TI1 LP2996LQX Ddr termination regulator Datasheet

LP2996
DDR Termination Regulator
General Description
Features
The LP2996 linear regulator is designed to meet the JEDEC
SSTL-2 specifications for termination of DDR-SDRAM. The
device contains a high-speed operational amplifier to provide
excellent response to load transients. The output stage prevents shoot through while delivering 1.5A continuous current
and transient peaks up to 3A in the application as required for
DDR-SDRAM termination. The LP2996 also incorporates 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 LP2996 is an active low
shutdown (SD) pin that provides Suspend To RAM (STR)
functionality. When SD is pulled low the VTT output will tristate providing a high impedance output, but, VREF will remain
active. A power savings advantage can be obtained in this
mode through lower quiescent current.
■
■
■
■
■
■
■
■
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 or LLP-16 packages
Applications
■ DDR-I and DDR-II Termination Voltage
■ SSTL-2 and SSTL-3 Termination
■ HSTL Termination
Typical Application Circuit
20057518
© 2012 Texas Instruments Incorporated
200575 SNOSA40I
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LP2996 DDR Termination Regulator
June 29, 2012
LP2996
Connection Diagrams
Top View
20057503
PSOP-8 Layout
20057502
LLP-16 Layout
20057504
SO-8 Layout
Pin Descriptions
SO-8 Pin or
PSOP-8 Pin
LLP Pin
Name
1
2
GND
2
4
SD
3
5
VSENSE
4
7
VREF
Buffered internal reference voltage of VDDQ/2
5
8
VDDQ
Input for internal reference equal to VDDQ/2
6
10
AVIN
Analog input pin
7
11, 12
PVIN
Power input pin
8
14, 15
VTT
Output voltage for connection to termination resistors
-
1, 3, 6, 9, 13, 16
NC
No internal connection
EP
EP
Exposed pad thermal connection. Connect to Ground.
Function
Ground
Shutdown
Feedback pin for regulating VTT.
Ordering Information
Order Number
Package Type
NSC Package Drawing
LP2996M
SO-8
M08A
95 Units per Rail
Supplied As
2500 Units Tape and Reel
LP2996MX
SO-8
M08A
LP2996MR
PSOP-8
MRA08A
95 Units per Rail
LP2996MRX
PSOP-8
MRA08A
2500 Units Tape and Reel
LP2996LQ
LLP-16
LQA16A
1000 Units Tape and Reel
LP2996LQX
LLP-16
LQA16A
4500 Units Tape and Reel
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2
If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for
availability and specifications.
AVIN to GND
−0.3V to +6V
PSOP-8 Thermal Resistance (θJA)
43°C/W
PVIN to GND
-0.3V to AVIN
LLP-16 Thermal Resistance (θJA)
51°C/W
VDDQ (Note 2)
−0.3V to +6V
Lead Temperature (Soldering, 10 sec)
260°C
Storage Temp. Range
−65°C to +150°C
ESD Rating (Note 3)
1kV
Junction Temperature
150°C
SO-8 Thermal Resistance (θJA)
151°C/W
Operating Range
Junction Temp. Range (Note 4)
AVIN to GND
PVIN Supply Voltage
SD Input Voltage
0°C to +125°C
2.2V to 5.5V
0 to AVIN
0 to AVIN
Electrical Characteristics
Specifications with standard typeface are for TJ = 25°C and limits in boldface type
apply over the full Operating Temperature Range (TJ = 0°C to +125°C) (Note 5). Unless otherwise specified,
AVIN = PVIN = 2.5V, VDDQ = 2.5V (Note 6).
Symbol
VREF
Parameter
VREF Voltage
Conditions
VIN = VDDQ = 2.3V
VIN = VDDQ = 2.5V
VIN = VDDQ = 2.7V
Min
Typ
Max
Units
1.135
1.235
1.335
1.158
1.258
1.358
1.185
1.285
1.385
V
ZVREF
VREF Output Impedance
IREF = -30 to +30 μA
VTT
VTT Output Voltage
IOUT = 0A
VIN = VDDQ = 2.3V
VIN = VDDQ = 2.5V
VIN = VDDQ = 2.7V
1.125
1.225
1.325
1.159
1.259
1.359
1.190
1.290
1.390
IOUT = ±1.5A (Note 9)
VIN = VDDQ = 2.3V
VIN = VDDQ = 2.5V
VIN = VDDQ = 2.7V
1.125
1.225
1.325
1.159
1.259
1.359
1.190
1.290
1.390
VTT Output Voltage Offset
(VREF-VTT)
IOUT = 0A
IOUT = -1.5A (Note 9)
IOUT = +1.5A (Note 9)
-20
-25
-25
0
0
0
20
25
25
mV
IQ
Quiscent Current (Note 7)
IOUT = 0A (Note 5)
320
500
µA
ZVDDQ
VDDQ Input Impedance
ISD
Quiescent Current in
Shutdown (Note 7)
SD = 0V
115
150
µA
IQ_SD
Shutdown Leakage Current
SD = 0V
2
5
µA
VIH
Minimum Shutdown High
Level
VIL
Maximum Shutdown Low
Level
IV
VTT Leakage Current in
Shutdown
ISENSE
VSENSE Input Current
TSD
Thermal Shutdown
TSD_HYS
Thermal Shutdown
Hysteresis
VosTT/VTT
2.5
kΩ
100
kΩ
V
1.9
SD = 0V
VTT = 1.25V
1
(Note 8)
3
V
0.8
V
10
µA
13
nA
165
Celcius
10
Celcius
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LP2996
Absolute Maximum Ratings (Note 1)
LP2996
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 100pF capacitor discharged through a 1.5kΩ 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: VIN is defined as VIN = AVIN = PVIN.
Note 7: Quiescent current defined as the current flow into AVIN.
Note 8: 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 9: VTT load regulation is tested by using a 10 ms current pulse and measuring VTT.
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LP2996
Typical Performance Characteristics
Iq vs AVIN in SD
Iq vs AVIN
20057520
20057521
VIH and VIL
VREF vs IREF
20057522
20057523
VREF vs VDDQ
VTT vs IOUT
20057524
20057525
5
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LP2996
VTT vs VDDQ
Iq vs AVIN in SD Temperature
20057526
20057527
Iq vs AVIN Temperature
Maximum Sourcing Current vs AVIN
(VDDQ = 2.5V, PVIN = 1.8V)
20057528
20057531
Maximum Sourcing Current vs AVIN
(VDDQ = 2.5V, PVIN = 2.5V)
Maximum Sourcing Current vs AVIN
(VDDQ = 2.5V, PVIN = 3.3V)
20057532
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20057533
6
LP2996
Maximum Sinking Current vs AVIN
(VDDQ = 2.5V)
Maximum Sourcing Current vs AVIN
(VDDQ = 1.8V, PVIN = 1.8V)
20057534
20057535
Maximum Sinking Current vs AVIN
(VDDQ = 1.8V)
Maximum Sourcing Current vs AVIN
(VDDQ = 1.8V, PVIN = 3.3V)
20057536
20057537
Block Diagram
20057505
7
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LP2996
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 LP2996. This implementation can be seen below in Figure 1.
Description
The LP2996 is a linear bus termination regulator designed to
meet the JEDEC requirements of SSTL-2. 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 LP2996 also incorporates two distinct
power rails that separates the analog circuitry from the power
output stage. This allows a split rail approach to be utilized to
decrease internal power dissipation. It also permits the
LP2996 to provide a termination solution for the next generation of DDR-SDRAM memory (DDRII). For new designs, the
LP2997 or LP2998 is recommended for DDR-II applications.
The LP2996 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 improve 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-
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20057506
FIGURE 1. SSTL-Termination Scheme
8
pull-up current, therefore to turn the part on the shutdown pin
can either be connected to AVIN or left open.
AVIN AND PVIN
AVIN and PVIN are the input supply pins for the LP2996. AVIN
is used to supply all the internal control circuitry. PVIN, however, is used exclusively to provide the rail voltage for the
output stage used to create VTT. These pins have the capability to work off separate supplies depending on the application. Higher voltages on PVIN will increase the maximum
continuous output current because of output RDSON limitations at voltages close to VTT. The disadvantage of high
values of PVIN is that the internal power loss will also increase, thermally limiting the design. For SSTL-2 applications, a good compromise would be to connect the AVIN and
PVIN directly together at 2.5V. This eliminates the need for
bypassing the two supply pins separately. The only limitation
on input voltage selection is that PVIN must be equal to or
lower than AVIN. It is recommended to connect PVIN to voltage rails equal to or less than 3.3V to prevent the thermal limit
from tripping because of excessive internal power dissipation.
If the junction temperature exceeds the thermal shutdown
than the part will enter a shutdown state identical to the manual shutdown where VTT is tri-stated and VREF remains active.
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 these inputs are typically an extremely high impedance,
there should be little current drawn from VREF. For improved
performance, an output bypass capacitor can be used, located 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
remains 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 LP2996 is
designed to handle peak transient currents of up to ± 3A with
a fast transient response. The maximum continuous current
is a function of VIN and can be viewed in the TYPICAL PERFORMANCE CHARACTERISTICS section. If a transient is
expected to last above the maximum continuous current rating for a significant amount of time then the output capacitor
should be sized large enough to prevent an excessive voltage
drop. Despite the fact that the LP2996 is designed to handle
large transient output currents it is not capable of handling
these for long durations, under all conditions. The reason for
this is the standard packages are not able to thermally dissipate the heat as a result of the internal power loss. If large
currents are required for longer durations, then care should
be taken to ensure that the maximum junction temperature is
not exceeded. Proper thermal derating should always be
used (please refer to the Thermal Dissipation section). If the
junction temperature exceeds the thermal shutdown point
than VTT will tri-state until the part returns below the hysteretic
trip-point.
VDDQ
VDDQ is the input used to create the internal reference voltage for regulating VTT. The reference voltage is generated
from a resistor divider of two internal 50kΩ resistors. This
guarantees that VTT will track VDDQ / 2 precisely. The optimal
implementation of VDDQ is as a remote sense. This can be
achieved by connecting VDDQ directly to the 2.5V rail at the
DIMM instead of AVIN and PVIN. This ensures that the reference voltage tracks the DDR memory rails precisely without
a large voltage drop from the power lines. For SSTL-2 applications VDDQ will be a 2.5V signal, which will create a 1.25V
termination 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 termination resistors will connect to VTT in a long plane. If the output
voltage was regulated only at the output of the LP2996 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 connected to VTT. Care should be taken when a long VSENSE trace
is implemented in close proximity to the memory. Noise pickup 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 preventing errors.
Component Selections
INPUT CAPACITOR
The LP2996 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 dependent on the
application required. A typical value recommended for AL
electrolytic capacitors is 50 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 reduced if
the LP2996 is placed close to the bulk capacitance from the
output of the 2.5V DC-DC converter. If the two supply rails
(AVIN and PVIN) are separated then the 47uF capacitor
should be placed as close to possible to the PVIN rail. An
additional 0.1uF ceramic capacitor can be placed on the AVIN
rail to prevent excessive noise from coupling into the device.
SHUTDOWN
The LP2996 contains an active low shutdown pin that can be
used to tri-state VTT. During shutdown VTT should not be exposed to voltages that exceed AVIN. With the shutdown pin
asserted low the quiescent current of the LP2996 will drop,
however, VDDQ 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 considered. For more information refer to the Thermal
Dissipation section. The shutdown pin also has an internal
OUTPUT CAPACITOR
The LP2996 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 application 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
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LP2996
Pin Descriptions
LP2996
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 frequency (between 20 kHz and 100 kHz) should be used for the
LP2996. 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 can have
their ESR rapidly increase at cold temperatures.
Ceramic - Ceramic capacitors typically have a low capacitance, in the range of 10 to 100 µF range, but they 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 function of voltage and temperature. Because of the typically low value of capacitance it is recommended to use
ceramic capacitors in parallel with another capacitor such as
an aluminum electrolytic. A dielectric of X5R or better is recommended 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.
20057507
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 possible to reduce the θJA further than the nominal values shown
in Figure 2
Layout is also extremely critical to maximize the output current with the LLP package. By simply placing vias under the
DAP the θJA can be lowered significantly. Figure 3 shows the
LLP thermal data when placed on a 4-layer JEDEC board with
copper thickness of 0.5/1/1/0.5 oz. The number of vias, with
a pitch of 1.27 mm, has been increased to the maximum of 4
where a θJA of 50.41°C/W can be obtained. Via wall thickness
for this calculation is 0.036 mm for 1oz. Copper.
Thermal Dissipation
Since the LP2996 is a linear regulator any current flow from
VTT will result in internal power dissipation generating heat.
To prevent damaging the part from exceeding the maximum
allowable junction temperature, care should be taken to derate the part dependent 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 application 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 LP2996 will be dependent 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
standard. Figure 2 shows how the θJA varies with airflow for
the two boards mentioned.
20057508
FIGURE 3. LLP-16 θJA vs # of Vias (4 Layer JEDEC Board))
Additional improvements in lowering the θJA can also be
achieved with a constant airflow across the package. Maintaining the same conditions as above and utilizing the 2x2 via
array, Figure 4 shows how the θJA varies with airflow.
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PD = PAVIN + PVDDQ + PVTT
Where,
PAVIN = IAVIN * VAVIN
PVDDQ = VVDDQ * IVDDQ = VVDDQ2 x RVDDQ
To calculate the maximum power dissipation at VTT both conditions at VTT need to be examined, sinking and sourcing
current. 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 LP2996 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 reduced quiescent current at AVIN and the constant impedance
that is seen at the VDDQ pin.
20057509
FIGURE 4. θJA vs Airflow Speed (JEDEC Board with 4
Vias)
Optimizing the θJA and placing the LP2996 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 sourcing, and quiescent current at AVIN and VDDQ. During the
PD = PAVIN + PVDDQ
PAVIN = IAVIN x VAVIN
PVDDQ = VVDDQ * IVDDQ = VVDDQ2 x RVDDQ
11
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LP2996
active state (when shutdown is not held low) the total internal
power dissipation can be calculated from the following equations:
LP2996
Typical Application Circuits
Several different application circuits have been shown in Figure 5 through Figure 14 to illustrate some of the options that
are possible in configuring the LP2996. Graphs of the individual circuit performance can be found in the Typical Performance Characteristics section in the beginning 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 between power dissipation and component count and selection.
An example of this circuit can be seen in Figure 5.
20057510
FIGURE 5. Recommended SSTL-2 Implementation
If power dissipation or efficiency is a major concern then the
LP2996 has the ability to operate on split power rails. The
output stage (PVIN) can be operated on a lower rail such as
1.8V and the analog circuitry (AVIN) can be connected 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 encountered.
20057511
FIGURE 6. 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 LP2996 power rail to 3.3V. 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 current at the down-
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side of higher thermal dissipation. Care should be taken to
prevent the LP2996 from experiencing large current levels
which cause the junction temperature to exceed the maximum. Because of this risk it is not recommended to supply
the output stage with a voltage higher than a nominal 3.3V
rail.
12
LP2996
20057512
FIGURE 7. SSTL-2 Implementation with higher voltage rails
the recommended circuit configuration for DDR-II applications. The output stage is connected to the 1.8V rail and the
AVIN pin can be connected to either a 3.3V or 5V rail. For new
designs, the LP2997 or LP2998 is recommended for DDR-II
applications.
DDR-II APPLICATIONS
With the separate VDDQ pin and an internal resistor divider
it is possible to use the LP2996 in applications utilizing DDRII memory. Figure 6 and Figure 7 show several implementations of recommended circuits with output curves displayed
in the Typical Performance Characteristics. Figure 6 shows
20057513
FIGURE 8. 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 junction temperature as the thermal
dissipation increases with lower VTT output voltages. For this
reason it is not recommended to power PVIN off a rail higher
than the nominal 3.3V. The advantage of this configuration is
that it has the ability to source and sink a higher maximum
continuous current.
20057514
FIGURE 9. DDR-II Termination with higher voltage rails
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LP2996
from VTT to the VSENSE pin. This has been illustrated in Figures
10 and 11. Figure 10 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.
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
VTT = VDDQ/2 ( 1 + R1/R2)
20057515
FIGURE 10. Increasing VTT by Level Shifting
VTT = VDDQ/2 (1 - R1/R2)
Conversely, the R2 resistor can be placed between VSENSE
and VDDQ to shift the VTT output lower than the internal reference voltage of VDDQ/2. The equations relating VTT and the
resistors can be seen below:
20057516
FIGURE 11. Decreasing VTT by Level Shifting
VREF voltage of approximately 0.75V for the termination resistors. AVIN and PVIN should be connected to a 2.5V rail for
optimal performance.
HSTL APPLICATIONS
The LP2996 can be easily adapted for HSTL applications by
connecting VDDQ to the 1.5V rail. This will produce a VTT and
20057517
FIGURE 12. HSTL Application
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OUTPUT CAPACITOR SELECTION
For applications utilizing the LP2996 to terminate SSTL-2 I/O
signals the typical application circuit shown in Figure 11 can
be implemented.
20057518
FIGURE 13. Typical SSTL-2 Application Circuit
This circuit permits termination in a minimum amount of board
space and component count. Capacitor selection can be varied 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 advisable to use multiple bulk capacitors and addition to
high frequency decoupling. Figure 12 shown below depicts
an example circuit where 2 bulk output capacitors could be
situated at both ends of the VTT plane for optimal placement.
Large aluminum electrolytic capacitors are used for their low
ESR and low cost.
20057519
FIGURE 14. 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.
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LP2996
tor divider or one of the LP2996 signals. Because VREF and
VTT are expected to track and the part to part variations are
minor, there should be little difference between the reference
signals of each LP2996.
QDR APPLICATIONS
Quad data rate (QDR) applications utilize multiple channels
for improved memory performance. However, this increase in
bus lines has the effect of increasing the current levels required for termination. The recommended approach in terminating multiple channels is to use a dedicated LP2996 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 reference provided by a local resis-
LP2996
PCB Layout Considerations
1.
2.
3.
4.
The input capacitor for the power rail should be placed
as close as possible to the PVIN pin.
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.
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.
For improved thermal performance excessive top side
copper should be used to dissipate heat from the
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5.
6.
16
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.
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.
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.
LP2996
Physical Dimensions inches (millimeters) unless otherwise noted
8-Lead Small Outline Package (M8)
NS Package Number M08A
17
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LP2996
16-Lead LLP Package (LD)
NS Package Number LQA16A
8-Lead PSOP Package (PSOP-8)
NS Package Number MRA08A
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18
LP2996
Notes
19
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LP2996 DDR Termination Regulator
Notes
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