In the process of people’s continuous pursuit of higher system efficiency and performance, the working input voltage of digital and mixed-signal components in data storage and communication systems is showing an increasingly lower trend. In many cases, the maximum input voltage required by most of the components inside such systems is now only 3.3V. In this case, the traditional 5V or 12V intermediate voltage rail can be bypassed, and the 24VDC or 48VDC backplane distribution voltage can be directly converted into a 3.3V dual-purpose bus and power rail. Many suppliers of high-power DC/DC brick modules (such as Emerson and TDK-Lambda) have recognized this development trend, and they have easily achieved 92% of the performance by greatly improving their performance in high step-down ratio operations. Efficiency indicators. Using this 3.3V intermediate bus, subsequent point-of-load regulators can generate lower voltages (ie: 2.5V, 1.2V, 1.0V, etc.) for power supply memory, ASIC/FPGA core and high-speed I/O Wait for power supply.
Direct conversion from the intermediate bus provides another advantage, which is the reduction in the number of copper foil layers required to complete the power rail to load wiring on the printed circuit board (PCB). Take a PCB with a 5V voltage rail used only as an intermediate bus as an example. It contains two DC/DC converters for supporting 3.3V and 1.8V voltage rails. The same circuit board redesigned with a 3.3V intermediate bus and a single 3.3V to 1.2V converter will most likely have fewer copper layers (3 voltage rails are now reduced to 2). The size of the overall solution finally formed on the circuit board is very attractive, while eliminating the need to transmit the 5V potential to an integral part of the PCB. The option of reducing the number of copper foil layers as much as possible in the PCB manufacturing process has the potential to reduce costs and save materials, and is expected to improve yield and reliability.
In addition, for system operation from backup power sources such as supercapacitors, a lower voltage intermediate bus rail is also very suitable. Compared with batteries, supercapacitors can support higher peak current, power density, wider operating temperature range and lower ESR, so they are increasingly used as short-term power supplies to provide battery backup systems Replenish. Since the maximum charging voltage of the supercapacitor is only 2.3V to 2.7V, the use of a high-efficiency low-input voltage step-down converter can maximize the system preparation time to achieve rapid system recovery after the main power supply is turned on again .
Limitations of traditional solutions
When using traditional DC/DC step-down solutions, the switching regulator or switching controller requires a minimum input voltage or bias voltage of about 5V to drive N-channel power MOSFETs. During current conduction, this minimum voltage needs to be used to drive the power MOSFET to a low on-resistance area. For efforts to improve work efficiency (especially under high current conditions often encountered in network and storage systems), any increase in on-resistance is disadvantageous. For those systems that try to improve work efficiency and reduce production costs by minimizing the intermediate rail voltage to the component input supply voltage (such as 3.3V), the challenge is how to best support the current consumption, which is usually only 50mA ~100mA bias power supply-add a 5V output high voltage step-down regulator; add a boost converter (from 3.3V); or continue to use the existing 5V intermediate bus. In terms of the number of components, design workload, PCB complexity, reliability, cost, and work efficiency, the above options require some unpleasant compromises.
A better alternative solution
Another alternative that aims to solve the problem of low operating input voltage mentioned earlier in this article is the LTM4611 step-down µModule® regulator. This device belongs to a new series of DC/DC converters, which is developed from traditional switching power management solutions. Almost all components of the switching converter (including inductors) are integrated into a compact surface mount type In the package. The LTM4611 power module uses a single working input voltage rail from 1.5V to 5.5V, and steps it down to an output voltage as low as 0.8V, and can provide up to 15A of output current. The self-generated bias power supply completely built into an LGA package can support operation from a single low-voltage power supply. Figure 1 shows a schematic diagram of the LTM4611 circuit for a fully operational 15A step-down solution. It can be clearly seen from the figure: The circuit requires very few external components, which can achieve a compact solution and simple PCB layout.
Figure 1: A complete voltage converter schematic (used to operate from 2.1V to 5.5V single input to provide a 1.8V/15A output)
Work efficiency comparison
It is very tricky to prove the rationality of the traditional three-stage step-down architecture from the efficiency point of view, because the efficiency of each step-down stage between the distribution voltage rail and the load must be much higher than that of the two-stage solution. Figure 2 shows the previously proposed 5V intermediate bus option and the 3.3V intermediate bus implemented using the LTM4611 µModule regulator. In both cases, the 48V step-down is simulated as a 75W Emerson (formerly Artesyn) 1/8-brick single output converter whose 1.8V and 3.3V voltage rails bear a 10A load. In the traditional three-stage step-down architecture, the 5V to 3.3V and 5V to 1.8V step-down converters are simulated as another device in the µModule regulator series.
Figure 2: Schematic diagram of three-stage and two-stage step-down architectures
Total power loss during 48VDC to 3.3VDC and 1.8VDC conversion)
Figure 3 compares the efficiency and total power loss of the three-level solution and the two-level solution using the LTM4611 in a wide output current range (assuming the output current on each voltage rail is the same). Since the maximum rated power of the brick module is 75W, for the 3.3V and 1.8V voltage rails, the maximum output current that the three-stage solution can provide is limited to 13A, while the two-stage solution can each support up to 14A output current . As shown by the curve in the figure, the difference in total power loss between the two solutions in the process of returning to the 48V distribution voltage will be quite large, and may therefore further push up the cost-due to the increase in the copper foil area in the PCB , The increase in the size of the actual system, the use of radiators, and even the forced cooling airflow that must be provided in order to maintain reliable system operation.
Figure 3: Comparison of efficiency and power loss between three-stage and two-stage conversion (from 48VDC to 3.3VDC and 1.8VDC)
For more and more products, reducing power loss at light loads is equally important than reducing power loss at heavy loads—if not more important. The subsystem is designed to work as long as possible in a standby or sleep state with lower power consumption (in order to save energy), and only draw peak power (full load) when needed. LTM4611 supports pulse skip mode and burst mode (Burst Mode®) operation. Compared with continuous conduction mode, its efficiency level under the condition of less than 3A load current has been greatly improved.
Current sharing of multiple power supplies to provide an output current of 60A or more
For power rails that need to provide up to 60A output, it can support current sharing of up to 4 LTM4611 µModule regulators. The current mode control makes the current sharing of the modules particularly reliable and easy to implement. At the same time, it can even ensure the current sharing between the modules during startup, transient and steady-state operation.
In contrast, many voltage mode modules use a master-slave configuration or “droop-sharing” (also known as “load line sharing”) to achieve current sharing. Under start-up and transient load conditions, the master-slave mode is prone to over-current jumps, and even the voltage drop will cause the load regulation index to drop, and it is almost impossible to guarantee a good module-to-module current during the transient load step. match. The LTM4611 usually provides better than 0.2% load regulation from no load to full load-0.5% (maximum) over the entire internal module temperature range from -40ºC to 125ºC.
Accurate regulation on load
High current and low voltage FPGAs, ASICs, microprocessors (μP), etc. often need to be connected to the package terminals (for example: VDD And DGND pin) provide an extremely accurate voltage that has been precisely adjusted-nominal VOUT ±3% of (or better). At such high current levels and low voltage levels, the resistive distribution losses in the PCB traces may affect the voltage on the load. In order to meet this strict regulation requirement for low output voltage, LTM4611 provides a unity gain differential amplifier for remote sampling on the load terminal when the voltage is lower than or equal to 3.7V. It can be seen from Figure 1 that the differential feedback signal (VOSNS+ – VOSNS−) In DIFF_VOUT The upper is reconstructed (relative to the local SGND of the module) so that the control loop can compensate for any voltage drop in the power delivery path between the output pin of the module and the POL device.
When the output voltage of LTM4611 is at nominal VOUT Within ±5%, an internal output voltage power good (PGOOD) indicator pin will provide a logic high open-drain signal; otherwise, the PGOOD pin will be pulled to a logic low level. When the output voltage exceeds 107.5% of the nominal value, the output over-voltage protection circuit will be triggered and the internal low-side MOSFET will be turned on until the over-voltage condition is cleared. Foldback current limit protects the upstream power supply and the device itself in the event of an output short circuit.
Thermally enhanced package
The LGA package of the device allows heat to be dissipated from the top and bottom, thus facilitating the use of a metal chassis or BGA heat sink. Regardless of the presence or absence of cooling airflow, this package shape is conducive to achieving excellent heat dissipation. Figure 4 shows the IR thermal image of the top surface of the LTM4611. It can be seen from the figure: When the 1.8V input to 1.5V/15A output conversion is performed and there is no cooling air flow, the power loss measured on the test bench is only 3.2W.
As mentioned above, under the condition of low input voltage of 1.8V, in order to drive the gate with sufficient amplitude to fully saturate the power MOSFET, the traditional power IC solution without bias power will be very difficult. Therefore, its thermal performance will be lower than the level that LTM4611 can provide (as shown in Figure 4), because the latter has an internal micro-power bias generator.
Figure 4: The top thermal image of the LTM4611 regulator when it produces a 1.5V/15A output from a 1.8V input.
The power loss is 3.2W. The bench test without cooling air flow produced a 65ºC surface temperature hot spot.
Reduce board area
The LTM4611 is built in a thermally enhanced LGA (Land Grid Array) package, with a small land pattern (only 15mm x 15mm) and actual volume (only 4.32mm in height-only 1cm of space occupied3), can provide compelling efficiency. In addition to high efficiency, under a given input voltage condition, the power consumption curve of the LTM4611 is relatively flat, which makes the thermal design of the LTM4611 and the repeated use in subsequent products simple and easy-even when the intermediate bus voltage is due to IC chips are shrinking and declining, and this is no exception.
A reliable solution
Linear Technology’s µModule regulators (such as LTM4611) are tested according to the same stringent standards as other packaged integrated circuits in the product series. Before being released to the public, the product must successfully pass a series of tests, such as: working life test according to JEDEC specifications, +85℃/85% temperature-humidity bias, temperature cycling, mechanical shock, vibration, etc. This principle gives engineers full confidence: these highly integrated solutions can provide reliability comparable to traditional switching converters, while the latter requires many associated external components, which must be purchased and manufactured Purchase, assemble and inspect with the quality department.
The industry urgently needs to improve speed performance and reduce power consumption, which promotes the continuous reduction of the operating voltage of digital components. In order to adapt to this development trend, DC/DC brick module suppliers are introducing new devices that can directly step down the distribution voltage rail (24V or 48V) to an output voltage lower than 5V with high efficiency. The practice of generating a 5V bias voltage rail solely for efficient operation of traditional switching converters can increase undesirable cost, power consumption, complexity, or components. The LTM4611 is built into a single LGA package (many other integrated circuits use this package), which maintains high efficiency and excellent thermal performance over the entire input voltage range. LTM4611 is a simple and highly reliable step-down regulator, which can easily adapt to those load point applications that need to provide high output current from an input voltage as low as 1.5V, and reduces the need for “extra” voltage rails .