72V inductorless DC/DC controller delivers >500W

Article By : Vivek Nanda

LTC7820 is a fixed-ratio DC/DC controller that eliminates the power inductor in a non-isolated intermediate bus converter, allowing 50% reduction in circuit size and 4kW per cubic inch power density.

Linear Technology, Analog Devices Inc.'s acquisition in March, has introduced a fixed ratio inductor-less DC/DC controller. The LTC7820 operates over a 6V to 72V (80V abs max) input voltage range and can produce output currents in the 10's of amps, depending on the topology and choice of external components.

LTC7820 is a fixed ratio charge pump DC/DC controller that eliminates the power inductor in a non-isolated intermediate bus converter, allowing up to a 50% reduction in circuit size and up to 4kW/in3 power density, claims the company. The device drives external MOSFETs in voltage divider, doubler or inverter configurations with an efficiency of up to 99%.

Its open-loop fixed 50% duty cycle operation is suitable for non-isolated intermediate bus applications in power distribution, datacom, telecom, computing and industrial systems. With extended and industrial temperature versions available from -40°C to 125°C, the device is also suitable for the automotive 42V rail.

The LTC7820 can be configured for a 2:1 step-down ratio from an input voltage up to 72V or a 1:2 step-up ratio or a 1: –1 ratio as an inverter from an input voltage as high as 36V. Multiple devices can be cascaded for higher conversion ratios (48V to 12V or 54V to 9V). External MOSFETs switch at a fixed frequency, programmable from 100kHz to 1MHz. The LTC7820’s 1.1Ω N-channel MOSFET gate drivers  can drive multiple MOSFETs in parallel for higher power applications.

The LTC7820 monitors system voltage, current and temperature for faults, and uses a sense resistor for overcurrent protection. It stops switching and pulls the FAULT pin low when a fault condition occurs. An on-board timer can be set for appropriate restart/retry times. The EXTVCC pin permits the LTC7820 to be powered from the lower voltage output of the converter or other sources up to 40V. Additional features include soft switching for low EMI, a power good output signal, undervoltage lockout and internal charge balance circuitry.

"We have a set of customers that run off a nominal 48V rail," said Tony Armstrong, Director of Product Marketing, Product Products with Analog Devices' Linear Technology team, during a product briefing with EDN Asia. "Think telecommunications, backbone infrastructure. They are tired of doing conversions in multiple stages because it adds design complexity, efficiency issues, thermal issues, space issues."

Armstrong: *"This is now a step function higher [for charge pump converters] in terms of power levels from half a watt to 500W."*

Armstrong added that using a charge pump for step-down conversion isn't new but it hasn't been available in this voltage range and power level. "Traditionally, in switched cap solutions or charge pumps current is usually 100's of mA… 500mA is pretty big. Now, you can go to half a kilowatt or even higher. The technique has been around for a long time… 30-40 years. This is now a step function higher in terms of power levels from half a watt to 500W," said Armstrong.

Comparison with inductor-armed devices

Talking about the alternative approach of using an inductor, Armstrong said that inductors are basically transmitters and customers have to worry about potential noise interference issues in their systems. "The Japanese don't like inductors because of spurious noise they have to filter out… 80% of our charge pump business is in Japan," he added. There's a trade-off from using an inductor-less solution, however. Since there's no transformer, there is no isolation as noted in the specs above.

Armstrong admitted the output regulation accuracy is "somewhere between 3% and 4% but, if it's an intermediate bus, that's usually within tolerance and so a lot of people want to use this as an intermediate bus converter, whether it’s a 24V rail or 12V rail, and then use a point load to power your downstream digital cores—whatever they are—with very tight regulation." That wide input voltage range makes the device particularly suited to the intermediate bus conversion application.

"Stepping down from 48V to, say 1V, you end up with efficiency and thermal problems along with a bulky solution when using a device with an inductor," Armstrong explained. "By using an intermediate bus converter to step down from 48V to 24V or 12V with very high efficiency before stepping down to a much lower voltage, you overcome those issues.

Figure 1: A typical distributed power architecture. The LTC7820 is used as an intermediate bus converter.

The best we see in magnetics based solutions going from 48V to 24V or 12V is about 90% [efficiency]. So 10% of 500W is 50W… big thermal problem. If efficiency is 99% [as of the LTC7820], 5W is not so difficult [to dissipate]," he said.

Size = cost

Armstrong said engineers will need to use more capacitors, but that capacitors are relatively low in cost compared to magnetics and heat sinking. He claims that the solution is significantly smaller, typically anywhere between 25% and 50% smaller than a magnetics-based solution.

Figure 2: The company offers the LTC7820 in a 4mm x 5mm QFN-28 package.

Giving an example of a typical telecom company, like Cisco, Armstrong said they would use 14-layer boards in telecommunications systems. "Those boards unpopulated are $75 a square inch. Populated, they are tens of thousands of dollars because of the digital content. So, they have big thermal problem and if I can reduce their area by 2 square inches, I save them $150. This part sells for $4.70 in 1k," he concluded.

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