LP2998MAX/NOPB - NATIONAL SEMICONDUCTOR

LP2998

LP2998 DDR-I and DDR-II Termination Regulator

Literature Number: SNVS521G

LP2998

March 28, 2011

DDR-I and DDR-II Termination Regulator

General Description

The LP2998 linear regulator is designed to meet JEDEC

SSTL-2 and JEDEC SSTL-18 specifications for termination of

DDR1-SDRAM and DDR-II memory. The device contains a

high-speed operational amplifier to provide excellent re-

sponse to load transients. The output stage prevents shoot

through while delivering 1.5A continuous current as required

for DDR1-SDRAM termination, and 0.5A continuous current

as required for DDR-II termination. The LP2998 also incor-

porates a VSENSE pin to provide superior load regulation and

a VREF output as a reference for the chipset and DIMMs.

An additional feature found on the LP2998 is an active low

shutdown (SD) pin that provides Suspend To RAM (STR)

functionality. When SD is pulled low, the VTT output will tri-

state providing a high impedance output, while VREF remains

active. A power savings advantage can be obtained in this

mode through lower quiescent current.

Features

■Source and sink current

■Low output voltage offset

■No external resistors required

■Linear topology

■Suspend to Ram (STR) functionality

■Low external component count

■Thermal Shutdown

■Available in SO-8, PSOP-8 packages

Applications

■DDR-I, DDR-II and DDR-III Termination Voltage

■SSTL-18 Termination

■SSTL-2 and SSTL-3 Termination

■HSTL Termination

Typical Application Circuit

30026918

LP2998 DDR-I and DDR-II Termination Regulator

Connection Diagrams

30026903

Top View

PSOP-8 Layout

30026904

Top View

SO-8 Layout

Pin Descriptions

SO-8 Pin or PSOP-8 Pin Name Function

1 GND Ground.

2 SD Shutdown.

3 VSENSE Feedback pin for regulating VTT.

4 VREF Buffered internal reference voltage of VDDQ /2.

5 VDDQ Input for internal reference equal to VDDQ/2.

6 AVIN Analog input pin.

7 PVIN Power input pin.

8 VTT Output voltage for connection to termination resistors.

EP Exposed pad thermal connection. Connect to soft Ground (PSOP-8 only).

Ordering Information

Order Number Package Type NSC Package Drawing Supplied As

LP2998MA SO-8 M08A 95 Units per Rail

LP2998MAX SO-8 M08A 2500 Units Tape and Reel

LP2998MAE SO-8 M08A 250 Units Tape and Reel

LP2998MR PSOP-8 MRA08A 95 Units per Rail

LP2998MRX PSOP-8 MRA08A 2500 Units Tape and Reel

LP2998MRE PSOP-8 MRA08A 250 Units Tape and Reel

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LP2998

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

AVIN to GND −0.3V to +6V

PVIN to GND -0.3V to AVIN

VDDQ (Note 2) −0.3V to +6V

Storage Temp. Range −65°C to +150°C

Junction Temperature 150°C

Lead Temperature (Soldering, 10 sec) 260°C

SO-8 Thermal Resistance (θJA)151°C/W

PSOP-8 Thermal Resistance (θJA)43°C/W

Minimum ESD Rating (Note 3) 1kV

Operating Range

Junction Temp. Range (Note 4) -40°C to +125°C

AVIN to GND 2.2V to 5.5V

Electrical Characteristics Specifications with standard typeface are for TJ = 25°C and limits in boldface type

apply over the full Operating Temperature Range (TJ = -40°C to +125°C) (Note 5). Unless otherwise specified,

VIN = AVIN = PVIN = 2.5V.

Symbol Parameter Conditions Min Typ Max Units

VREF

VREF Voltage (DDR I) VIN = VDDQ = 2.3V 1.135 1.158 1.185 V

VIN = VDDQ = 2.5V 1.235 1.258 1.285 V

VIN = VDDQ = 2.7V 1.335 1.358 1.385 V

VREF Voltage (DDR II) PVIN = VDDQ = 1.7V 0.837 0.860 0.887 V

PVIN = VDDQ = 1.8V 0.887 0.910 0.937 V

PVIN = VDDQ = 1.9V 0.936 0.959 0.986 V

VREF Voltage (DDR III) PVIN = VDDQ = 1.35V 0.669 0.684 0.699 V

PVIN = VDDQ = 1.5V 0.743 0.758 0.773 V

PVIN = VDDQ = 1.6V 0.793 0.808 0.823 V

ZVREF VREF Output Impedance IREF = -30 to +30 µA 2.5 kΩ

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LP2998

Symbol Parameter Conditions Min Typ Max Units

VTT

VTT Output Voltage (DDR I) (Note 8)

IOUT = 0A

VIN = VDDQ = 2.3V 1.120 1.159 1.190 V

VIN = VDDQ = 2.5V 1.210 1.259 1.290 V

VIN = VDDQ = 2.7V 1.320 1.359 1.390 V

IOUT = +/- 1.5A

VIN = VDDQ = 2.3V 1.125 1.159 1.190 V

VIN = VDDQ = 2.5V 1.225 1.259 1.290 V

VIN = VDDQ = 2.7V 1.325 1.359 1.390 V

VTT Output Voltage (DDR II) (Note 8)

IOUT = 0A, AVIN = 2.5V

PVIN = VDDQ = 1.7V 0.822 0.856 0.887 V

PVIN = VDDQ = 1.8V 0.874 0.908 0.939 V

PVIN = VDDQ = 1.9V 0.923 0.957 0.988 V

IOUT = +/- 0.5A, AVIN = 2.5V

PVIN = VDDQ = 1.7V 0.820 0.856 0.890 V

PVIN = VDDQ = 1.8V 0.870 0.908 0.940 V

PVIN = VDDQ = 1.9V 0.920 0.957 0.990 V

VTT Output Voltage (DDR III) (Note 8)

IOUT = 0A, AVIN = 2.5V

PVIN = VDDQ = 1.35V 0.656 0.677 0.698 V

PVIN = VDDQ = 1.5V 0.731 0.752 0.773 V

PVIN = VDDQ = 1.6V 0.781 0.802 0.823 V

IOUT = +0.2A, AVIN = 2.5V

PVIN = VDDQ = 1.35V

0.667 0.688 0.710 V

IOUT = -0.2A, AVIN = 2.5V

PVIN = VDDQ = 1.35V

0.641 0.673 0.694 V

IOUT = +0.4A, AVIN = 2.5V

PVIN = VDDQ = 1.5V

0.740 0.763 0.786 V

IOUT = -0.4A, AVIN = 2.5V

PVIN = VDDQ = 1.5V

0.731 0.752 0.773 V

IOUT = +0.5A, AVIN = 2.5V

PVIN = VDDQ = 1.6V

0.790 0.813 0.836 V

IOUT = -0.5A, AVIN = 2.5V

PVIN = VDDQ = 1.6V

0.781 0.802 0.823 V

VOSVtt

VTT Output Voltage Offset (VREF – VTT) for DDR I (Note 8) IOUT = 0A -30 030 mV

IOUT = -1.5A -30 030 mV

IOUT = +1.5A -30 030 mV

VTT Output Voltage Offset (VREF – VTT) for DDR II (Note 8)

IOUT = 0A -30 030 mV

IOUT = -0.5A -30 030 mV

IOUT = +0.5A -30 030 mV

VTT Output Voltage Offset (VREF – VTT) for DDR III (Note 8) IOUT = 0A -30 030 mV

IOUT = ±0.2A -30 030 mV

IOUT = ±0.4A -30 030 mV

IOUT = ±0.5A -30 030 mV

IQQuiescent Current (Note 6) IOUT = 0A 320 500 µA

ZVDDQ VDDQ Input Impedance 100 kΩ

ISD Quiescent current in shutdown (Note 6) SD = 0V 115 150 µA

IQ_SD Shutdown leakage current SD = 0V 2 5µA

VIH Minimum Shutdown High Level 1.9 V

VIL Maximum Shutdown Low Level 0.8 V

Iv VTT leakage current in shutdown SD = 0V

VTT = 1.25V

1 10 µA

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LP2998

Symbol Parameter Conditions Min Typ Max Units

ISENSE VSENSE Input current 13 nA

TSD Thermal Shutdown (Note 7) 165 °C

TSD_HYS Thermal Shutdown Hysteresis 10 °C

Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. Operating range indicates conditions for which the device is

intended to be functional, but does not guarantee specific performance limits. For guaranteed specifications and test conditions see Electrical Characteristics.

The guaranteed specifications apply only for the test conditions listed. Some performance characteristics may degrade when the device is not operated under

the listed test conditions.

Note 2: VDDQ voltage must be less than 2 x (AVIN - 1) or 6V, whichever is smaller.

Note 3: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin.

Note 4: At elevated temperatures, devices must be derated based on thermal resistance. The device in the SO-8 package must be derated at θJA = 151.2° C/W

junction to ambient with no heat sink.

Note 5: Limits are 100% production tested at 25°C. Limits over the operating temperature range are guaranteed through correlation using Statistical Quality

Control (SQC) methods. The limits are used to calculate National's Average Outgoing Quality Level (AOQL).

Note 6: Quiescent current is defined as the current flow into AVIN.

Note 7: The maximum allowable power dissipation is a function of the maximum junction temperature, TJ(MAX), the junction to ambient thermal resistance, θJA,

and the ambient temperature, TA. Exceeding the maximum allowable power dissipation will cause excessive die temperature and the regulator will go into thermal

shutdown.

Note 8: VTT load regulation is tested by using a 10 ms current pulse and measuring VTT.

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LP2998

Typical Performance Characteristics Unless otherwise specified VIN = AVIN = PVIN = 2.5V

Iq vs AVIN in SD

30026920

Iq vs AVIN

30026921

VIH and VIL

30026922

VREF vs VDDQ

30026924

VTT vs VDDQ

30026926

ISD vs AVIN over Temperature

30026927

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LP2998

Iq vs AVIN over Temperature

30026928

Maximum Sourcing Current vs AVIN

(VDDQ = 1.8V, PVIN = 1.8V)

30026935

Maximum Sinking Current vs AVIN

(VDDQ = 1.8V)

30026936

Maximum Sourcing Current vs AVIN

(VDDQ = 2.5V, PVIN = 1.8V)

30026931

Maximum Sourcing Current vs AVIN

(VDDQ = 2.5V, PVIN = 2.5V)

30026932

Maximum Sourcing Current vs AVIN

(VDDQ = 2.5V, PVIN = 3.3V)

30026933

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LP2998

Maximum Sinking Current vs AVIN

(VDDQ = 2.5V)

30026934

Maximum Sourcing Current vs AVIN

(VDDQ = 1.8V, PVIN = 3.3V)

30026937

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LP2998

Block Diagram

30026905

Description

The LP2998 is a linear bus termination regulator designed to

meet the JEDEC requirements of SSTL-2 and SSTL-18. The

output, VTT is capable of sinking and sourcing current while

regulating the output voltage equal to VDDQ / 2. The output

stage has been designed to maintain excellent load regulation

while preventing shoot through. The LP2998 also incorpo-

rates two distinct power rails that separates the analog cir-

cuitry from the power output stage. This allows a split rail

approach to be utilized to decrease internal power dissipation.

It also permits the LP2998 to provide a termination solution

for the next generation of DDR-SDRAM memory (DDRII). The

LP2998 can also be used to provide a termination voltage for

other logic schemes such as SSTL-3 or HSTL.

Series Stub Termination Logic (SSTL) was created to im-

prove signal integrity of the data transmission across the

memory bus. This termination scheme is essential to prevent

data error from signal reflections while transmitting at high

frequencies encountered with DDR-SDRAM. The most com-

mon form of termination is Class II single parallel termination.

This involves one RS series resistor from the chipset to the

memory and one RT termination resistor. Typical values for

RS and RT are 25 Ohms, although these can be changed to

scale the current requirements from the LP2998. This imple-

mentation can be seen below in .

30026906

FIGURE 1. SSTL-Termination Scheme

Pin Descriptions

AVIN AND PVIN

AVIN and PVIN are the input supply pins for the LP2998. AVIN

is used to supply all the internal control circuitry. PVIN, how-

ever, is used exclusively to provide the rail voltage for the

output stage used to create VTT. These pins have the capa-

bility to work off separate supplies, under the condition that

AVIN is always greater than or equal to PVIN. For SSTL-18

applications, it is recommended to connect PVIN to the 1.8V

rail used for the memory core and AVIN to a rail within its

operating range of 2.2V to 5.5V (typically a 2.5V supply). PVIN

should always be used with either a 1.8V or 2.5V rail. This

prevents the thermal limit from tripping because of excessive

internal power dissipation. If the junction temperature ex-

ceeds the thermal shutdown threshold, the part will enter a

shutdown state identical to the manual shutdown where VTT

is tri-stated and VREF remains active. A lower rail, such as

1.5V can be used but it will reduce the maximum output cur-

rent. Therefore it is not recommended for most termination

schemes.

VDDQ

VDDQ is the input used to create the internal reference volt-

age for regulating VTT. The reference voltage is generated

from a resistor divider of two internal 50kΩ resistors. This

guarantees that VTT will precisely track VDDQ / 2. The optimal

implementation of VDDQ is as a remote sense. This can be

achieved by connecting VDDQ directly to the 1.8V rail at the

DIMM instead of PVIN. This ensures that the reference volt-

age precisely tracks the DDR memory rails without a large

voltage drop from the power lines. For SSTL-18 applications,

VDDQ will be a 1.8V signal, which will create a 0.9V termina-

tion voltage at VTT (See Electrical Characteristics Table for

exact values of VTT over temperature).

VSENSE

The purpose of the sense pin is to provide improved remote

load regulation. In most motherboard applications, the termi-

nation resistors will connect to VTT in a long plane. If the output

voltage was regulated only at the output of the LP2998, then

the long trace will cause a significant IR drop resulting in a

termination voltage lower at one end of the bus than the other.

The VSENSE pin can be used to improve this performance by

connecting it to the middle of the bus. This will provide a better

distribution across the entire termination bus. If remote load

regulation is not used, then the VSENSE pin must still be con-

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LP2998

nected to VTT. Care should be taken when a long VSENSE trace

is implemented in close proximity to the memory. Noise pick-

up in the VSENSE trace can cause problems with precise

regulation of VTT. A small 0.1uF ceramic capacitor placed next

to the VSENSE pin can help filter any high frequency signals

and prevent errors.

SHUTDOWN

The LP2998 contains an active low shutdown pin that can be

used for suspend to RAM functionality. In this condition, the

VTT output will tri-state while the VREF output remains active

providing a constant reference signal for the memory and

chipset. During shutdown, VTT should not be exposed to volt-

ages that exceed PVIN. With the shutdown pin asserted low

the quiescent current of the LP2998 will drop. However, VD-

DQ will always maintain its constant impedance of 100kΩ for

generating the internal reference. Therefore, to calculate the

total power loss in shutdown, both currents need to be con-

sidered. For more information refer to the Thermal Dissipation

section. The shutdown pin also has an internal pull-up current.

Therefore, to turn the part on, the shutdown pin can either be

connected to AVIN or left open.

VREF

VREF provides the buffered output of the internal reference

voltage VDDQ / 2. This output should be used to provide the

reference voltage for the Northbridge chipset and memory.

Since the inputs typically have an extremely high impedance,

there should be little current drawn from VREF. For improved

performance, an output bypass capacitor can be placed,

close to the pin, to help with noise. A ceramic capacitor in the

range of 0.1 µF to 0.01 µF is recommended. This output re-

mains active during the shutdown state and thermal shutdown

events for the suspend to RAM functionality.

VTT

VTT is the regulated output that is used to terminate the bus

resistors. It is capable of sinking and sourcing current while

regulating the output precisely to VDDQ / 2. The LP2998 is

designed to handle continuous currents of up to +/- 1.5A with

excellent load regulation. If a transient is expected to last

above the maximum continuous current rating for a significant

amount of time, then the bulk output capacitor should be sized

large enough to prevent an excessive voltage drop. If the

LP2998 is to operate in elevated temperatures for long dura-

tions, care should be taken to ensure that the maximum

operating junction temperature is not exceeded. Proper ther-

mal de-rating should always be used (Please refer to the

Thermal Dissipation section). If the junction temperature ex-

ceeds the thermal shutdown threshold, VTT will tri-state until

the part returns below the temperature hysteresis trip-point.

Component Selections

INPUT CAPACITOR

The LP2998 does not require a capacitor for input stability,

but it is recommended for improved performance during large

load transients to prevent the input rail from dropping. The

input capacitor should be located as close as possible to the

PVIN pin. Several recommendations exist and is dependent

on the application required. A typical value recommended for

AL electrolytic capacitors is 22 µF. Ceramic capacitors can

also be used. A value in the range of 10 µF with X5R or better

would be an ideal choice. The input capacitance can be re-

duced if the LP2998 is placed close to the bulk capacitance

from the output of the 1.8V DC-DC converter. For the AVIN

pin, a small 0.1uF ceramic capacitor is sufficient to prevent

excessive noise from coupling into the device.

OUTPUT CAPACITOR

The LP2998 has been designed to be insensitive of output

capacitor size or ESR (Equivalent Series Resistance). This

allows the flexibility to use any capacitor desired. The choice

for output capacitor will be determined solely on the applica-

tion and the requirements for load transient response of VTT.

As a general recommendation the output capacitor should be

sized above 100 µF with a low ESR for SSTL applications with

DDR-SDRAM. The value of ESR should be determined by the

maximum current spikes expected and the extent at which the

output voltage is allowed to droop. Several capacitor options

are available on the market and a few of these are highlighted

below:

AL - It should be noted that many aluminum electrolytics only

specify impedance at a frequency of 120 Hz, which indicates

they have poor high frequency performance. Only aluminum

electrolytics that have an impedance specified at a higher fre-

quency (100 kHz) should be used for the LP2998. To improve

the ESR several AL electrolytics can be combined in parallel

for an overall reduction. An important note to be aware of is

the extent at which the ESR will change over temperature.

Aluminum electrolytic capacitors tend to have rapidly increas-

ing ESR at cold temperatures.

Ceramic - Ceramic capacitors typically have a low capaci-

tance, in the range of 10 to 100 µF. They also have excellent

AC performance for bypassing noise because of very low

ESR (typically less than 10 mΩ). However, some dielectric

types do not have good capacitance characteristics as a func-

tion of voltage and temperature. Because of the typically low

value of capacitance, it is recommended to use ceramic ca-

pacitors in parallel with another capacitor such as an alu-

minum electrolytic. A dielectric of X5R or better is recom-

mended for all ceramic capacitors.

Hybrid - Several hybrid capacitors such as OS-CON and SP

are available from several manufacturers. These offer a large

capacitance while maintaining a low ESR. These are the best

solution when size and performance are critical, although

their cost is typically higher than any other capacitor type.

Thermal Dissipation

Since the LP2998 is a linear regulator, any current flow from

VTT will result in internal power dissipation and heat genera-

tion. To prevent damaging the part by exceeding the maxi-

mum allowable operating junction temperature, care should

be taken to derate the part based on the maximum expected

ambient temperature and power dissipation. The maximum

allowable internal temperature rise (TRmax) can be calculated

given the maximum ambient temperature (TAmax) of the ap-

plication and the maximum allowable junction temperature

(TJmax).

TRmax = TJmax − TAmax

From this equation, the maximum power dissipation (PDmax)

of the part can be calculated:

PDmax = TRmax / θJA

The θJA of the LP2998 will depend on several variables: the

package used; the thickness of copper; the number of vias

and the airflow. For instance, the θJA of the SO-8 is 163°C/W

with the package mounted to a standard 8x4 2-layer board

with 1oz. copper, no airflow, and 0.5W dissipation at room

temperature. This value can be reduced to 151.2°C/W by

changing to a 3x4 board with 2 oz. copper that is the JEDEC

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LP2998

standard. Figure 2 shows how the θJA varies with airflow for

the two boards mentioned.

30026907

FIGURE 2. θJA vs Airflow (SO-8)

Additional improvements can be made by the judicious use of

vias to connect the part and dissipate heat to an internal

ground plane. Using larger traces and more copper on the top

side of the board can also help. With careful layout, it is pos-

sible to reduce the θJA further than the nominal values shown

in Figure 2

Optimizing the θJA and placing the LP2998 in a section of a

board exposed to lower ambient temperature allows the part

to operate with higher power dissipation. The internal power

dissipation can be calculated by summing the three main

sources of loss: output current at VTT, either sinking or sourc-

ing, and quiescent currents at AVIN and VDDQ. During the

active state (when shutdown is not held low) the total internal

power dissipation can be calculated from the following equa-

tions:

PD = PAVIN + PVDDQ + PVTT

Where,

PAVIN = IAVIN * VAVIN

PVDDQ = VVDDQ * IVDDQ = VVDDQ2 x RVDDQ

To calculate the maximum power dissipation at VTT, both the

sinking and sourcing current conditions need to be examined.

Although only one equation will add into the total, VTT cannot

source and sink current simultaneously.

PVTT = VVTT x ILOAD (Sinking) or

PVTT = ( VPVIN - VVTT) x ILOAD (Sourcing)

The power dissipation of the LP2998 can also be calculated

during the shutdown state. During this condition the output

VTT will tri-state. Therefore, that term in the power equation

will disappear as it cannot sink or source any current (leakage

is negligible). The only losses during shutdown will be the re-

duced quiescent current at AVIN and the constant impedance

that is seen at the VDDQ pin.

PD = PAVIN + PVDDQ

PAVIN = IAVIN x VAVIN

PVDDQ = VVDDQ * IVDDQ = VVDDQ2 x RVDDQ

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LP2998

Typical Application Circuits

Several different application circuits have been shown in Fig-

ure 3 through Figure 12 to illustrate some of the options that

are possible in configuring the LP2998. Graphs of the indi-

vidual circuit performance can be found in the Typical Perfor-

mance Characteristics section of the datasheet. These

curves illustrate how the maximum output current is affected

by changes in AVIN and PVIN.

SSTL-2 APPLICATIONS

For the majority of applications that implement the SSTL-2

termination scheme it is recommended to connect all the input

rails to the 2.5V rail. This provides an optimal trade-off be-

tween power dissipation and component count and selection.

An example of this circuit can be seen in Figure 3.

30026910

FIGURE 3. Recommended SSTL-2 Implementation

If power dissipation or efficiency is a major concern then the

LP2998 has the ability to operate on split power rails (see

Figure 4). The output stage (PVIN) can be operated on a lower

rail such as 1.8V and the analog circuitry (AVIN) can be con-

nected to a higher rail such as 2.5V, 3.3V or 5V. This allows

the internal power dissipation to be lowered when sourcing

current from VTT. The disadvantage of this circuit is that the

maximum continuous current is reduced because of the lower

rail voltage, although it is adequate for all motherboard

SSTL-2 applications. Increasing the output capacitance can

also help if periods of large load transients will be encoun-

tered.

30026911

FIGURE 4. Lower Power Dissipation SSTL-2 Implementation

The third option for SSTL-2 applications in the situation that

a 1.8V rail is not available and it is not desirable to use 2.5V,

is to connect the LP2998 power rail to 3.3V (see Figure 5). In

this situation AVIN will be limited to operation on the 3.3V or

5V rail as PVIN can never exceed AVIN. This configuration

has the ability to provide the maximum continuous output cur-

rent at the downside of higher thermal dissipation. Care

should be taken to prevent the LP2998 from experiencing

large current levels which cause the device to exceed the

maximum operating junction temperature. Because of this

risk it is not recommended to supply the output stage with a

voltage higher than a nominal 3.3V rail.

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LP2998

30026912

FIGURE 5. SSTL-2 Implementation With Higher Voltage Rails

DDR-II APPLICATIONS

With the separate VDDQ pin and an internal resistor divider

it is possible to use the LP2998 in applications utilizing DDR-

II memory. Figure 6 and Figure 7 show several implementa-

tions of recommended circuits with output curves displayed

in the Typical Performance Characteristics. Figure 6 shows

the recommended circuit configuration for DDR-II applica-

tions. The output stage is connected to the 1.8V rail and the

AVIN pin can be connected to either a 2.5, 3.3V or 5.5V rail.

30026913

FIGURE 6. Recommended DDR-II Termination

If it is not desirable to use the 1.8V rail it is possible to connect

the output stage to a 3.3V rail. Care should be taken to not

exceed the maximum operating junction temperature as the

thermal dissipation increases with lower VTT output voltages.

For this reason it is not recommended to power PVIN with a

rail higher than the nominal 3.3V. The advantage of this con-

figuration is that it has the ability to source and sink a higher

maximum continuous current.

30026914

FIGURE 7. DDR-II Termination With Higher Voltage Rails

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LP2998

LEVEL SHIFTING

If standards other than SSTL-2 are required, such as SSTL-3,

it may be necessary to use a different scaling factor than 0.5

times VDDQ for regulating the output voltage. Several options

are available to scale the output to any voltage required. One

method is to level shift the output by using feedback resistors

from VTT to the VSENSE pin. This has been illustrated in Figure

8 and Figure 9. Figure 8 shows how to use two resistors to

level shift VTT above the internal reference voltage of VDDQ/

2. To calculate the exact voltage at VTT the following equation

can be used.

VTT = VDDQ/2 (1 + R1/R2)

30026915

FIGURE 8. Increasing VTT by Level Shifting

Conversely, the R2 resistor can be placed between VSENSE

and VDDQ to shift the VTT output lower than the internal refer-

ence voltage of VDDQ/2. The equation relating to VTT and

the resistors can be used as shown:

VTT = VDDQ/2 (1 - R1/R2)

30026916

FIGURE 9. Decreasing VTT by Level Shifting

HSTL APPLICATIONS

The LP2998 can be easily adapted for HSTL applications by

connecting VDDQ to the 1.5V rail. This will produce a VTT and

VREF voltage of approximately 0.75V for the termination re-

sistors. AVIN and PVIN should be connected to a 2.5V rail for

optimal performance.

30026917

FIGURE 10. HSTL Application

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LP2998

QDR APPLICATIONS

Quad data rate (QDR) applications utilize multiple channels

for improved memory performance. However, this increase in

bus lines increases the current levels required for termination.

The recommended approach in terminating multiple channels

is to use a dedicated LP2998 for each channel. This simplifies

layout and reduces the internal power dissipation for each

regulator. Separate VREF signals can be used for each DIMM

bank from the corresponding regulator with the chipset refer-

ence provided by a local resistor divider or one of the LP2998

signals. Because VREF and VTT are expected to track and the

part to part variations are minor, there should be little differ-

ence between the reference signals of each LP2998.

OUTPUT CAPACITOR SELECTION

For applications utilizing the LP2998 to terminate SSTL-2 I/O

signals the typical application circuit shown in Figure 11 can

be implemented.

30026918

FIGURE 11. Typical SSTL-2 Application Circuit

This circuit permits termination in a minimum amount of board

space and component count. Capacitor selection can be var-

ied depending on the number of lines terminated and the

maximum load transient. However, with motherboards and

other applications where VTT is distributed across a long

plane, it is recommended to use multiple bulk capacitors in

addition to high frequency decoupling. Figure 12 depicts an

example circuit where 2 bulk output capacitors could be situ-

ated at both ends of the VTT plane for optimal placement.

Large aluminum electrolytic capacitors are typically used for

their low ESR and low cost.

30026919

FIGURE 12. Typical SSTL-2 Application Circuit for Motherboards

In most PC applications, an extensive amount of decoupling

is required because of the long interconnects encountered

with the DDR-SDRAM DIMMs mounted on modules. As a re-

sult, bulk aluminum electrolytic capacitors in the range of

1000uF are typically used.

15 www.national.com

LP2998

PCB Layout Considerations

1. The input capacitor for the power rail should be placed

as close as possible to the PVIN pin.

2. VSENSE should be connected to the VTT termination bus

at the point where regulation is required. For

motherboard applications an ideal location would be at

the center of the termination bus.

3. VDDQ can be connected remotely to the VDDQ rail input at

either the DIMM or the Chipset. This provides the most

accurate point for creating the reference voltage.

4. For improved thermal performance excessive top side

copper should be used to dissipate heat from the

package. Numerous vias from the ground connection to

the internal ground plane will help. Additionally these can

be located underneath the package if manufacturing

standards permit.

5. Care should be taken when routing the VSENSE trace to

avoid noise pickup from switching I/O signals. A 0.1uF

ceramic capacitor located close to the SENSE can also be

used to filter any unwanted high frequency signal. This

can be an issue especially if long SENSE traces are used.

6. VREF should be bypassed with a 0.01 µF or 0.1 µF

ceramic capacitor for improved performance. This

capacitor should be located as close as possible to the

VREF pin.

www.national.com 16

LP2998

Physical Dimensions inches (millimeters) unless otherwise noted

8-Lead Small Outline Package (M8)

NS Package Number M08A

8-Lead PSOP Package (PSOP-8)

NS Package Number MRA08A

17 www.national.com

LP2998

Notes

LP2998 DDR-I and DDR-II Termination Regulator

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