Design Note 551: Introduction
Delivering the highest RTR (random transaction rate) of QDR (quad data rate) SRAMs, QDR-IV provides up to 400Gbps data transfer for high bandwidth networking, high performance computing and intensive data processing applications. A key challenge at these faster data rates is maintaining the integrity of the data transferred between the SRAM and devices such as high speed FPGAs and processors.
A good solution is to place the SRAM—QDR-IV, QDR or DDR, for example—very close to the interfaced devices on the PCB’s topside. To conserve PCB area and minimize induced PCB parasitic noise on data bus lines, the DC/DC regulator circuit powering the QDR-IV SRAM data bus drivers should be placed nearby. The challenge is finding space for regulators on a densely populated PCB.
Using a complete DC/DC regulator with on-board inductor and MOSFETs housed in a compact package is one solution. But the scarcity of area on the top of the PCB can render even compact solutions insufficient. If the footprint, height and weight of the DC/DC regulator solution can be reduced enough, it can be placed on the backside of the PCB where space is available.
VTT, VDDQ, VREF from 12VIN in a Tiny Ultrathin Package
The LTM4632 is a complete triple output step-down μModule® regulator specifically designed to support all three voltage rails required by the new QDR-IV and older DDR RAMs, housed in a 0.21g miniature ultrathin profile LGA package (6.25mm × 6.25mm × 1.82mm).
Included in the package are the switching controllers, divide-by-2 circuit, power FETs, inductors and support components. Its tiny footprint and low external component count (as low as one resistor and three capacitors) occupies only 0.5cm2 (dual-sided) or 1cm2 (single-sided) while its thin profile enables mounting on the PCB bottom side to free up space on the topside for super-compact board designs.
The LTM4632 operates from an input voltage between 3.3V and 15V, providing precision output rail voltages between 0.6V and 2.5V. Its two switching regulator outputs, VOUT1 and VOUT2, provide up to 3A for VDDQ and ±3A for VTT bus termination rails, respectively. Its third output provides a low noise buffered 10mA output for the termination reference (VTTR) tracking voltage. Figure 1 shows the LTM4632 circuit in a typical DDR3 application, illustrating its simple solution and small component count.
Powering More SRAM Modules
The LTM4632’s design flexibility enables it to support a broad range of application requirements. For example, its VDDQIN input allows the VTT and VREF rail voltages to be set as either a typical 1/2 × VDDQ voltage or programmed by an external reference voltage for other values. The LTM4632 can be configured as a two phase single output rail for VTT in applications needing more than a ±3A termination rail current. These features allow the LTM4632 to support voltage requirements for many different SRAMs and increase load current requirements for larger memory arrays.
Figure 2 illustrates the flexibility of the LTM4632. The two switching regulator outputs of the LTM4632 are connected in a PolyPhase® current sharing configuration to provide up ±6A VTT for larger memory banks. For more than 6A VDDQ, the LTM4632 can be combined with other μModule regulators, such as the LTM4630, to provide up to 36A for large SRAM arrays. Efficiency and power loss are shown in Figure 3, with thermal performance for the LTM4632 shown in Figure 4.
Tight Regulation with Fast Transient Response
The LTM4632’s unique controlled on-time current mode architecture and internal loop compensation allow for a fast transient with good loop stability over a wide range of operating conditions and output capacitance. Voltage regulation for its switching regulator outputs is precise, with guaranteed low ±1.5% maximum total DC output voltage error over line, load and temperature.
Figures 5 and 6 show the fast transient performance and tight load regulation of the LTM4632 VTT rail of the Figure 2 circuit.
The ultrathin LTM4632 provides a complete high performance regulator solution for all three rails required in DDR/QDR RAM applications. Its wide operating range, features and compact solution size make it highly flexible and robust, and capable of fitting into the tightest spaces on the topside and backside of a PCB.