English
Language : 

LP2998MRX Datasheet, PDF (11/26 Pages) Texas Instruments – LP2998 DDR-I and DDR-II Termination Regulator
LP2998
www.ti.com
SNVS521I – DECEMBER 2007 – REVISED APRIL 2013
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 application and the requirements for load transient response of VTT. 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 (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 increasing
ESR at cold temperatures.
Ceramic - Ceramic capacitors typically have a low capacitance, 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 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 capacitor type.
Thermal Dissipation
Since the LP2998 is a linear regulator, any current flow from VTT will result in internal power dissipation and heat
generation. To prevent damaging the part by exceeding the maximum 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 (TRmax) can be calculated given the maximum
ambient temperature (TAmax) of the application and the maximum allowable junction temperature (TJmax).
TRmax = TJmax − TAmax
(1)
From this equation, the maximum power dissipation (PDmax) of the part can be calculated:
PDmax = TRmax / θJA
(2)
The θJA of the LP2998 will depend on several variables: the package used; the thickness of copper; the number
of vias and the airflow. For instance, the θJA of the SOIC-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 18
shows how the θJA varies with airflow for the two boards mentioned.
Copyright © 2007–2013, Texas Instruments Incorporated
Product Folder Links: LP2998
Submit Documentation Feedback
11