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G2996P1UF Datasheet(PDF) 9 Page - Global Mixed-mode Technology Inc |
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G2996P1UF Datasheet(HTML) 9 Page - Global Mixed-mode Technology Inc |
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9 / 13 page  Ver: 2.3 May 16, 2006 TEL: 886-3-5788833 http://www.gmt.com.tw 9 G2996 Global Mixed-mode Technology Inc. VDDQ A voltage divider of two 50k Ω is connected between VDDQ and ground, to create the internal reference voltage (VDDQ/2). This guarantees that VTT will track VDDQ/2 precisely. The optimal implementation of VDDQ is as a remote sensing. This can be achieved by connecting VDDQ directly to the 2.5V rail (SSTL-2 applications) at the DIMM instead of AVIN and PVIN. This will ensure that the reference voltage tracks the DDR memory rails precisely without a large voltage drop from the power lines. Vsense The VSENSE pin is the feedback sensing pin of the op- eration amplifier which regulates the VTT voltage. In most motherboard applications, the termination resis- tors will connect VTT in a long plane. If using the re- mote sensing pin – VSENSE to the middle of the bus, the significant long-trace IR drop resulting in a termination voltage which is lower at one end than the other can be avoided. This will provide a better distribution across the entire termination bus. If the remote load regulation is not used, the VSENSE pin must still be connected to VTT for correct regulation. Care should be taken when a long VSENSE trace is implemented in close proximity to the memory. Noise pickup in the VSENSE trace can cause problems with precise regula- tion of VTT. A small 0.1µF ceramic capacitor placed next to the VSENSE pin can help to filter any high fre- quency signals and preventing errors. VREF VREF provides a buffered output of the internal refer- ence voltage (VDDQ/2). It can support the reference voltage of Northbridge chipset and memory. This out- put remains active during the shutdown state and thermal shutdown events to support the suspend to RAM (STR) functionality. For better performance, us- ing an output bypass capacitor close this pin is more helpful for the noise. A ceramic capacitor in the range of 0.1µF to 0.01µF is recommended. VTT VTT is the regulated output that is used to terminate the bus resistors of DDR-SDRAM. It can precisely track the VDDQ/2 voltage with the sinking and sourcing current capability. The G2996 is designed to deliver 1.5A continuous current and peak current up to 3A with a fast transient response @ 2.5V supply rail. The maximum continuous current sourcing from VTT is a function of PVIN. Using a higher PVIN will increase the source current from VTT, but it also increase the inter- nal power dissipation and reduce the efficiency. Al- though the G2996 can deliver the larger current, care should be taken for the thermal dissipation when lar- ger current is required. The G2996 is packaged with Power-Pad to increase the power dissipation capability. When driving larger current, the larger heat-sink in the PCB is strongly recommended to have a better ther- mal performance. The RDS of MOS will increase when the junction temperature increases. If the heat is not dealt with well, the maximum output current will be degraded. When the temperature exceeds the junction temperature, the thermal shutdown protection is acti- vated. That will drive the VTT output into tri-state until the temperature returns below the hysteretic trigger point. Capacitors The G2996 does not require the capacitors for input stability, but it is recommended for improving the per- formance during large load transition to prevent the input power rail from dropping, especially for PVIN. The input capacitor for PVIN should be as close as possible. The typical recommended value is 50µF for AL electrolytic capacitors, 10µF with X5R for the ce- ramic capacitors. To prevent the excessive noise cou- pling into this device, an additional 0.1µF ceramic ca- pacitor can be placed on the AVIN power rail for the better performance. The output capacitor of the G2996 is suggested to use the capacitors with low ESR. Using the capacitors with low ESR (as ceramic, OS-CON, tantalum) will have the better transition performance which is with smaller voltage drop when the peak current occurring at the transition. As a general recommendation the output capacitor should be sized above 220µF with the low ESR for SSTL applications with DDR-SDRAM. Thermal Dissipation When the current is sinking to or sourcing from VTT, the G2996 will generate internal power dissipation resulting in the heat. Care should be taken to prevent the device from damages caused by the junction tem- perature exceeding the maximum rating. The maxi- mum allowable internal temperature rise (TRMAX) can be calculated under the given maximum ambient temperature (TAMAX) of the application and the maxi- mum allowable junction temperature (TJMAX). TRMAX= TJMAX - TAMAX From this equation, the maximum power dissipation (PDMAX) of the G2996 can be calculated: PDMAX = TRMAX /θJA θ JA of the G2996 will be dependent on several vari- ables: the packages used, the thickness and size of the copper, the number of vias and the airflow. In the package, the G2996 use the SOP-8 with Power-PAD to improve the θJA . If the layout of the PCB can put a larger size of copper to contact the Power-PAD of this device, the θJA will be further improved. The better θJA is not only protecting the device well, but also increas- ing the maximum current capability at the same ambi- ent temperature. |
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