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MIC5167 Datasheet(PDF) 11 Page - Micrel Semiconductor |
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MIC5167 Datasheet(HTML) 11 Page - Micrel Semiconductor |
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11 / 23 page  Micrel, Inc. MIC5167 January 2012 11 M9999-012312-B Application Information DDR memory requires two power supplies, one for the memory chip, referred to as VDDQ and the other for a termination supply VTT, which is one-half VDDQ. With memory speeds in excess of 300MHz, the memory system bus must be treated as transmission lines. To maintain good signal integrity the memory bus must be terminated to minimize signal reflections. Figure 2 shows the simplified termination circuit. Each control, address and data lines have these termination resistors RS and RT connected to them. - + DDR MEMORY PVIN SW FB 22µF x4 PGND 100µF x3 SGND SVIN VDDQ EP VREF 100pF 0.4µH RT RS VIN 5.0V VDDQ 1.8V MIC5167 - + CHIP SET RT RS VDDQ VDDQ VDDQ VTT/0.9V VREF + Figure 2. DDR Memory Termination Circuit Bus termination provides a means to increase signaling speed while maintaining good signal integrity. The termination network consists of a series resistor (RS) and a terminating resistor (RT). Values of RS range between 10Ω to 30Ω with a typical of 22Ω, while RT ranges from 22Ω to 28Ω with a typical value of 25Ω. VTT will dynamically sink and source current to maintain a termination voltage under all conditions. This method of bus termination reduces common mode noise, settling time, voltage swings, EMI/RFI and improves slew rates. VDDQ powers all the memory IC’s, memory drivers and receivers for all the memory bits in the DDR memory system. When the driver is logic low VTT sources current. When the driver is logic high VTT sinks current. The MIC5167 regulates VTT to VDDQ/2 during sourcing or sinking current. The power dissipated in RS (bus resistance) and RT (termination resistance) is VTT squared divided by their respective resistance. Since the voltage across the resistance is VDDQ/2, the power dissipated is one-quarter the power of a termination voltage of VDDQ. The memory bits are not usually all at a logic high or logic low at the same time so the VTT supply is usually not sinking or sourcing much current. VTT VTT is regulated to VREF. Due to high speed signaling, the load current seen by VTT is constantly changing. To maintain adequate transient response, large OS-CON and ceramics are recommended on VTT. The proper combination and placement of the OS-CON and ceramic capacitors is important to reduce both ESR and ESL such that high-current and high-speed transients do not exceed the dynamic voltage tolerance requirement of VTT. The larger OS-CON capacitors provide bulk charge storage while the smaller ceramic capacitors provide current during the fast edges of the bus transition. Using several smaller ceramic capacitors distributed near the termination resistors is important to reduce the effects of PCB trace inductance. When VTT is sinking current, the external power supply that powers the MIC5167 (PVIN) must be able to sink current-to-ground; otherwise, the supply voltage will start to increase. It is crucial that this external power supply must also be able to source and sink current. VDDQ The VDDQ input on the MIC5167 is used to create the internal reference voltage for VTT. The reference voltage is generated from an internal resistor divider network of two 60kΩ resistors, generating a reference voltage VREF that is VDDQ/2. The VDDQ input should be Kelvin connected as close as possible to the memory supply voltage. Since the reference is simply VDDQ/2, any perturbations on VDDQ will also appear at half the amplitude on the reference. For this reason, both ceramics and low-ESR bulk capacitors such as OS-CON are recommended on the VDDQ supply. This will aid in performance by improving the source impedance over a wide frequency range. Feedback The feedback (FB) pin provides the path for the error amplifier to regulate VTT. The FB input must also be Kelvin connected to the VTT bypass capacitors. If the FB input is connected to close to the MIC5167, the IR drop of the PCB trace can cause the VTT voltage at the memory chip to be too low. Placing the MIC5167 as close as possible to the DDR memory will improve the load regulation performance. |
Similar Part No. - MIC5167_12 |
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Similar Description - MIC5167_12 |
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