent Switcher 2 device. The switching converter has a regulated value of about 600 mV for the FB pin, and when the FB pin and VIOC pin are connected, the voltage difference across the LDO regulator is about 600 mV. The LT8648S EV kit generates a 5 V output voltage, and the LT3041 EV kit outputs 33 v。The following section compares the performance of the system without VIOC and with VIOC. Each experiment uses 12 V DC from the power supply to power the LT8648S. The experimental results are shown in Tables 1 and 2.
Figure 7Evaluation board connection.
In the first experiment, the VIOC pins were not connected, and the switching converter regulated the voltage to close to 5 V to power the LDO regulator. Table 1 shows that the efficiency of the LDO regulator is approximately 67%, which is the same as expected in Figure 3, as the primary function of the LDO regulator is to post-regulate the output of the switching converter. While this solution produces clean power rails, it is inefficient. As mentioned earlier, the reason for the low efficiency is that the LDO regulator consumes a lot of power due to the voltage difference.
Table 1The LT3041 rear adjustable LT8648S does not use VIOC
In the second experiment, the VIOC connection between the LT8648S and LT3041 caused the switching power supply to regulate its output voltage to VOUT(LDO) +VVIOC. When the VIOC pin is connected to the feedback pin, VVIOC = VFB = 600 mV. The LT3041 has a vout of 33 V, so the input voltage of the LDO regulator turns out to be about 39 v。Table 2 shows the resulting LDO regulator input voltage.
Table 2LT3041 rear adjustment LT8648S with VIOC
The LT3041 with VIOC successfully reduces the voltage differential across the LDO regulator, thereby improving efficiency. Instead of passing a 5 V signal from the switching converter, the VIOC pin forces the switching converter to produce about 39 V voltage. With the VIOC connection, the LDO regulator differential voltage drops to about 600 mV, compared to 17 V voltage difference. The input voltage of the LDO regulator is reduced to an efficiency of about 84% (as shown in Table 2), which is a 17% increase in efficiency and a 2% reduction in power consumption compared to the previous experiment7 times. Although the two systems output the same power, the power consumption is very different. For any given load, an LDO regulator using a VIOC will outperform an LDO regulator without a VIOC. With VIOC, the system is able to provide the ideal voltage to the LDO regulator.
The connection between the VIOC and the feedback pins of the switching converter does not guarantee the power-saving benefits of the VIOC. The VIOC can reduce, but not increase, the output voltage of the switching converter. Following the inequality VOUT(switcher) >VOUT(LDO) +VVIOC ensures that the VIOC brings power-saving benefits. If the above inequality is violated, the LT3041 will still regulate its output voltage, but will not optimize the output voltage of the switching converter.
The following experiment is an example of a system breaking its threshold to ensure power savings. In this test, the output voltage of the LDO regulator changes, resulting in a nominal 432 V output. As can be seen from Table 3, VOUT(LDO)+VVIOC has not yet exceeded the 5 V regulated output voltage of the switching converter, which allows the VIOC to be optimized for power savings. Note that the input voltage provided by the switching regulator satisfies VIN(LDO) = VOUT(LDO) + VVIOC. In addition, the LDO regulator maintains a voltage drop of approximately 600 mV through the VIOC. If a VIOC is not used, the LDO regulator will pass an input voltage of about 5 V. Table 4 shows a system without a VIOC with a switching converter output of 5 V. Note that the input voltage of the LDO regulator is closer to 5 V than the value in Table 3. While the efficiency of LDO regulators with VIOC is slightly improved, the data in Tables 3 and 4 show that VIOC reduces power consumption, even if by a smaller amount.
Table 3LT3041 rear adjustment LT8648S with VIOC
Table 4The input voltage of the VIOC is not used.
Some applications with variable load voltages can cause VOUT LDO + VVIOC to exceed the regulated output voltage of the switching regulator. Consider the LT8648S regulator with a regulated 5 V output and 600 mV FB, paired with the LT3041, but the latter now outputs 5 V. When using VIOC, based on the formula VIN (LDO) = VOUT(LDO) + VVIOC, the combination of these devices results in an input voltage of 5 for the LDO regulator6 v。This value is much larger than the 5 V output of the switching regulator. This situation invalidates the power-saving function of the LDO regulator.
Power saving for variable loads.
With a variable load, the VIOC can be programmed with three resistors, as shown in Figure 8. This configuration can program the input-to-output differential voltage by setting resistors R1, R2, and R3. To properly size these three resistors, refer to the LT3041 data sheet. While this method is not as effective in saving power as connecting the VIOC directly to the feedback pin of the switching converter, it is still reliable for applications with variable loads. By programming the voltage difference to the set voltage, the user is able to take advantage of the constant voltage drop across the LDO regulator despite the variable output voltage. Figure 8 is an example of a variable load scenario with and without these resistors.
Figure 8Variable load circuit configuration.
Consider the block diagram in Figure 9, which represents an LDO regulator that does not use VIOC to post-regulate the switching converter. Switching converters produce 65 V output, LDO regulator produces 5 V output.
Figure 95 V LDO regulator output without VIOC.
This system results in a voltage drop of 1 on the LDO regulator5 V with a power loss of 15 w。Since the load is variable, the output voltage of the LDO regulator changes. In this example, the output voltage of the LDO regulator drops to 33 V, as shown in Figure 10.
Figure 103. Without using VIOC3 V output.
NEW 3A 3 V load results in a voltage drop of 3 on the LDO regulator2 V, the power loss is 32 W, LDO regulator efficiency from 799% to 508%。
In contrast, setting the resistors shown in Figure 8 eliminates fluctuations in power dissipation and efficiency at variable loads. Consider the previous scenario, as shown in Figure 10, but the LDO regulator uses VIOC with three resistors setting the voltage difference to 15 v。The switching converter will output vout(switcher) = vdifferntial(LDO) + vout(LDO). When a variable load causes the output voltage to drop from 5 V to 3At 3 V, the output voltage of the switching converter drops to 48 V, not 65 V output, as shown in Figure 11.
Figure 113. Use VIOC 33 V output.
The voltage difference is programmed with three resistors to set the voltage drop across the LDO regulator to a constant 15 v。For a 1 A load, the LDO regulator loses 15 W of power instead of 32 W of power. With the help of VIOC and three resistors, when the load voltage drops to 3At 3 V, the LDO regulator can more than double the power consumption. The connection makes 3The efficiency of the 3 V load reaches 688%, while for the same load, the efficiency of the previous scenario was 508%。While both systems provide the same power, LDO regulators with VIOC are more efficient.
Conclusion. Overall, LDO regulators with VIOC outperform LDO regulators without VIOC. ADI's ultra-low noise LDO regulators with VIOC provide the ideal balance of efficiency and output signal quality. The combination of VIOC and LDO regulator PSRR makes the LT3041 a dual-purpose tool that can handle noisy inputs while optimizing system efficiency. When the load changes, the VIOC pins automatically adjust to optimize the system. LDO regulators using VIOC have proven to be superior under all conditions. It also improves efficiency and reduces power consumption. The main difference between an LDO regulator using a VIOC and an LDO regulator without a VIOC is the control function introduced by the VIOC. With automatic control, the LDO regulator can dynamically adjust the upstream DC-DC converter in real time for optimal efficiency. Automatic control pushes ADI technology to keep up with telemetry trends in power circuits. As people increasingly use PMBus and other methods to collect data to improve power systems, VIOC provides another layer of automatic power control.
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