【Introduction】This article introduces the latest driver + MOSFET (DrMOS) technology and its advantages in Voltage regulator module (VRM) applications. Monolithic DrMOS devices enable power systems to dramatically increase power density, efficiency and thermal performance, thereby enhancing the overall performance of the end application.
As technology advances, multi-core architectures allow microprocessors to become denser and faster on a horizontal scale. Consequently, the power required by these devices has increased dramatically. This power required by the microprocessor is provided by the Voltage Regulator Module (VRM).
There are two main parameters driving the development of voltage regulators in this field. The first is the power density (power per unit volume) of the regulator, which must be greatly increased in order to meet the high power requirements of the system in a limited space. Another parameter is power conversion efficiency, which reduces power losses and improves thermal management.
As development challenges continue to evolve, the power industry will find ways to meet those requirements. One solution is to integrate advanced switching MOSFETs (the main building blocks of voltage regulators) and their corresponding drivers into a single chip in an advanced package, enabling compact and efficient power conversion. This DrMOS power stage optimizes high-speed power conversion.
As demand for such power stages, known as smart power stages, steadily increased, and power switching technology continued to advance, Analog Devices introduced a DrMOS version of smart power modules. The LTC705x DrMOS family utilizes ADI’s patented Silent Switcher® 2 architecture and integrates a bootstrap circuit that enables DrMOS modules to switch at ultra-fast speeds while reducing power loss and switch node voltage overshoot for improved performance. The LTC705x DrMOS devices also provide safety features such as over-temperature protection (OTP), input over-voltage protection (VIN OVP) and under-voltage lockout (UVLO) protection.
LTC7051 SilentMOS Smart Power Stage
The LTC7051, part of the LTC705x DrMOS family, is a 140A monolithic smart power module that successfully integrates high-speed drivers with high figure of merit (FOM) top and bottom power MOSFETs and comprehensive monitoring and protection circuitry into one electrically and thermally optimized package . Together with the appropriate PWM controller, this smart power stage provides market-leading high-efficiency, low-noise, high-density power conversion. This combination enables high-current regulator modules with the latest efficiency and transient response technologies. A typical application of the LTC7051 is shown in Figure 1. It acts as the main switching circuit of the buck converter and is paired with the LTC3861, a dual-channel multiphase step-down voltage-mode DC-DC controller with accurate current sharing.
To demonstrate the key features of the LTC7051, Analog Devices created an evaluation board to demonstrate the performance of the LTC7051 compared to competing products. This demonstration platform facilitates an unbiased and accurate comparison of the LTC7051 DrMOS with competing products on basic parameters such as efficiency, power loss, telemetry accuracy, thermal and electrical performance. The purpose of the comparison is to remove any doubts about the validity of the results. This demo platform is used to highlight best-in-class DrMOS performance metrics regardless of manufacturer.
Figure 1. Dual-Phase POL Converter
DrMOS Analysis Evaluation Hardware
This analysis demonstrates the following important features of the hardware:
● A PWM controller that can operate over a wide range of input and output voltages and switching frequencies. In this application, the controller is the LTC7883, a quad-output multiphase step-down DC-DC voltage-mode controller, as shown in Figure 2.
● The LTC7051 uses the same power stage design as competing devices.
● The LTpowerPlay® power system management environment is used to comprehensively telemetry the system performance provided by the LTC7883.
● Can withstand extended ambient temperatures based on the specified operating temperature range of ADI and competing devices.
● The circuit board is designed to easily capture and measure heat.
Figure 2. Analysis Demo Board Block Diagram
The DrMOS analysis demo board is shown in Figure 3. The board has been carefully designed with the key features mentioned earlier. components are placed symmetrically and systematically on each power rail and have the same PCB size and area to limit variation between power rails. Placement and routing and layer stacking are also performed symmetrically.
Figure 3. DrMOS evaluation board, top and bottom. PCB Dimensions: 203 mm × 152 mm × 1.67 mm (L × H × W), 2 oz copper thickness
DrMOS Analysis Test Method and Software
In addition to the demo board itself, the test setup and test methodology are equally important for the impartiality of the data and results. To this end, the team also created a companion evaluation software with a graphical user interface (GUI), as shown in Figure 4, to allow users to conduct tests and collect data more easily. The user only needs to specify the input and output parameters, and the software will take care of the automated testing. The software automatically controls the appropriate test and measurement equipment, such as DC power supplies, Electronic loads, and multiplexed data acquisition devices (DAQs), to measure temperature, current, and voltage data directly from the demo board, and then graph the measurement results on the GUI. The software also collects vital telemetry data from the onboard devices via the PMBus/I2C protocol. All of this information is important for comparing system efficiencies and power losses.
Figure 4. DrMOS evaluation software showing configuration and thermal analysis tabs
Data and Results
The following test results cover steady-state performance measurements, functional performance waveforms, thermal measurements, and output noise measurements. The demo board was tested with the following configuration:
● Input voltage: 12 V
● Output voltage: 1 V
● Output load: 0 A to 60 A
● Switching frequency: 500 kHz and 1 MHz
● Performance data
Efficiency and Power Loss
The test results in Figure 5 show that the LTC7051 is more efficient (0.70% higher) than competing devices at a switching frequency of 500 kHz. The efficiency of the LTC7051 also gets better (0.95% improvement) as the switching frequency is further increased from 500 kHz to 1 MHz.
Figure 5. Efficiency and Power Loss at 1 V, Load 0 A to 60 A, Switching Frequency 500 kHz and 1 MHz
Notably, the LTC7051’s efficiency performance outperforms the competition at high output load currents and higher switching frequencies. This is the advantage of ADI’s patented Silent Switcher technology, which improves switching edge rates and reduces dead time, thereby reducing overall power loss. This enables smaller form factor solutions to operate at higher switching frequencies without significantly impacting overall efficiency. The lower the total power loss, the lower the operating temperature, the higher the output current and the higher the power density.
The LTC7051’s advantages in efficiency and power loss also contribute to its better thermal performance. A temperature difference of approximately 3°C to 10°C was observed between the LTC7051 and the competitor, with the former being lower, as shown in Figure 6. This better performance of the LTC7051 is due to its carefully designed thermally enhanced package.
Figure 6. Typical performance at 1 V output, 60 A load, 500 kHz and 1.0 MHz switching frequencies
As the ambient temperature increases from 25°C to 80°C, the temperature difference between the LTC7051 and the competition widens to approximately 15°C, which is also cooler.
Device Switch Node Performance
As can be seen in Figure 7, the peak drain-source voltage (VDS) of the LTC7051 is lower than competing devices. Furthermore, when the load is increased to 60 A, the VDS measured on the competing device is at its peak, while a long period of oscillation can be seen. However, the LTC7051 manages to reduce spikes and oscillations, again thanks to the Silent Switcher 2 architecture of the LTC705x DrMOS family and the integrated bootstrap capacitors inside. As a result, lower overshoot on the switch node means lower EMI and radiated and conducted noise, and higher reliability due to reduced switch node overvoltage stress.
Figure 7. Switching Node Waveforms at 1 V, Evaluated at 0 A and 60 A Loads
Device Output Ripple Performance
Another parameter is the output voltage ripple shown in Figure 8. As can be seen, the LTC7051 has less noise than competing devices. The noise reduction is due to the Silent Switcher technology resulting in lower VDS spikes and less oscillations on the switch node. If no switching node spikes are generated, there will be no conducted noise at the output.
Figure 8. Output Ripple Waveforms at 1 V, Evaluated at 0 A and 60 A Loads
Similarly, the LTC7051 and competing devices also performed output noise spread-spectrum measurements, as shown in Figure 9. The LTC7051 outperforms other DrMOS devices and has been shown to generate lower noise at switching frequencies than competing devices. The noise difference is about 1 mV rms.
Figure 9. Output Noise Spectral Response: Voltage 1 V, Load 60 A, Switching Frequency 1 MHz
The LTC7051 DrMOS Demonstration Platform can be used to fairly compare competing products. The LTC7051 integrates the SilentMOS™ architecture and bootstrap capacitors into a single thermally enhanced package to significantly improve power conversion efficiency and thermal performance when operating at high switching frequencies. Additionally, the LTC7051 reduces ringing and spike energy that not only manifests at the switch node but also propagates to the output. In practical applications, the output load requires tight tolerances, one of which is the nominal DC. However, noise due to high spike energy and ripple (which also appears on the output) can eat into the overall budget. Power-hungry data centers will yield considerable power and cost savings, not to mention the additional benefits of less thermal management and EMI (which can be significantly reduced and eventually eliminated), along with filter design and component placement Regulations will still be properly followed. All in all, the LTC7051 should be your first choice for power stage and DrMOS devices for your VRM design and application needs.
Source: ADI, by Christan Cruz, Joseph Viernes, Kareem Atout, Gary Sapia, and Marvin Neil Solis Cabueñas