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Texas Instruments DC/DC Converter Solutions for Hardware Accelerators in Data Center Applications Application notes
Application Report
SLVAEG2 – August 2019
DC/DC Converter Solutions for Hardware Accelerators in
Data Center Applications
Richard Nowakowski
ABSTRACT
Hardware accelerators are custom-made hardware designs on a circuit board that perform specific
functions better than software. Hardware accelerators use advanced processors, such as:
• FPGAs
• ASICs
• SoC
• GPUs
These processors are very suitable for performing specific, computation-intensive algorithms. Hardware
acceleration helps enable artificial intelligence, including special functionalities such as machine learning,
brain simulation, and neural engines. These functions use statistical techniques that allow computer
systems to learn from data without being programmed, similar to our understanding of how the brain
operates. Examples include 360° camera-view image recognition and speech recognition. The advanced
processors used in hardware accelerator applications need special attention from point-of-load power
management solutions, with features such as:
• Margining
• Adaptive Voltage Scaling (AVS)
• High temperature
• Safe Operating Area (SOA)
• High-current capability
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Contents
Suggested DC/DC Converters ............................................................................................
Adaptive Voltage Scaling ...................................................................................................
High Efficiency and Thermal Performance ...............................................................................
Current Sharing ..............................................................................................................
Voltage Regulation Accuracy ..............................................................................................
Conclusion ....................................................................................................................
Resources ....................................................................................................................
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2
4
5
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7
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List of Figures
1
Using LM10011 for AVS .................................................................................................... 3
2
TPS549B22 with Integrated PMBus Interface ........................................................................... 3
3
TPS543C20A SOA Curve
4
TPS543C20A Power Loss Curve .......................................................................................... 4
5
TPS543C20A in Stackable Configuration Supports up to 80 A
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7
.................................................................................................
.......................................................
Remote Sense Without Feedback Resistors .............................................................................
Remote Sense With Feedback Resistors ................................................................................
4
5
6
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List of Tables
1
Suggested Point-of-Load Converters ..................................................................................... 2
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1
Suggested DC/DC Converters
2
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...............................................................................
TPS54824 Feedback Voltage Regulation
5
Trademarks
PMBus is a trademark of SMIF Inc..
1
Suggested DC/DC Converters
Table 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. Converters with
similar note markings are pin-compatible.
Table 1. Suggested Point-of-Load Converters
2
Output Current
Converter
Converter with PMBus
Converter with PMBus and
Telemetry
≤4A
TPS54424*
-
-
4A–8A
TPS54824*
TPS53915#
-
8 A – 10 A
TPS54A24
TPS53915#
TPS544B20
8 A – 12 A
TPS53515#
TPS53915#
TPS544B20
#
#
12 A – 15 A
TPS548A20
TPS549A20
TPS546D24
15 A – 25 A
TPS543B20%
TPS549B22+
TPS546D24
25 A – 40 A
TPS543C20A%
TPS549D22+
TPS546D24
%
40 A – 80 A
TPS543C20A (x2)
TPS546D24 (x2)
TPS546D24 (x2)
80 A – 160 A
-
TPS546D24 (x4)
TPS546D24 (x4)
Adaptive Voltage Scaling
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 1, implementing a parallel
identification scheme by placing the LM10011 VID voltage programmer in the feedback loop of any
DC/DC converter.
2
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Adaptive Voltage Scaling
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Figure 1. Using LM10011 for AVS
(1)
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 InterIntegrated 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 datasheet 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 shows the TPS549B22
with PMB_DATA and PMB_CLK pins for serial communication, which support AVS and other
programmable features.
Figure 2. TPS549B22 with Integrated PMBus Interface
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High Efficiency and Thermal Performance
3
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High Efficiency and Thermal Performance
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, and the power-loss
plot in Figure 4, 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 RθJA to ψJT. 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, JEDECreferenced thermal numbers (1).
Figure 3. TPS543C20A SOA Curve
Figure 4. TPS543C20A Power Loss Curve
(1)
4
Analog Design Journal: "Understanding the thermal-resistance specification of DC/DC converters with integrated power MOSFETs"
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Current Sharing
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4
Current Sharing
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 5 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 TPS546D24 can be stacked up to four devices to support up to 160 A. For more
information, see reference design PMP21814.
Figure 5. TPS543C20A in Stackable Configuration Supports up to 80 A
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Voltage Regulation Accuracy
As semiconductor process technology advances, processors require tighter voltage accuracy and lower
operating voltages. The processor datasheet specifies the voltage tolerance as either a percentage or
value in mV, which includes DC, AC and ripple variations over the entire operating temperature range.
Designers must also consider the tolerance of the resistor divider used by the DC/DC converter, the
routing and trace losses of the circuit board, and variations in the application. These include input voltage
variations, temperature swings, and fast changes in the load.
It is important to check the feedback voltage accuracy of the DC/DC converter in the datasheet rather than
the front page. Table 2 shows the regulated feedback voltage specification of the TPS54824, which is a
4.5 to 17-V, 8-A converter, and shows that the reference accuracy is ±5mV, or ±0.83%, over input voltage
and temperature variations. The total output voltage accuracy is improved by choosing tighter tolerance
resistors. If more headroom is needed, designers can choose 0.1% or 0.5% resistors (2), even though they
may cost a little bit more. The additional headroom will allow the total ±3% or ±5% output voltage variation
to be met with less bulk and bypass capacitance.
Table 2. TPS54824 Feedback Voltage Regulation
Parameter
Regulated FB
Voltage
(2)
Test Condition
Min.
Typ.
Max.
Unit
TJ = -40°C to
150°C, VIN = 4.5 V
to 17 V
595
600
605
mV
TJ = 25°C, VIN = 4.5
V to 17 V
596
600
604
mV
Power Tip #18: Your regulator's output-voltage accuracy may not be as bad as you think
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Voltage Regulation Accuracy
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Often, layout constraints, connectors, and board density requirements affect the total output voltage
accuracy. A remote-sense feature of a DC/DC converter compensates for voltage drops from long-trace
lines to accommodate processors that need high-accuracy output voltage. This feature is especially useful
when routing higher currents, as the voltage drop can be a large portion of the overall DC error. Figure 6
shows the TPS543B20 using the remote-sense feature with voltage feedback resistors used to program
the output voltage. Figure 7 shows the TPS543B20 using the remote-sense feature, without voltage
feedback resistors, when the VSEL pin selects the reference voltage. The RSP and RSN pins are
extremely high-impedance input terminals of a true-differential, remote-sense amplifier.
Figure 6. Remote Sense Without Feedback Resistors
6
DC/DC Converter Solutions for Hardware Accelerators in Data Center
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Figure 7. Remote Sense With Feedback Resistors
6
Conclusion
Hardware accelerators need DC/DC converters that offer:
• Improved output voltage accuracy
• Fast transient response
• Adaptive voltage scaling
• High efficiency
• Excellent thermal performance
Texas Instruments offers high-performance point-of-load solutions to address these requirements. Visit
TI's end equipment page to learn more about power management solutions for hardware accelerator
applications.
7
Resources
•
•
•
Texas Instruments Training Video, How to meet DC voltage accuracy and AC load transient
specifications
Texas Instruments, PMP21814: 4-Phase, 160-A synchronous buck converter reference design using
TPS546D24
Texas Instruments, Hardware accelerator card end equipment page
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