LP2995M/NOPB National Semiconductor, LP2995M/NOPB Datasheet - Page 7

• Manufacturer Part Number: LP2995M/NOPB
• Description: IC REGULATOR DDR TERM 8-SOIC
• Manufacturer: National Semiconductor

Specifications of LP2995M/NOPB

• Applications: Converter, DDR
• Voltage - Input: 2.5 ~ 5.5 V
• Number Of Outputs: 1
• Operating Temperature: 0°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Primary Input Voltage: 2.5V
• No. Of Outputs: 1
• No. Of Pins: 8
• Output Current: 1.5A
• Operating Temperature Range: 0°C To +125°C
• Msl: MSL 1 - Unlimited
• Filter Terminals: SMD
• Rohs Compliant: Yes
• Current Rating: 1.5A
• For Use With: LP2995M-EVAL - BOARD EVALUATION LP2995M
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Other names: *LP2995M; LP2995M

Pin Descriptions
AVIN AND PVIN
AVIN and PVIN are the input supply pins for the LP2995. AVIN
is used to supply all the internal control circuitry for the two
op-amps and the output stage of V
sively to provide the rail voltage for the output stage on the
power operational amplifier used to create V
applications AVIN and PVIN pins should be connected di-
rectly and tied to the 2.5V rail for optimal performance. This
eliminates the need for bypassing the two supply pins sepa-
rately.
VDDQ
VDDQ is the input that is used to create the internal reference
voltage for regulating V
by two internal 50kΩ resistors. This guarantees that V
V
tion of VDDQ is as a remote sense for the reference input.
This can be achieved by connecting VDDQ directly to the 2.5V
rail at the DIMM. 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 reference voltage
on V
applications it may be desirable to have a different scaling
factor for creating the internal reference voltage besides 0.5.
For instance a typical value that is commonly used is to have
the reference voltage equal VDDQ*0.45. This can be
achieved by placing a resistor in series with the VDDQ pin to
effectively change the resistor divider.
V
The purpose of the sense pin is to provide improved remote
load regulation. In most motherboard applications the termi-
nation resistors will connect to V
voltage was regulated only at the output of the LP2995, 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 V
connecting it to the middle of the bus. This will provide a better
distribution across the entire termination bus.
Note: If remote load regulation is not used, then the V
V
V
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 V
performance, an output bypass capacitor can be used, locat-
ed close to the pin, to help with noise. A ceramic capacitor in
the range of 0.1 µF to 0.01 µF is recommended.
V
V
resistors. It is capable of sinking and sourcing current while
regulating the output precisely to VDDQ / 2. The LP2995 is
designed to handle peak transient currents of up to ± 3A with
a fast transient response. The maximum continuous current
is a function of V
FORMANCE CHARACTERISTICS section. If a transient is
expected to last above the maximum continuous current rat-
ing 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 LP2995 is designed to handle
REF
SENSE
REF
REF
TT
TT
is the regulated output that is used to terminate the bus
REF
will track VDDQ / 2 precisely. The optimal implementa-
connected to V
provides the buffered output of the internal reference
SENSE
and a 1.25V termination voltage at V
pin can be used to improve this performance, by
IN
TT
and can be viewed in the TYPICAL PER-
.
TT
and V
REF
TT
in a long plane. If the output
. This voltage is generated
REF
. PVIN is used exclu-
REF
SENSE
TT
TT
. For improved
. For SSTL-2
. For SSTL-3
pin must still be
TT
and
7
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 dissi-
pate 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).
Component Selection
INPUT CAPACITOR
The LP2995 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 LP2995 is placed close to the bulk capacitance from the
output of the 2.5V DC-DC converter.
OUTPUT CAPACITOR
The LP2995 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 V
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 (between 20 kHz and 100 kHz) should be used for the
LP2995. 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 capaci-
tance, 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 typ-
ically 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 rec-
ommended 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.
Capacitor recommendations for different application circuits
can be seen in the accompanying application notes with sup-
porting evaluation boards.
www.national.com
TT
.