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High-power density systems require high-current converters
Nov 15, 2017

Background Information

In 2016, the expanding market for high-current, low-voltage digital ICs reached 9.2 billion U.S. dollars [Source: Intense Research]. Such digital ICs include microcontrollers and microprocessors (μC and μP), programmable logic devices (PLDs), digital signal processors (DSPs), application specific integrated circuits (ASICs), and graphics processor units (GPUs). In addition, let's look at the expectations of a larger market segment, Field Programmable Gate Array (FPGA) IC, in this segment, which was $ 3.92 billion in 2014. It is estimated that by 2022 Reaching 7.23 billion U.S. dollars, a CAGR of 7.41% from 2016 to 2022 [data source: marketsandmarkets.com]. High-power density digital ICs have entered almost all embedded systems. Such systems include, but are not limited to: industrial, communications, telecommunications, server, medical, gaming, consumer audio / video and automotive systems. In these markets, FPGAs are making advanced applications a reality, for example, automotive applications such as Advanced Driver Assistance Systems (ADAS) and collision avoidance systems that eliminate human error. In addition, the government required safety features, such as anti-lock braking systems, stability control and electrical control of independent suspension systems must use FPGA. Demand for IoT and M2M communications and data and server centric growth in consumer electronics are also factors driving the FPGA market growth, requiring large amounts of data to be stored and cloud computing to be data And the server driver of growth.

These systems based on high-power-density digital ICs have a unique set of requirements for power supplies. For the current generation of FPGAs and ASIC processors, the combination of high current, low voltage, and fast transient response puts increasingly stringent demands on the power supplies that power these devices. These digital ICs are powerful, but unstable from a power point of view. Traditionally, the devices used to power these devices have been high-efficiency switching regulator controllers with separate high-power MOSFETs, but such controllers have potential noise interference, slow transient response, and layout limitations. As a result, low dropout regulators (LDOs) that minimize heat in recent years have been used as an alternative solution, but such regulators are not without their own limitations. However, due to the latest product innovations in this area, trends are changing. Newer, high-power monolithic switching regulators no longer need performance compromises and are rapidly being adopted.

Switching Regulator with Charge Pump and LDO

Low-voltage, high-current step-down conversion and regulation can be achieved through various methods and various design trade-offs. For starters, the switching regulator controller operates at high efficiency, delivering high currents over a wide voltage range, but requires external components such as inductors and capacitors (and FETs in the case of controllers) To run. Inductor-less charge pumps (or switched capacitor voltage converters) can also be used to achieve lower voltage conversions, but with limited output current capability and poor transient performance, requiring more external circuitry than current linear regulators Component. As a result, the charge pump is infrequent in digital IC power applications. In contrast, linear regulators, especially LDOs, are very simple because they require only two external capacitors for operation. However, the linear regulator may have limited power, depending on the amount of input-to-output voltage difference across the IC and how much current the load requires, as well as the package's thermal resistance characteristics. This places restrictions on the linear regulator into the field of digital IC power supply.

Design Challenges for High Current Monolithic Buck Converters

Chip manufacturing technology, narrowing the line width, strictly follow Moore's Law (originally conceived in 1965), thus requiring digital IC to run at lower voltage. Smaller geometries allow more functionality in the final product to be integrated with more power. For example, modern computer servers and communication routing systems require more bandwidth to handle more computing data and Internet traffic. Cars have more automotive electronics to provide entertainment, navigation, self-driving functions and even engine control. As a result, the system current consumption and the total power required increase. As a result, state-of-the-art packaging and innovative internal power stage designs are needed to dissipate heat from the power IC while delivering unprecedented power.

Larger power supply rejection ratio (PSRR) and lower output voltage noise or ripple requirements are two other challenges to consider. Devices with a large power supply rejection ratio can more easily filter and reject noise at the input, resulting in a clean and stable output. In addition, devices with lower output voltage noise or lower output ripple over a wide bandwidth are conducive to powering today's new low-noise rails, where noise sensitivity is a major design consideration. As the speed requirements for high-end FPGAs increase, power supply noise margins continue to drop, minimizing bit errors. For such high-speed PLDs, such noise-induced digital faults greatly reduce the effective data throughput rate. At high currents, input supply noise is clearly an important but demanding performance specification.

Higher transceiver rates (such as in FPGAs) determine larger currents because circuits of smaller geometries consume more power when switching. Such ICs are fast and may increase the load current from nearly zero to several amperes in tens to hundreds of nanoseconds, thus requiring a regulator with an ultrafast transient response.

As the area of the circuit board reserved for power regulators becomes smaller and smaller, it is also becoming increasingly commonplace that monolithic switching regulators with higher switching frequencies reduce the size of the external components and therefore reduce the number of solutions The overall size, with the attendant trade-off, is that there is some slight loss of efficiency due to switching losses at higher frequencies. However, the new generation of monolithic switching regulators offers some unique features that significantly reduce switching losses even at higher frequencies. That is, the simultaneous operation of the integrated high-side and low-side switches allows better control of their gate voltage, which greatly reduces the dead time and therefore enables more efficient operation.

One of the biggest challenges of high-current monolithic switching regulators is their ability to dissipate heat, which draws a lot of power from the IC. This challenge is addressed by using a thermally enhanced ball-in-glass array (BGA) package where most solder balls are dedicated to the power supply pins (VIN, SW, GND) so that the heat can be very easy From the IC to the circuit board. The larger copper plane on the board connected to these power pins allows for more uniform heat dissipation.

New High Current Step-Down Regulator

Obviously, the buck converters solutions that address the issues mentioned in this article need to have the following properties:

· Higher Switching Frequency - Reduced external component size

Dead time zero design - to improve efficiency

· Monolithic - Built-in power devices to achieve smaller size solutions

Synchronous operation - higher efficiency and lower power consumption

Simple design - requires minimal external components

Very low output ripple

Fast transient response

Operating over wide input / output voltage range

Can provide a large output current

Excellent thermal performance

Compact footprint

To meet these specific needs, Linear Technology has introduced the LTC71xx family of monolithic high-current step-down regulators. The newest member of this family is the LTC7150S, a 20V / 20A monolithic synchronous buck converter with differential VOUT remote sensing. The unique phase-locked controlled on-time, constant frequency current mode architecture to reduce the burden on the compensation, ideal for high frequency operation while requiring fast transient response of high buck ratio applications. The LTC7150S utilizes Silent Switcher® 2 technology, including an integrated bypass capacitor, to provide a high-efficiency solution with superior EMI performance at high frequencies. Multiphase operation of up to 12 phases allows multiple devices to be directly connected in parallel to provide greater current with minimal input and output capacitance. VOUT remote sensing ensures accurate load side voltage regulation regardless of load current or board layout. Its wide 3.1V to 20V input range supports a wide range of applications including most intermediate bus voltages and is compatible with many battery types. The integrated N-Channel MOSFETs deliver up to 20A of continuous load current with minimal thermal derating over the 0.6V to VIN output voltage range, making them ideal for point-of-load applications such as high current / low voltage DSP / FPGA / ASIC reference designs. Other applications include telecommunications / data communications systems, distributed power architectures and general high-power density systems. Figure 1 shows a typical application schematic showing the simplicity of the design.

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The very short 25ns minimum on-time of the LTC7150S allows high step-down ratios to be achieved at high frequencies. The operating frequency is user-selectable from 400kHz to 3MHz and can be synchronized to an external clock. The LTC7150S's total differential output voltage accuracy is ± 1% over the operating junction temperature range of -40 ° C to 125 ° C. Other features include high speed differential remote sense amplifiers, PHMODE phase selector pins, accurate 1.2V RUN Pin Threshold, VIN Overvoltage Protection, Power Good Marking and Programmable Soft-Start / Tracking.

Finally, the LTC7150S is available in a thermally enhanced 42-lead 6mm x 5mm x 1.3mm BGA package and is available in lead-free and lead-free SnPb (63/37) RoHS. Class E and Class I versions operate over the -40 ° C to 125 ° C junction temperature range.

High efficiency, lower EMI and fast transient response

Part Numbering "S" in the LTC7150S refers to the second generation of Silent Switcher technology. The IC integrates ceramic capacitors for VIN and BOOST to keep all fast AC current loops small, thereby improving EMI performance. In addition, the device allows faster switching of switching edges, which greatly improves efficiency at high switching frequencies.

The LTC7150S's unique controlled on-time architecture allows the IC to respond quickly to transient steps. This is done during transient steps - the switching frequency comes with an acceleration capability, which allows the inductor current to better follow the output of the error amplifier (ITH). This allows more aggressive setting of ITH compensation, which can increase the total loop bandwidth.

The LTC7150S allows high efficiency at high frequencies because of the key feature of the device, which is a significant reduction in dead time. The servo loop inside the IC locks the dead time to <1ns before the rising edge of SW. The dead-time reduction minimizes / eliminates the need for the bottom switching body diode conduction. This essentially eliminates the effect of the reverse recovery of the bottom switching body diode when the top switch is on. Due to this characteristic, the power consumption is considerably reduced.

Lower ripple current reduces inductor core loss, output capacitor ESR loss, and output voltage ripple. Low frequency, small ripple current can achieve high efficiency operation. However, achieving this requires a large inductor. There is a trade-off between component size, efficiency and operating frequency. The curves in Figure 2 show the high efficiency of the LTC7150S.

This unique constant frequency / controlled turn-on time architecture is ideal for high step-down applications operating at high frequencies while requiring fast transient response. Figure 3 shows the transient response of the LTC7150S.

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Ultra-low DCR current sensing applications

The LTC7130 is a constant frequency, peak current mode control, synchronous step-down DC / DC converter, ultra-low DCR current sensing and clock synchronization with temperature compensation. The device's unique architecture alleviates the burden of compensation and enables direct parallel connection for greater output current capability. The LTC7130 also increases the signal-to-noise ratio of the current sense signal, allowing the use of very low DC resistance power inductors to maximize efficiency in high current applications. This feature also reduces the switching jitter commonly found in low DCR applications and increases the accuracy of the current limit. The LTC7130's 4.5V to 20V input range supports a wide range of applications, including most intermediate bus voltages and is compatible with many battery types. The integrated N-channel MOSFET delivers up to 20A of continuous load current over the 0.6V to 5.5V output voltage range, making the device ideal for point-of-load applications such as high current / low voltage DSP / FPGA / ASIC reference designs. Other applications include telecommunications / data communications systems, distributed power architectures and general high-power density systems. Figure 4 shows a typical application circuit.

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In Conclusion

The continuous development trend of high performance digital ICs such as FPGAs and microprocessors is that the current is getting larger and larger and the operating voltages are getting lower and lower accordingly. This is achieved through the increasingly narrow linewidth chip manufacturing technology. However, along with these advances are other application needs, in the field of power management, such needs include the need for fast transient response, low noise / low ripple, and high efficiency operation to minimize heat. Traditionally, powering these digital ICs has been done with LDO or inductor-based switching regulator controllers and external power devices. However, Linear Technology offers a new generation of monolithic, high current step-down switching regulators in thermally efficient BGA packages to address these issues. These products include the LTC7150S and LTC7130, both of which have unique capabilities to address the power of digital ICs in a variety of applications.