LP2995 Datasheet, PDF (8/24 Pages) National Semiconductor (TI) – DDR Termination Regulator

LP2995
SNVS190M – FEBRUARY 2002 – REVISED MARCH 2013
NOTE
If remote load regulation is not used, then the V_SENSE pin must still be connected to V_TT.
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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, 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.
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 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_IN 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 LP2995 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).
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 application 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 frequency (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 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
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