LP2998MR/NOPB National Semiconductor, LP2998MR/NOPB Datasheet - Page 10

• Manufacturer Part Number: LP2998MR/NOPB
• Description: IC REG TERM VOLT DDRI/II 8-PSOP
• Manufacturer: National Semiconductor

Specifications of LP2998MR/NOPB

• Applications: Converter, DDR, DDR2, DDR3
• Voltage - Input: 2.2 ~ 5.5 V
• Number Of Outputs: 1
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-PSOP
• Primary Input Voltage: 2.5V
• No. Of Outputs: 1
• No. Of Pins: 8
• Output Current: 1.5A
• Operating Temperature Range: -40°C To +125°C
• Msl: MSL 3 - 168 Hours
• Filter Terminals: SMD
• Rohs Compliant: Yes
• For Use With: LP2998EVAL - BOARD EVALUATION FOR LP2998
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Other names: LP2998MR

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nected to V
is implemented in close proximity to the memory. Noise pick-
up in the V
regulation of V
to the V
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
V
providing a constant reference signal for the memory and
chipset. During shutdown, V
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.
V
V
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 V
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.
V
V
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, V
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
TT
REF
REF
TT
TT
output will tri-state while the V
is the regulated output that is used to terminate the bus
provides the buffered output of the internal reference
SENSE
TT
SENSE
. Care should be taken when a long V
TT
pin can help filter any high frequency signals
. A small 0.1uF ceramic capacitor placed next
trace can cause problems with precise
TT
should not be exposed to volt-
REF
output remains active
TT
REF
will tri-state until
. For improved
SENSE
trace
10
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 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 (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
V
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 (T
given the maximum ambient temperature (T
plication and the maximum allowable junction temperature
(T
From this equation, the maximum power dissipation (P
of the part can be calculated:
The θ
package used; the thickness of copper; the number of vias
and the airflow. For instance, the θ
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
TT
Jmax
will result in internal power dissipation and heat genera-
JA
).
of the LP2998 will depend on several variables: the
T
P
Rmax
Dmax
= T
= T
Jmax
Rmax
− T
JA
/ θ
Amax
of the SO-8 is 163°C/W
Rmax
JA
) can be calculated
Amax
) of the ap-
Dmax
TT
)
.