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Application Note

Point-of-Load Solutions for Network Interface Cards (NIC)

Richard Nowakowski
ABSTRACT
Network Interface Cards (NIC) are crucial for business continuity and reliable communications by connecting
physical layer circuirty with data link layer standards, such as wired Ethernet or wireless networking. The
performance of the NIC card directly affects the data transfer rate of the overall network. Texas Instruments (TI)
provides performance power management solutions for use in NICs enabling low noise, high efficiency, reduced
external components and good thermal management when powering the NIC's network processors, memory,and other circuitry.

Table of Contents
1 Suggested DC/DC Converters............................................................................................................................................... 22 Low Ripple Noise....................................................................................................................................................................33 Voltage Regulation Accuracy................................................................................................................................................ 44 High Efficiency and Thermal Performance.......................................................................................................................... 65 Load Transient Response...................................................................................................................................................... 76 Linear Regulator with Adjustable Soft-Start........................................................................................................................ 97 Conclusion............................................................................................................................................................................ 108 Additional Resources........................................................................................................................................................... 11

List of Figures
Figure 2-1. TPS62913 Output Voltage Ripple After the Second-stage L-C Filter........................................................................3Figure 3-1. TPS543B20 Remote Sense Without Feedback Resistors........................................................................................ 4
Figure 3-2. TPS543B20 Remote Sense With Feedback Resistors............................................................................................. 5
Figure 3-3. TPS548A29 Differential Remote Sense Implementation...........................................................................................5Figure 4-1. TPS548A29 SOA Curve............................................................................................................................................6Figure 4-2. TPS548A29 Efficiency Curve.................................................................................................................................... 6Figure 5-1. Load Transient Response of a D-CAP3 Converter................................................................................................... 7
Figure 5-2. Asynchronous Pulse Injection and Body Braking......................................................................................................8Figure 6-1. TPS74801 Start-up Waveforms.................................................................................................................................9

List of Tables
Table 1-1. Suggested Point-of-Load Converters..........................................................................................................................2
Table 3-1. Internal Voltage Reference of the TPS548A29........................................................................................................... 4Trademarks
D-CAP3™ is a trademark of Texas Instruments.
All trademarks are the property of their respective owners.

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1

Suggested DC/DC Converters

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1 Suggested DC/DC Converters
Table 1-1 highlights the latest point-of-load DC/DC converters with integrated MOSFETs, linear regulators andpower modules applicable to powering NICs; however, they are designed to accommodate the requirements fora wide range of markets. These devices are designed to achieve fast transient response, low output voltage
ripple, high efficiency, good thermal performance, and high output voltage accuracy. Notice that different control
architectures are suggested in the table below.
Fixed-frequency control architectures provide a predictable switching frequency and can be synchronized to an
external clock. Current mode or voltage mode control is desirable in noise-sensitive applications that use data
converters and high-speed analog circuits. On the other hand, devices implementing constant on-time control
deliver a faster transient response than voltage or current mode control to quickly changing load profiles, since
there is no internal clock to control the switching frequency. Several devices feature PMBus or I2C with adaptive
voltage scaling and margining. Devices integrating PMBus or I2C with telemetry report voltage, current, andtemperature information to a host.
Table 1-1. Suggested Point-of-Load Converters
IOUT RangeConstant On-TimeControl

Fixed Frequency
Control

PMBus / I2C without
Telemetry

PMBus / I2C withTelemetry

Power Module with
Integrated Inductor

TPS62912

N/A

N/A

TPS82120

Switch Mode Power Supplies≤2A

TPS62148

2A–3A

TPS62135

TPS62913

N/A

N/A

TPS82130

3A–6A

TPS54J061

TPS543620

TPS542A50

TPS546A24A

TPSM84624

6 A – 10 A

TPS54JA20

TPS54A24

TPS542A50

TPS546A24A

LMZ31710

10 A – 15 A

TPS548A29

TPS542A52

TPS542A50

TPS546B24A

TPSM41615

15 A - 20 A

TPS548B28

TPS543B20

TPS549B22

TPS546B24A

TPSM41625

20 A – 25 A

TPS548B22

TPS543B20

TPS549B22

TPS546D24A

TPSM41625

25 A – 40 A

TPS548D22

TPS543C20A

TPS549D22

TPS546D24A

TPSM846C24

Low Drop Out Regulators

2IOUT

Device

1.5-A

TPS74801

Description
Linear Regulator with PG and Enable

2-A

TPS7A52

High-Accuracy (0.5%), Low-Noise (4.4 μVRMS), LDO Voltage Regulator with 65 mV Dropout

3-A

TPS7A53

High-Accuracy (0.5%), Low-Noise (4.4 μVRMS), LDO Voltage Regulator with 110 mVDropout

4-A

TPS7A54

High-Accuracy (0.5%), Low-Noise (4.4 μVRMS), LDO Voltage Regulator with 175 mVDropout

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Low Ripple Noise

2 Low Ripple Noise
Several Ethernet controllers on the market integrate SERDES, PLLs and I/O ports on-chip that require low
voltage ripple noise levels. The traditional method for achieving low output voltage ripple noise is is to use aDC/DC converter followed by a low-dropout regulator (LDO) such as the TPS7A52, TPS7A53 or TPS7A54.
This family of LDOs supports output currents up to 4-A with a low dropout voltage as low as 65 mV. As theoutput current increases, a linear regulator becomes less feasible due to added cost, board space, and power
loss. Besides a linear regulator, another method to achieve low noise along with a switching DC/DC converter
is to implement a second-stage L-C filter following the DC/DC converter. Every DC/DC converter generates
an output voltage ripple at its switching frequency. Ethernet controllers with integrated noise-sensitive circuitry
need low voltage ripple to minimize frequency spurs in the spectrum, which typically varies with switchingfrequency, inductor value, output capacitance, and equivalent series inductance and resistance. Low-ripple buckconverters such as the 2-A TPS62912 and 3-A TPS62913 leverage an external ferrite-bead filter by integrating
the compensation to accommodate the ferrite-bead. Using the inductance of the ferrite-bead along with anadditional output capacitor removes the high frequency components of the output voltage ripple and achieves
less than 10 uVrms ripple. Figure 2-1 shows the output voltage ripple with an input voltage of 12 V, an output
voltage of 1.2 V, and output current of 1-A.

Figure 2-1. TPS62913 Output Voltage Ripple After the Second-stage L-C Filter

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3Voltage Regulation Accuracy

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3 Voltage Regulation Accuracy
As the semiconductor process technology advances, processors require tighter voltage accuracy and loweroperating voltages. The processor data sheet specifies the voltage tolerance as either a percentage or as avalue in mV, which includes DC, AC and ripple variations over the entire operating temperature range. Designers
also consider the tolerance of the resistor divider used by the DC/DC converter, the routing and trace losses of
the circuit board, and the variations of the application, like the input voltage variations, temperature swings, andfast changes in the load.
Check the feedback voltage accuracy of the DC/DC converter in the data sheet rather than the front page. Table3-1 shows the regulated feedback voltage, or the internal voltage reference specification of the TPS548A29,
which is a 2.7 V to 16 V, 15-A converter, and shows that the reference accuracy is ±6 mV or ±1% over the fulltemperature range. The total output voltage accuracy is improved by choosing tighter tolerance resistors. If moreheadroom is needed, designers can choose 0.1% or 0.5% resistors, even though they may cost a little bit more.
The additional headroom allows a total ±3% or ±5% output voltage variation to be met with less bulk and bypass
capacitance. 1
Table 3-1. Internal Voltage Reference of the TPS548A29
Parameter
Internal Voltage
Reference

Test Condition

Minimum

TYP

Maximum

Unit

TJ = –40°C to 125°C, VCC = 3 V594

600606mV

TJ = 0°C to 85°C, VCC = 3 V

597

600603mV

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 toaccommodate processors needing high accuracy output voltage. This feature is especially useful when routinghigher currents since the voltage drop can be a large portion of the overall DC error.
Figure 3-1 shows the TPS543B20 using the remote sense feature with voltage feedback resistors used toprogram the output voltage. Figure 3-2 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 3-1. TPS543B20 Remote Sense Without Feedback Resistors

14

Power Tip #18: The output-voltage accuracy of the regulator may not be as bad as perceived.
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Voltage Regulation Accuracy

Figure 3-2. TPS543B20 Remote Sense With Feedback Resistors
The TPS548A29 also offers a differential remote sense function through the FB and VSNS– pins. Connecting
the FB voltage divider resistors to the remote location allows output voltage sensing at a remote location. The
ground connection of the remote sensing signal is connected to the VSNS– pin, and the VOUT connection of
the remote sensing signal is connected to the feedback resistor divider with the lower feedback resistor, RFB_LS
terminated at the VSNS– pin. To maintain a stable output voltage and minimize the ripple, the remote sensing
lines should stay away from any noise sources such as inductor and switch nodes, or high frequency clock lines.
It is recommended to shield the pair of remote sensing lines with ground planes above and below.

Figure 3-3. TPS548A29 Differential Remote Sense Implementation

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High Efficiency and Thermal Performance

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4 High Efficiency and Thermal Performance
NICs employ a circuit board with many PCB layers, depending on the form factor constraints. Since the NIC
is designed to be used in tight spaces, consider the thermal environment and available airflow when selecting
DC/DC converters. The ambient temperature inside a rack server, for example, is typically between 65°C and70°C since forced airflow is usually available. Higher circuit board temperatures translate to lower reliability, andhigher current DC/DC converters contribute to the overall thermal budget.
Figure 4-1 shows the safe operating curve and Figure 4-2 shows the efficiency plot of TPS548A29. The
TPS548A29 DC/DC converter it a 3x4mm QFN package safely delivers 15-A and 1 V output with the ambient
temperature of 81°C, without airflow. At 15-A and 1 V output, the total efficiency is about 86% at 800 kHz, whichtranslates to the entire solution dissipating less than 2.5 W and the IC dissipating about 1.7 W, knowing thejunction-to-ambient thermal resistance RθJA is 26.6° C/W for the evaluation board measured.
The TPS548A29 measured junction-to-ambient thermal resistance of 26.6° C/W is based on a 4-layer, 2-oz
Cu per layer and a 2.75 inch by 3 inch board size, which demonstrates low thermal resistance. However,many thermal metrics exist for semiconductor packages, which range from RθJA to RψJT. Often, designers
misapply these thermal metrics when trying to estimate the junction temperatures in their systems. Ultimately,
thermal performance depends on the circuit-board layout rather than using standard JEDEC-referenced thermalnumbers. 2

Figure 4-1. TPS548A29 SOA Curve

Figure 4-2. TPS548A29 Efficiency Curve

26Understanding the thermal-resistance specification of DC/DC converters with integrated power MOSFETs
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Load Transient Response

5 Load Transient Response
Since the load profile can change dramatically in an enterprise rack server, it is important to consider ACtransient performance. Choosing a DC/DC converter with a fast transient response using non-linear control
techniques, such as constant on-time or D-CAP3™, allows a fast transient response with minimal output
capacitance. A typical D-CAP3 converter design has three primary considerations for deciding the value ofthe output capacitance: transient (which includes load step and slew rate of the load step), output ripple, andstability. In applications where the load transient is stringent, the output capacitance is predominantly driven by
the transient requirement.
Today, a tight ripple requirement is becoming important in some high-end, high-performance application-specificintegrated circuit (ASIC) and field-programmable gate array (FPGA) designs. The LC output power stage
must be designed to meet the ripple criteria. For a D-CAP3-based design, there is a minimum capacitancerequirement in terms of small signal stability. This requirement prevents subharmonic, multiple-pulsing behavior
in all D-CAP modulators.
Figure 5-1 shows how D-CAP3 control mode works in a load transient case. The Calculating Output Capacitanceto Meet Transient and Ripple Requirements of an Integrated POL Converter Design Based on D-CAPx™
Modulators application report shows how to design the output filter for a 1-V output to accommodate a ±10 mV
ripple requirement and a ±30 mV transient voltage deviation using the D-CAP3 control mode.

Figure 5-1. Load Transient Response of a D-CAP3 Converter
The TPS543B20 converter features a new control mode called Advanced Current Mode (ACM), which isan internally compensated emulated peak-current-mode control, with a clock synchronizable, fixed-frequency
modulator. The internal integrator and directly amplifying ramp tracking loop eliminate the need for external
compensation over a wide range of frequencies. The TPS543B20 also features Asynchronous Pulse Injection(API) and body braking to improve transient performance by significantly reducing undershoot and overshoot,respectively, reducing the external capacitance requirement. When the API and body diode braking areactivated, ACM delivers a similar transient performance as D-CAP3. Figure 5-2 shows examples of output
voltage undershoot and overshoot reduction with API and body braking. 3

3Comparing internally-compensated advanced current mode (ACM) with D-CAP3™ control

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Load Transient Response

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Figure 5-2. Asynchronous Pulse Injection and Body Braking

8

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Linear Regulator with Adjustable Soft-Start

6 Linear Regulator with Adjustable Soft-Start
For low current rails, a linear regulator with a user-programmable soft-start minimizes stress on the input powersource by reducing the capacitive inrush current on start-up, especially from highly capacitive loads or withprocessors requiring many bypass capacitors. The TPS74801 adjustable linear regulator provides a monotonic(non-negative slope) soft-start that is well-suited for powering many different types of processors. Figure 6-1
illustrates the start-up waveform showing the start-up times varying over several soft start capacitor values.

Figure 6-1. TPS74801 Start-up Waveforms

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7 Conclusion
The performance of network interface cards benefits from DC/DC converters that offer improved output
voltage accuracy, low output voltage ripple, fast transient response, reduced external component count, highefficiency and excellent thermal performance. TI offers high performance point-of-load solutions including DC/DCconverters, power modules, and linear regulators to address these design requirements.

10Point-of-Load Solutions for Network Interface Cards (NIC)
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Additional Resources

8 Additional Resources
••
•
•
•

Texas Instruments, How to Meet DC Voltage Accuracy and AC Load Transient Specification? TI training
video.
Texas Instruments, Minimize Noise and Ripple with a Low-noise Buck Converter blog.
Texas Instruments, Accuracy-Enhanced Ramp-Generation Design for D-CAP3 Modulation application report.
Texas Instruments, Internally Compensated Advanced Current Mode, white paper.
Texas Instruments, Control Mode Quick Reference Guide, Step-Down Non-Isolated DC/DCselection guide.
Texas Instruments, TI Network Interface Card.

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