LP2998 Datasheet, PDF (9/18 Pages) Intersil Corporation – DDR-II and DDR-I Termination Regulator

SHUTDOWN
The LP2998 contains an active low shutdown pin that can be
used for suspend to RAM functionality. In this condition the
V_TT output will tri-state while the V_REF output remains active
providing a constant reference signal for the memory and
chipset. During shutdown V_TT 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
V_REF
V_REF 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 V_REF. For improved
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. This output
remains active during the shutdown state and thermal shut-
down events for the suspend to RAM functionality.
V_TT
V_TT 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 junc-
tion temperature is not exceeded. Proper thermal de-rating
should always be used. (Please refer to the Thermal Dissi-
pation section) If the junction temperature exceeds the ther-
mal shutdown point than V_TT 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 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 V_TT.
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 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 capacitors.
Thermal Dissipation
Since the LP2998 is a linear regulator any current flow from
V_TT will result in internal power dissipation generating heat.
To prevent damaging the part by exceeding the maximum al-
lowable junction temperature, care should be taken to derate
the part dependent on the maximum expected ambient tem-
perature and power dissipation. The maximum allowable in-
ternal temperature rise (T_Rmax) can be calculated given the
maximum ambient temperature (T_Amax) of the application and
the maximum allowable junction temperature (T_Jmax).
T_Rmax = T_Jmax − T_Amax
From this equation, the maximum power dissipation (P_Dmax)
of the part can be calculated:
P_Dmax = T_Rmax / θ_JA
The θ_JA of the LP2998 will be dependent on several variables:
the package used; the thickness of copper; the number of vias
and the airflow. For instance, the θ_JA 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 θ_JA varies with airflow for
the two boards mentioned.
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