Linear applies switched-capacitor step-down topology for inductor-less DC/DC

June 21, 2017 // By Graham Prophet
LTC7820 is, a high power 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 4000 W/in³ power density. The 72V fixed-ratio DC/DC controller can deliver 500W.

Non-isolated regulators and voltage level converters that employ energy storage in capacitors rather than inductors are well-known, but the majority of DC/DC designs in recent times have used magnetics. Indeed, a feature of various companies’ presentation of devices that use ever-higher switching speeds is the ability to reduce the size of the inductor. Now, Linear Technology (now part of Analog Devices) has eliminated the inductor and introduced the LTC7820, that employs energy storage on capacitors and that drives external MOSFETs in voltage divider, doubler or inverter configurations with an efficiency of up to 99%. As the energy storage is not in an inductance, there cannot be a transformer so the topologies available – which include the ‘Dickson converter’ - are necessarily non-isolated. The IC’s open loop fixed duty is suitable for non-isolated intermediate bus applications in power distribution, datacom, telecom, high-end computing and industrial systems.

 

The LTC7820 charge-pump controller operates over a 6V to 72V (80V abs max) input voltage range and can produce output currents in multiple-tens of Amps, depending on the topology and choice of external components (FETs/capacitors). It can be configured for a 2:1 step-down ratio from an input voltage up to 72V. Two of the LTC7820 ICs can be cascaded for a 4:1 step-down ratio. For even higher step-down ratios, such as a 6:1, the Dickson converter topology is recommended and is suitable for 54V input to a 9V outputs, requiring only a single LTC7820. The LTC7820 can also be configured for a 1:2 step-up ratio or a 1:1 inverter ratio from an input voltage as high as 36V. External MOSFETs switch at a fixed frequency, programmable from 100 kHz to 1 MHz. The LTC7820’s 1.1Ω N-channel MOSFET gate drivers maximize efficiency and 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 onboard 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 available sources up to 40V, reducing power dissipation and improving efficiency. Additional features include soft switching for low EMI, a power-good output signal, undervoltage lockout and internal charge balance circuitry.

 

In a 4 x 5mm QFN-28 package, and in extended and industrial temperature versions, pricing starts at $4.70 (1000).

 

Analog Devices/Linear Technology; www.linear.com/product/LTC7820

 

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