LP2997MX National Semiconductor, LP2997MX Datasheet - Page 8

• Manufacturer Part Number: LP2997MX
• Description: DDR-II Termination Regulator
• Manufacturer: National Semiconductor

www.national.com
Pin Descriptions
This output remains active during the shutdown state and
thermal shutdown events for the suspend to RAM function-
ality.
V
V
resistors. It is capable of sinking and sourcing current while
regulating the output precisely to VDDQ / 2. The LP2997 is
designed to handle continuous currents of up to +/- 0.5A with
excellent load regulation. If a transient is expected to last
above the maximum continuous current rating for a signifi-
cant amount of time, then the bulk output capacitor should
be sized large enough to prevent an excessive voltage drop.
If the LP2997 is to operate in elevated temperatures for long
durations care should be taken to ensure that the maximum
junction temperature is not exceeded. Proper thermal de-
rating should always be used. (Please refer to the Thermal
Dissipation section) If the junction temperature exceeds the
thermal shutdown point than V
returns below the temperature hysteresis trip-point
Component Selection
INPUT CAPACITOR
The LP2997 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 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
reduced if the LP2997 is placed close to the bulk capaci-
tance 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 LP2997 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 capaci-
tor 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 (100 kHz) should be used for the LP2997. 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
TT
TT
is the regulated output that is used to terminate the bus
(Continued)
TT
will tri-state until the part
TT
.
8
very low ESR (typically less than 10 mΩ). However, some
dielectric types do not have good capacitance characteris-
tics 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 capaci-
tors.
Thermal Dissipation
Since the LP2997 is a linear regulator any current flow from
V
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 ambi-
ent temperature and power dissipation. The maximum allow-
able internal temperature rise (T
given the maximum ambient temperature (T
application and the maximum allowable junction temperature
(T
From this equation, the maximum power dissipation (P
of the part can be calculated:
The θ
ables: the package used; the thickness of copper; the num-
ber of vias and the airflow. For instance, the θ
is 163˚C/W with the package mounted to a standard 8x4
2-layer board with 1oz. copper, no airflow, and 0.5W dissi-
pation 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 1 shows how the θ
with airflow for the two boards mentioned.
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
TT
Jmax
will result in internal power dissipation generating heat.
JA
).
of the LP2997 will be dependent on several vari-
FIGURE 1. θ
T
Rmax
P
Dmax
= T
JA
= T
Jmax
vs Airflow (SO-8)
Rmax
− T
Rmax
/ θ
Amax
JA
) can be calculated
JA
Amax
of the SO-8
20109407
JA
) of the
varies
Dmax
)