Power system design optimization tips: Monolithic driver + MOSFET (DrMOS)

At present, multi-core architecture makes microprocessors denser and faster on a horizontal scale, which dramatically increases the power required by these devices. This directly leads to the need to upgrade the voltage regulator module (VRM) that powers the microprocessor: First, the power density (power per unit volume) of the voltage regulator must be upgraded. In order to meet the high power requirements of the system in a limited space, the power density must be greatly improved; the other is to improve the power conversion efficiency. High efficiency can reduce power loss and improve thermal management.

A widely accepted solution in the power supply industry is to integrate advanced switching MOSFETs (the main component of the regulator) and their corresponding drivers into a single chip and use advanced packaging to achieve compact and efficient power conversion. This DrMOS power stage optimizes high-speed power conversion.

As the demand for this power stage (known as the smart power stage) has steadily increased, and power switch technology has continued to advance, Analog Devices has introduced a DrMOS version of the smart power module. The LTC705x DrMOS family leverages ADI's patented Silent Switcher® 2 architecture and integrates a bootstrap circuit that enables the DrMOS module to switch at ultra-fast speeds while reducing power losses and switch node voltage overshoot for improved performance. LTC705x DrMOS devices also offer 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 belongs to the LTC705x DrMOS series and is a 140A monolithic intelligent power module that successfully integrates high-speed drivers with high-quality factor (FOM) top and bottom power MOSFETs and comprehensive monitoring and protection circuits into an electrically and thermally resistant package. Together with the appropriate PWM controller, this intelligent power stage provides market-leading high-efficiency, low-noise, and high-density power conversion. This combination enables high-current regulator modules to have the latest efficiency and transient response technology. The typical application of the LTC7051 is shown in Figure 1. It acts as the main switching circuit of the buck converter, and is matched with the LTC3861, a dual-channel multi-phase buck voltage mode DC-DC controller with precise current sharing characteristics.

Figure 1. Dual-Phase POL Converter

To demonstrate the key features of the LTC7051, Analog Devices created an evaluation board to showcase the performance of the LTC7051 versus competing products. This demonstration platform helps compare the basic parameters of the LTC7051 DrMOS with competing products in an unbiased and accurate manner, such as efficiency, power loss, telemetry accuracy, thermal and electrical performance. The purpose of the comparison is to eliminate any doubts about the validity of the results. The demonstration platform is used to highlight best-in-class DrMOS performance indicators, regardless of the manufacturer.

DrMOS Analysis and Evaluation Hardware

The analysis demonstration hardware has the following key features:

►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 and competing devices use the same power stage design.

► LTpowerPlay® power system management environment for comprehensive telemetry of system performance provided by the LTC7883.

►Can withstand extended ambient temperatures, as specified for ADI and competitive devices over their operating temperature range.

►Circuit boards are designed to capture and measure heat easily.

Figure 2. Block diagram of the analysis demo board

The DrMOS analysis demonstration board is shown in Figure 3. The board has been carefully designed with the key features mentioned previously. Components are placed symmetrically and systematically on each power rail with the same PCB size and area to limit the differences between the power rails. The layout and layer stackup are also done symmetrically.

Figure 3. DrMOS evaluation board, top and bottom. PCB dimensions: 203mm × 152mm × 1.67mm (L × H × W), 2oz copper thickness.

DrMOS analysis test method and software

In addition to the demo board itself, the test setup and test methods are equally important for the fairness 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 support users to conduct tests and collect data more easily. Users only need to specify the input and output parameters, and the software will be responsible for automated testing. The software automatically controls the corresponding test and measurement equipment, such as DC power supplies, electronic loads, and multiplexed data acquisition devices (DAQ) to measure temperature, current, and voltage data directly from the demo board, and then presents the measurement result curves on the GUI. The software also collects important telemetry data from on-board devices through the PMBus/I2C protocol. All this information is important for comparing system efficiency and power loss.

Figure 4. DrMOS evaluation software showing the 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 using the following configuration:

►Input voltage: 12V

►Output voltage: 1V

►Output load: 0A to 60A

►Switching frequency: 500kHz and 1MHz

Performance Data

 Efficiency and power loss

The test results in Figure 5 show that at a switching frequency of 500kHz, the LTC7051 has higher efficiency (0.70% higher) than competing devices. As the switching frequency is further increased from 500kHz to 1MHz, the efficiency of the LTC7051 also becomes better (0.95% higher).

Figure 5. Efficiency and power loss at 1V with loads from 0A to 60A and switching frequencies of 500kHz and 1MHz, respectively

Efficiency performance

It is worth noting that at high output load currents and higher switching frequencies, the LTC7051's efficiency performance outperforms competing products. This is the benefit of ADI's patented Silent Switcher technology, which improves switching edge rates and reduces dead time, thereby reducing overall power losses. This enables smaller solutions to operate at higher switching frequencies without significantly affecting overall efficiency. Lower total power losses allow for cooler operation, higher output currents, and significantly improved power density.

Thermal performance

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 is observed between the LTC7051 and competing products, with the former running cooler, as shown in Figure 6. This better performance of the LTC7051 is attributed to its carefully designed thermally enhanced package.

Figure 6. Typical performance at 1V output, 60A load, and 500kHz and 1.0MHz switching frequencies

As the ambient temperature increases from 25°C to 80°C, the temperature difference between the LTC7051 and the competition widens to about 15°C, with the former also running cooler.

Device switch node performance

As can be seen in Figure 7, the drain-source voltage (VDS) peak of the LTC7051 is lower than that of competing devices. In addition, when the load is increased to 60A, the VDS measured on the competing device is at a peak, and long-term ringing can be seen. However, the LTC7051 manages to reduce the spikes and ringing, again thanks to the Silent Switcher 2 architecture of the LTC705x DrMOS family and the internal integrated bootstrap capacitor. As a result, the overshoot on the switch node is lower, which means lower EMI and radiated and conducted noise, and higher reliability due to reduced switch node overvoltage stress.

Figure 7. Switch node waveform at 1V, evaluated at 0A and 60A load

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 reason for the reduced noise is that the Silent Switcher technology results in lower VDS spikes and less oscillation on the switch node. If there is no switch node spike, there will be no conducted noise on the output.

Figure 8. Output ripple waveform at 1V, evaluated at 0A and 60A loads

Likewise, output noise spread spectrum measurements were performed on the LTC7051 and competing devices, as shown in Figure 9. The LTC7051 outperforms the other DrMOS devices and is shown to produce less noise at the switching frequency than the competing devices. The noise difference is approximately 1 mV rms.

Figure 9. Output noise spectrum response: 1V, 60A load, 1MHz switching frequency

in conclusion

The LTC7051 DrMOS demonstration platform can be used to compare competitive products fairly. The LTC7051 integrates SilentMOS™ architecture and bootstrap capacitors into a single thermally enhanced package, which can significantly improve power conversion efficiency and thermal performance when operating at high switching frequencies. In addition, the LTC7051 can reduce ringing and spike energy, which not only appears on the switch node but also propagates to the output. In real applications, output loads require tight tolerances, one of which is nominal DC. However, the noise caused by high spike energy and ripple (which also appears at the output) will consume the overall budget. Data centers with huge power requirements will save considerable power and costs, not to mention the additional benefits of less thermal management and EMI (which will be significantly reduced or even eliminated), while filter design and component placement regulations are still correctly followed. In summary, the LTC7051 is clearly the preferred power stage and DrMOS device for enterprises today, providing strong support for VRM design and application needs.