BTX Computer Motherboard Power Management Solutions

The BTX specification outlines the necessary mechanical and electrical design interfaces for the chassis, motherboard, power supply, and other system component design, and provides component placement guidelines that enable more efficient system cooling for the CPU, graphics, chipsets, PCI Express slots, and their associated voltage regulators. This is accomplished, while pursuing the goal for a quieter PC.

In the BTX PC the CPU is placed at the front, right next to the intake fan, which is encased by what is known as a thermal module. The thermal module includes a heatsink and an air duct that guides the airflow to the hot components. As illustrated in Figure 1, the Memory Controller Hub (MCH), I/O Controller Hub (ICH), graphics, and PCI-Express slot are all located in the same line as the CPU to benefit from the thermal module airflow.

Figure 1 " BTX Motherboard Air Flow and Component Placement

Figure 2 illustrates three types of BTX motherboard form factors: picoBTX, microBTX, and regular BTX. The picoBTX standard can support either one or two expansion slots and has four mounting holes. PicoBTX cases will have one 3.5" and one 5.25" drive bay. The microBTX standard, which most are expecting to be the most popular of the three, has support for four expansion slots and has seven mounting holes. MicroBTX cases will feature one 3.5" bay and up to two 5.25" bays. The regular full-sized BTX motherboard supports up to 7 expansion slots and mounts to the case using 10 mounting screws. Regular BTX cases will be more like the mid-tower ATX cases, supporting three or more 3.5" bays and three or more 5.25" bays.

Figure 2 " BTX Motherboard Form Factors

The BTX Power Supply Unit (PSU) delivers +12V, -12V, +5V, +3.3V, and +5VSB (standby). The +5VSB is the only voltage available from the computer power supply when the other voltage supplies are off in any of the computer ACPI sleep states.

The first desktop systems with BTX motherboards will use Intel's Pentium 4 processor based on the VRD10.1 specification, and the latest Intel processors come with the new LGA775 CPU socket.

The Intersil approach to motherboard power system delivery is one where the voltage regulator architecture simplifies design and placement, minimizes the number of on-board voltage regulators and their associated BOM costs, and reduces the time-to-market by providing easy cut-and-paste solutions from one model to the next.

Addressing the VRD10.1 BTX power system led to the development of a complete power management solution as shown in Figure 3. The ISL6561 is a two-to-four-phase VRD10.X PWM controller and the ISL6612/13/14 are MOSFET drivers for the CPU core voltage regulator. The ISL6612/13 are single-channel FET drivers, driving two N-channel MOSFETs in a synchronous-rectified buck converter stage. The ISL6614 integrates two ISL6613 drivers, and drives four N-channel MOSFETs for two synchronous rectified bridges. The ISL6537A is a 5-in-1 DDR/chipset/FSB/VDAC voltage regulator. The ISL6506B is a linear dual-output ACPI voltage regulator.

Click to enlarge

Figure 3 " Intersil VRD10.1 BTX Motherboard Power Management System

The Intersil power solution takes advantage of the BTX airflow as shown in the motherboard photo in Figure 4. The CPU core regulator is located in front of the thermal module at the bottom of the photo, and the ISL6506 ACPI power controller is exposed to cooling, when placed near the DIMMs, NW of the CPU socket. The ISL6537A and ISL6612 are placed next to the DIMMs, and near the chipset in order to provide tight voltage regulation.

Figure 4: Intersil Power System Solution on BTX Motherboards
(Motherboard photo Courtesy of International Business Machines Corporation. Unauthorized use not permitted.)

The power requirements of the Pentium 4 processor require a multiphase buck voltage regulator, which uses the +12V PSU voltage source to regulate to a 6-bit VID-programmed CPU core output voltage. The Intersil Endura ISL6561 is a two-to-four phase VRD10.X synchronous PWM buck controller that drives a combination of Intersil's single and dual synchronous-rectified MOSFET drivers to form a multiphase buck dc/dc converter regulating Intel's VRD10.X Pentium 4 CPU core voltage. It comes in a 40-pad 6X6mm Quad Flat No-leads (QFN) package. The ISL6561 employs either low FET Rds(on) or DCR (inductor DC resistance) current sensing in each phase for channel current balance, adaptive voltage positioning (droop), and over-current protection. By utilizing either an internal or external thermistor, the ISL6561 nullifies the low FET Rds(on) temperature sensitivity and ensures droop accuracy and optimal core converter loadline performance. The thermistor integration reduces additional BOM cost and PCB area on the BTX motherboard. The differential current sensing feature of the PWM controller is a key feature in implementing precise channel current balance, over-current protection trip, and microprocessor loadline performance. The loadline window is a voltage tolerance band around the CPU core voltage that has to be met as load current increases, for the processor reliability and performance to be optimal. Intersil will also be sampling a multiphase PWM controller family for Intel's next generation VR specification later this year.

The ISL6561 is paired with Intersil's new ISL6612, ISL6613 or ISL6614 MOSFET drivers. These Endura driver ICs offer fast 2-MHz switching with up to 3-A drive current for synchronous-buck configured N-channel MOSFETs. The ISL6612 drives the upper MOSFET's gate to 12V, while the lower MOSFET's gate can be independently driven over a range from 5V to 12V. The ISL6613 drives both upper and lower MOSFET gates over a range of 5 V to 12 V. This drive-voltage flexibility provides optimization of applications involving trade-offs between gate charge and conduction losses. The ISL6614 further reduces BOM cost and PCB area, by combining two ISL6613 FET drivers in one chip.

The ISL6612/13/14 offer a unique pre-Power-On-Reset (pre-POR) over-voltage protection feature. The pre-POR over-voltage protection function becomes operational before the FET driver VCC exceeds its turn-on (rising) threshold. Upon DC/DC converter power-up, and at a very low voltage, the ISL6612/6613 look for over-voltage events, such as upper MOSFET shorts. When such an event is detected, the PHASE node is connected to the gate of the low side MOSFET (LGATE), limiting the output voltage of the converter to the gate-to-source threshold voltage (Vgs(th)) of the low-side MOSFET. For complete protection the lower MOSFET gate threshold should be below the maximum voltage rating of the load.

The ISL6612/13/14 also have a 36V Bootstrap (BOOT) pin rating that enables the FET drivers to withstand switching node spikes, typically found in switch-mode DC/DC converters. The near-chip-scale 10-pad 3X3mm Dual Flat No-leads (DFN) and 16-pad 4X4mm QFN packaging offer high thermal efficiency and space savings.

To speed time to market and reduce implementation costs, Intersil has developed a Drop-In Power Solution (DIPS) to implement the CPU core regulator with first pass success on a BTX MB VRD10.1 layout. The DIPS reference designs are optimized for the Pentium 4 LGA775 socket, as well as the previous generation mPGA478 socket (VRD10). For the LGA775 socket, the CPU core voltage regulator supplies current from the North and East side of the socket, with the ISL6561 located in the Southeast corner. Voltage regulator component height is kept to a 9mm maximum around the CPU, due to the Socket LGA775 Thermal Solution keep-out zone.

A design procedure, schematic, component layout, bill of materials, and test report of the ISL6561 + ISL6612x3 DIPS evaluation board are provided. Shown in Figure 5, layout takes into consideration the LGA775 power delivery path, along with the keep-out areas on the motherboard. The Intersil DIPS for the Pentium 4 processor in the LGA775 socket is capable of supplying 105 A of continuous current with airflow, while meeting the Pentium 4 CPU loadline specification.

Figure 5: Intersil DIPS (Drop-In Power Solution) for Pentium 4 LGA775 CPU on BTX Motherboard

As illustrated in Figure 6, taking temperature measurements on the power train components is very important as it allows the designer to select the proper FETs and inductors, the right package, optimize the component placement, and routing, and ensure that the proper amount of airflow is available for cooling. The BTX motherboard layout allows more air to get to the CPU core regulator and the other down-stream voltage regulators because the placement of the CPU and peripherals is streamlined.

Figure 6: Thermal Measurement Set-up for Pentium 4 LGA775 CPU Core Regulator

For the peripheral voltages, a pair of Intersil multi-output voltage regulators completes the power delivery. The ISL6537A and ISL6506 regulate a total of seven motherboard voltages: the DDR memory core, DDR termination, chipset core, 5V(dual), 3.3V(dual), VDAC, as well as the CPU front side bus termination voltage. As shown in Figure 7, the ISL6537A is a five-in-one voltage regulator that regulates the DDR/DDRII core voltage, DDR termination, chipset core, Front Side Bus (FSB), and DAC portion of the chipset.

Figure7: ISL6537A Power System Diagram

The ISL6537A provides a complete ACPI-compliant power solution for up to four dual in-line memory module (DIMM) dual-channel DDR/DDR2 memory systems. It comes in a 28-pad 6X6mm QFN package. The synchronous buck controller supplies the DDR/DDRII core (VDDQ) during the S0/S1 and S3 states. During the S0/S1 state, a fully integrated 3-A sink/source regulator generates an accurate (VDDQ/2) high-current DDR/DDRII termination voltage (VTT) without the need for a negative supply. The ISL6537A has a second PWM controller that regulates the chipset core voltage via an external ISL6612 MOSFET driver. This partitioning is optimal, as routing the MOSFET driver allows more flexibility in placement of the regulator, reduces thermal dissipation inside the ISL6537A, minimizes the risk for noise coupling between the ISL6612 driver gate trace and the MOSFET, and improves the overall voltage accuracy by having the driver right next to the point of load. Two more linear regulators are included in the ISL6537A: the FSB/VID_Vtt termination LDO controller, and the DAC LDO controller. The ISL6537A is fully compliant to the i915G, i915GV, and i915P chipset power sequence requirements.

On a BTX motherboard, the ISL6537A and the ISL6612 are located to the NW of the LGA775 P4 CPU socket, next to the DIMMs and close to the chipset in order to provide tight DDR/DDRII, FSB termination, chipset core, and VDAC voltage regulation (Figure8). The ISL6537 is a version of the ISL6537A, with the second chipset core PWM controller replaced by a dual-stage Low Drop-Out (LDO) regulator.

Figure 8: DDR/chipset/FSB/VDAC regulator and ACPI controller on BTX Motherboard
(Motherboard photo Courtesy of International Business Machines Corporation. Unauthorized use not permitted.)

Two other combination regulators that are related to the ISL6537A are the ISL6548 and ISL6548A. Both ICs are fully compliant to the high-end i925X chipset power-up timing. The ISL6548/48A differ from the ISL6537A in that their FSB/VID_Vtt LDO not only sources but also sinks current from the bus termination, an i925X chipset requirement.

Intersil will soon sample the ISL6548/48A 4-in-1 and 5-in-1 output voltage regulators that are fully-compliant not only to the i925X chipset, but also to the next-generation Intel Desktop chipset power sequencing and voltage rail regulation requirements. Shown in Figure 9, the ISL6548 family is optimized for lower voltage rails associated with DDRIII and next generation chipset and termination. The ISL6548/48A's backwards compatibility with the i925, as well as with DDR/DDRII memory-based systems enables seamless transition of power solutions from one chipset platform to the next.

Figure 9: ISL6548A 5-in-1 Voltage Regulator for the Next-generation Desktop Chipset

Illustrated in Figure 10, Intersil's ISL6506B dual ACPI-compliant regulator completes the BTX MB power system. Packaged in a space-saving and thermally enhanced 8-lead exposed-pad SOIC (8L EPSOIC), the ISL6506B regulates 5V dual, for the USB, keyboard and mouse, and 3.3V dual /3.3V standby for PCI, auxiliary and LAN circuits in S3/S5 ACPI modes.

Figure 10: ISL6506B Dual ACPI LDO Regulator/Controller

The ISL6506B uses a 2A integrated linear regulator to generate 3.3V dual from the ATX supply's 5VSB output during sleep states (S3, S4/S5). In active states, during S0 and S1/S2, the ISL6506B uses an external N-channel pass MOSFET to connect the outputs directly to the 3.3V input supplied by an ATX power supply, for minimal losses. The ISL6506B generates 5Vdual by switching in the ATX 5V output through an NMOS transistor in active states, or by switching in the ATX 5VSB through a PMOS (or PNP) transistor in S3 sleep state. In S4/S5 sleep states, the ISL6506B 5Vdual output is shut down.

Another member of the ISL6506 family, the ISL6506A, keeps the 5Vdual output on during S4/S5 sleep states. The ISL6506B operating mode (active outputs or sleep outputs) is selectable through two digital control pins, S3# and S5#. As illustrated in Figure 11, the ISL6506B provides the ISL6537A with the 5V dual power input required for down-conversion to the DDR core and the other voltages.

Figure 11: ISL6506 + ISL6537 Peripheral Voltage Regulator Pair

Conclusion

As BTX form factor motherboards are expected to become mainstream OEM/ODM designs in the coming year, a holistic, cut-and-paste approach to the motherboard power system is required. Intersil's CPU core, peripheral, and ACPI voltage regulator chipset optimizes the BTX motherboard design, by reducing the BOM cost, and PCB area, simplifying component placement, and complying with the current as well as next-generation Intel desktop chipset power sequencing requirements. Intersil's DIPS reference design tool significantly reduces time to market by providing an easy and reliable cut-and-paste layout of the BTX motherboard voltage regulators.

Authors
George Lakkas, Product Marketing Manager, Power Management Products, Durham, NC B.S.E.E, Northeastern University, Boston, MA

Charles Bailley, Staff Design Engineer, Power Management Products, Palm Bay, FL B.S.E.E, U.S. Naval Academy, Annapolis, MD

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