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LP2995LQ Datasheet(PDF) 7 Page - National Semiconductor (TI) |
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LP2995LQ Datasheet(HTML) 7 Page - National Semiconductor (TI) |
7 / 13 page Pin Descriptions AVIN AND PVIN AVIN and PVIN are the input supply pins for the LP2995. AVIN is used to supply all the internal control circuitry for the two op-amps and the output stage of V REF. PVIN is used exclusively to provide the rail voltage for the output stage on the power operational amplifier used to create V TT. For SSTL-2 applications AVIN and PVIN pins should be con- nected directly and tied to the 2.5V rail for optimal perfor- mance. This eliminates the need for bypassing the two sup- ply pins separately. VDDQ VDDQ is the input that is used to create the internal refer- ence voltage for regulating V TT and VREF. This voltage is generated by two internal 50k Ω resistors. This guarantees that V TT and VREF will track VDDQ / 2 precisely. The optimal implementation of VDDQ is as a remote sense for the refer- ence input. This can be achieved by connecting VDDQ directly to the 2.5V rail at the DIMM. This ensures that the reference voltage tracks the DDR memory rails precisely without a large voltage drop from the power lines. For SSTL-2 applications VDDQ will be a 2.5V signal, which will create a 1.25V reference voltage on V REF and a 1.25V termination voltage at V TT. For SSTL-3 applications it may be desirable to have a different scaling factor for creating the internal reference voltage besides 0.5. For instance a typical value that is commonly used is to have the reference voltage equal VDDQ*0.45. This can be achieved by placing a resis- tor in series with the VDDQ pin to effectively change the resistor divider. V SENSE The purpose of the sense pin is to provide improved remote load regulation. In most motherboard applications the termi- nation resistors will connect to V TT in a long plane. If the output voltage was regulated only at the output of the LP2995, then the long trace will cause a significant IR drop, resulting in a termination voltage lower at one end of the bus than the other. The V SENSE pin can be used to improve this performance, by connecting it to the middle of the bus. This will provide a better distribution across the entire termination bus. Note: If remote load regulation is not used, then the VSENSE pin must still be connected to VTT. 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 imped- ance, 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 tran- sient 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 exces- sive voltage drop. Despite the fact that the LP2995 is de- signed to handle large transient output currents it is not capable of handling these for long durations, under all con- ditions. 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 dura- tions, then care should be taken to ensure that the maximum junction temperature is not exceeded. Proper thermal derat- ing 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 capaci- tance 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 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 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 (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 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 m Ω). 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 www.national.com 7 |
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