Table 1-1 highlights the latest point-of-load DC/DC converters with integrated MOSFETs that are applicable for hardware accelerator applications from either a 5-V or 12-V input bus. These devices are designed to achieve high-output voltage accuracy, high efficiency, and good thermal performance. Several devices feature a Power Management Bus (PMBus™) with adaptive voltage scaling and margining. PMBus devices integrate telemetry report voltage, current, and temperature information to a host.

Adaptive voltage scaling (AVS) is the adaptation or modification of the supply voltage for a processor (given the processing strength). The supply voltage of the DC/DC converter can be adjusted to minimize power while still achieving desired performance. In hardware accelerator applications, the processor can allow the DC/DC converter to increase or decrease supply voltage based on the required performance. When the supply voltage is reduced, the power consumption of the processor is reduced. This is displayed in Equation 1, where C is the transistor capacitance of the processor. Since power consumption and heat generation are key concerns, especially in high ambient temperature environments, using AVS results in substantially improved processor thermal performance, energy savings, and long-term reliability. Advanced processors typically use a serial communication protocol to set the DC/DC converter’s output voltage, but there are other alternative approaches. These include using external MOSFETs to adjust the voltage divider resistors of the feedback loop, or, as shown in Figure 2-1, implementing a parallel identification scheme by placing the LM10011 VID voltage programmer in the feedback loop of any DC/DC converter.

Figure 2-1 Using LM10011 for AVS

DC/DC converters with an integrated serial bus supporting AVS are the easiest to implement. The PMBus protocol is a simple and powerful open-industry specification that unifies the communication standards for digital power-management systems and power conversion devices. It is the widely accepted Inter-Integrated Circuit (I²C) communication protocol for defining the physical layer. Many DC/DC converters from Texas Instruments with PMBus feature the VOUT_COMMAND to adjust the output voltage on-the-fly. The output voltage can also be adjusted on-the-fly by the VOUT_MARGIN_HIGH and VOUT_MARGIN_LOW commands. The support range for the VOUT_COMMAND of the 25-A TPS549D22, for example, is 0.5996 V to 1.1992 V. Please note that supported PMBus commands differ from one DC/DC converter to another, and it is wise to check the data sheet for the list of supported commands. Any power management device only needs one command to be PMBus-compliant. More DC/DC converters feature telemetry to read voltage, current, and temperature for improved thermal management capability and fault reporting, in addition to supporting AVS. Figure 2-2 shows the TPS549B22 with PMB_DATA and PMB_CLK pins for serial communication, which support AVS and other programmable features.

Figure 2-2 TPS549B22 with Integrated PMBus Interface

Depending on form factor constraints, hardware accelerators are built on circuit boards with many PCB layers. Since the hardware accelerator is typically designed for use in tight spaces, special attention must be taken in selecting DC/DC converters to ensure the application operates in thermally challenging environments with the available airflow. This is displayed in the SOA curve of Figure 3-1, and the power-loss plot in Figure 3-2, the TPS543C20A DC/DC converter delivers 40-A and 1-V output with an ambient temperature of 75°C without airflow. At 25-A, 12-V input and 1-V output, the entire solution dissipates less than 3 W, which translates to around 90% efficiency when switching at 500 kHz. The TPS543C20A measured junction-to-ambient thermal resistance is 12°C/W based on a six-layer, 2-ounce Cu per layer, and 2.75 inch by 3-inch board size, which demonstrates low thermal resistance. However, many thermal metrics exist for semiconductor and integrated circuit packages, which range from R_θJA to ψ_JT. Often, designers misapply these thermal metrics when trying to estimate the junction temperatures in a system. Ultimately, thermal performance depends on the circuit board layout and using standard, JEDEC-referenced thermal numbers ⁽¹⁾.

Figure 3-1 TPS543C20A SOA Curve

Figure 3-2 TPS543C20A Power Loss Curve

When two devices operate in a dual-phase, stackable application, a current-sharing loop maintains the current balance between devices. Both devices share the same internal control voltage through VSHARE pin. The sensed current in each phase is first compared in a current-share block by connecting the ISHARE pin of each device, and then the error current is added into the internal loop. Connect the SYNC pin of the master and slave converters to share switching frequency information. The resulting voltage is compared with the PWM ramp to generate the PWM pulse for DC/DC conversion. Figure 4-1 shows the TPS543C20A in a stackable configuration to current-share up to 80-A while operating 180 degrees out of phase. Not only does a stackable configuration support higher currents, but it also reduces input ripple with out-of-phase operation, and improves overall system thermal performance, since the generated heat is spread over more circuit board area. If higher current is desired from an integrated MOSFET DC/DC converter, the TPS546D24A can be stacked up to four devices to support up to 160 A. For more information, see the reference design PMP21814.

Figure 4-1 TPS543C20A in Stackable Configuration Supports up to 80 A