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Posted on 29 June 2019

Convert from Inputs Down to 1.5V, Deliver Up to 15A Output, without an Auxiliary Bias Supply

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Many next-generation designs for multi-ASIC embedded systems and dual-core single-board computers utilizing 48V or 24V distribution backplanes are migrating towards a 3.3V system bus -- away from 5V system bus architectures -- as the designing-out of legacy 5V digital devices leaves a need for only 3.3V DD voltages, and lower. Are there other reasons? -- Yes!

When implementing a 3.3V bus, overall power consumption of the system can be reduced--compared to incumbent architectural approaches, but only when proper DC/DC point-of-load (POL) converters are selected for subsequent down-conversion of the bus voltage to power DDR memory, FPGA core, or high-speed transceivers, for example.

By Jason Sekanina, Design Engineer, μModule Power Products and Alan Chern, Product Evaluation Engineer, μModule Power Products, Linear Technology Corp.

 

Running only one bus line (3.3V) simplifies circuit design and free-up board space, but is not always feasible. Often, higher-power DC/DC POL regulators require bias above 3.3V to drive their power switching MOSFETs.

Isolated bricks have improved their efficiency in converting 48V and 24V down to 3.3V — at ever-higher output power levels.

The challenge really appears when the loads that are downstream from the 3.3V bus require more than 5A to as much as 12A. Although this requirement seems rare, advances in FPGAs, processors and ASIC technologies enable designers to use more of these devices to boost performance in smaller circuit boards. Applications with 10A loads are increasingly common.

One recent customer asked for 30A from a 1V rail, powered from a 3.3V input bus. However, traditional low input voltage high power switchmode DC/DC converters with N-channel MOSFETs rely on a second regulator (housekeeping) circuit to provide higher-thanbus Vin voltage for MOSFET gate drive–increasing layout complexity, size and cost. When 5V (or higher) is not available, delivering high current to loads from a 3.3V input bus is usually very inefficient. The resulting excessive power dissipation increases the junction temperature of the regulator and surrounding components, and only serves to undermine system lifetime reliability.

To the rescue

The LTM®4611 is a low profile μModule step-down switch mode DC/DC converter in a compact 15mm × 15mm × 4.32mm LGA surface mount package. The switching controller, MOSFETs, inductor and support components are housed in the package, so design is reduced to selecting a few external components. The LTM4611 operates from an input voltage of 1.5V to 5.5V (6V, absolute maximum), making it suitable for a variety of power architectures, particularly data storage and RAID (redundant array of independent disks) systems, ATCA (advanced telecommunications computing architecture) and networking cards, where one or several commonly bussed voltages are 5V, 3.3V, 2.8V, and/or 2.5V.

While it is uncommon to see bus voltages lower than 2.5V due to the distribution losses (voltage drops) associated with relatively high bus currents, the ability of the LTM4611 to deliver full power to its load from a 1.5V input is particularly advantageous in applications where load voltages must be precisely regulated even as momentary or sustained electrical events induce input-bus line-sag. Transient events on the system bus can occur normally due to the operation of motors, transducers, defibrillators or an uptick in microprocessor activity. Fault events on a system’s distributed bus may leave the bus voltage compromised, but still above 1.5V. The LTM4611’s ability to deliver full power from as low as 1.5V input allows it to be considered for mission, critical, medical, and industrial instruments that have the highest standards for uptime and bus-sag ride-through capability. Precision-regulated power can even be provided by the LTM4611 to its load during so-called “dying-gasps”, which are sudden, unexpected losses of system power, such as those monitored by utility smart meters, where it is highly desirable to be able to operate from the decaying voltage provided by backup batteries or supercapacitors for as long as possible.

There is another advantage in the LTM4611’s ability to operate from as low as 1.5V: as the number of rails increases in today’s power system, so are the number of layers of copper in printed circuit boards (PCBs) required to route (distribute) the power effectively to the load. Consider a hypothetical example: it can be difficult to route a distributed 3.3V bus to both 3.3V-to-1.5V and 3.3V-to-1.2V DC/DC converters without increasing the number of layers of copper in the PCB. Alternatively, one LTM4611 could convert the 3.3V bus to a distributed 1.5V copper plane, while another LTM4611 could efficiently convert the 1.5V plane voltage to 1.2V at the POL. The resulting total solution size on the motherboard could be quite compelling, while eliminating the need to route 3.3V bus potential to an entire section of the PCB. The option to minimize the number of layers of copper in the manufacture of the PCB has potential for cost and material savings, and associated benefits to PCB yield in mass-production and PCB reliability.

Brains and Brawn - Self generated BIAS supply

The LTM4611 does not require an auxiliary bias supply to power its internal control IC or MOSFET-drive circuitry; it generates its own low power bias from the input-source supply. This internal bias supply enables the LTM4611 to operate from as low as 1.5V input — providing strong gate drive signals to its power MOSFETs at all line voltages — and realize high efficiency in systems utilizing 5V, 3.3V or lower bus voltages. The muscle behind the LTM4611 is a buck-converter topology that steps down its input voltage to deliver as low as 0.8V, up to 15A continuous, to its output. A voltage drop less than 0.3V from input-to-output and at 15A load is achievable, with proper selection of input-power source (dependent on source dynamic characteristics and transient load response) and local bypass capacitance. The LTM4611 employs a fixed-frequency peak-current-mode control buck-converter scheme, operating at 500kHz by default. Optionally, the switching frequencycon be adjusted to between 330kHz and 780kHz by resistor-pin strapping LTM4611´s PLLFLTR/fSET pin — or, synchronized to a 360kHz to 710kHz clock signal presented to its MODE_PLLIN pin.

Current Sharing of Multiple Supplies for 60A or More

Current sharing of four modules is supported for solutions up to 60A output. More modules can be paralleled for even higher output current — contact Linear Technology for details. Current mode control makes current sharing of modules especially reliable and easy to implement, and ensures module-to-module sharing of current during start-up, transient and steady-state operating conditions.

This is in contrast to many voltage mode modules, which achieve current-sharing by employing either master-slave configurations or by using “droop-sharing” (also called “load-line sharing”). Master-slave configurations can be vulnerable to nuisance overcurrent-tripping during start-up and transient load conditions, while droop-sharing results in compromised load regulation specifications while offering little assurance of good module-to-module current matching during transient load steps.

The LTM4611 typically provides better than 0.2% load regulation from no load to full load—0.5% maximum over the full internal module temperature range of -40°C to 125°C.

Easy POL Application: 1.8V–5.5V Input to 1.5V Output at 15A

The block diagram in Figure 1 shows the LTM4611 operating from 1.8V-to-5.5V input and delivering 1.5V output, up to 15A. The output voltage is programmed by a single resistor from VFB to GND. The control loop drives the power MOSFETs and output voltage such that VFB is equal to the lesser of 0.8V or the voltage on the TRACK/SS pin. A soft-start capacitor, CSS, on the TRACK/SS pin programs the start-up rate of the LTM4611’s output when the module’s RUN pin exceeds 1.22V (±10%). CSS assures monotonic output voltage waveform start-up and supports smooth power-up into pre-biased output voltage conditions. A resistor-divider from another rail can be applied to the TRACK/SS pin to program coincident or ratiometric tracking of the LTM4611’s output rail to the reference rail. This is a handy feature when powering digital devices with stringent rail-tracking requirements during system power-up and power-down.

Simplified block diagram of the LTM4611, and typical application

Remote Sensing for Accurate POL Regulation

Routinely, high current low voltage FPGAs, ASICs and microprocessors require extremely accurate voltages of ±3% of nominal VOUT (or better) regulated exactly at the POL terminals usually, VDD and DGND pins. To meet this regulation requirement where it is hardest to do so — for output voltages below 3.7V — the LTM4611 provides a unity gain buffer for remote sensing of the output voltage at the load’s terminals.

Voltage drops across the VOUT and GND copper planes in the PCB are an unavoidable result of resistive distribution losses physically between the module and the load. As shown in Figure 1, the differential feedback signal across the POL (VOSNS+ minus VOSNS-) is reconstructed at DIFF_VOUT with respect to the module’s local ground, SGND, thus allowing the control loop to compensate for any voltage drop in the power-delivery path between the module’s output pins and the POL device.

The LTM4611 includes an output voltage power good (PGOOD) indicator pin that supplies a logic high open-drain signal when output voltage is within 7.5% of nominal VOUT; otherwise, PGOOD pulls logic low. The LTM4611 provides foldback current-limiting to protect itself and upstream power sources from fault conditions on its output. The LTM4611 also includes an output overvoltage protection feature: when the output voltage exceeds 107.5% of nominal, the internal low side MOSFET is turned on until the condition is cleared.

How Green is Your Machine?

DC/DC power conversion efficiency and thermal management is as important today as ever. The LTM4611 provides compelling efficiency in a small land pattern (only 15mm × 15mm) and low physical volume (at only 4.32mm tall—it occupies only one cubic centimeter), in a thermally enhanced LGA (land grid array) package. Figure 2 shows the LTM4611 efficiency for various combinations of input and output voltage conditions. Besides high efficiency, the power dissipation envelope of the LTM4611 is relatively flat for a given input voltage and output loading condition, which makes the thermal design and re-use of the LTM4611 in follow-on products easy—even as rail voltages migrate to lower values due to IC die shrink.

LTM4611 efficiency vs load current for various input and output voltages

For an increasing number of applications, reducing power loss at light loads is as important as reducing power loss at heavy loads. Digital devices are increasingly and deliberately designed to operate in lower-power states for as long as possible and whenever practical (for energy conservation), and draw peak power (full load) only intermittently. The LTM4611 supports Pulse-Skip Mode and Burst Mode® operation, which yield substantially higher efficiency at lighter load currents (< 3A) than Forced Continuous Mode operation offers.

Thermally Enhanced Packaging

The device’s LGA packaging allows heat-sinking from both the top and bottom, facilitating the use of a metal chassis or a BGA heat sink. This form factor promotes excellent thermal dissipation with or without airflow. Figure 3 shows an infrared thermal image of the top surface of the LTM4611, demonstrating a power-loss of 3.5W with no airflow, tested on a lab bench, converting a 5V input to a 1.5V output at 15A. The hottest surface temperature measures about 65°C.

Top thermal image of an LTM4611 regulator producing 1.5V at 15A from at 5V Input. Power loss is 3.5W.

LTM4611 is a complete low Vin DC/DC regulator system in a small surface mount package

Conclusion

The LTM4611 is a μModule buck regulator that easily fits into POL applications needing high output current from low voltage inputs down to 1.5V. Efficiency and thermal performance remain high across the entire input voltage range, simplifying placement in POL applications. Providing up to 15A of load current and easily parallelable for generating up to 60A, the LTM4611 can help simplify and enable board-mount power solutions for next generation 3.3V system bus architectures and beyond.

 

 

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