COVER STORY FEATURE


suitable for a range of loads by sizing the power switching MOSFETs accordingly. Higher output power can be achieved by arranging multiple circuits in parallel. The operating frequency is selectable between 100kHz and 400kHz and can be synchronised to an external clock. The LT8705 employs a proprietary current- mode control architecture for constant frequency operation in buck or boost mode and has powerful onboard quad N- channel MOSFET gate drivers. The user can select among forced continuous, discontinuous and Burst Mode operation to maximise light load efficiency. Additional features include servo pins


to indicate which feedback loops are active, a 3.3V/12mA LDO, adjustable soft- start, onboard die temperature monitor and ±1% reference voltage accuracy over an operating junction temperature range of -40°C to 125°C. The LT8705 is available in a 38-pin 5mm x 7mm QFN and also a 38-lead TSSOP package with additional pin spacing for high voltage operation at altitude. Both lead-free and tin-lead terminal finish are available.


APPLICATIONS The versatility of this device with its “anything in, anything out” flexibility lends itself to a variety of applications. In figure 1 this device is configured as a Voltage Stabiliser, with a nominal 28V input and 12V output to an intermediate bus it achieves >95% efficiency. Super capacitors can be used to supply


large peak currents required by powerful servo systems and actuators used in military hardware or to act as a temporary hold up circuit if power is interrupted. In this scenario this device can be used as a single component Bidirectional Supercap Charger to replace two traditional pulse width modulated supplies, in one direction to charge the supercaps and in the other to deliver


energy to the load. No additional power routing/management are needed, reducing the parts count and power losses incurred by the additional circuitry. Forward operating bases want to utilise


solar energy to reduce the consumption of diesel fuel in electrical generators and the LT8705 can provide a Solar Panel Battery Charger solution. The solar cell produces current in proportion to the illumination level, while the open circuit voltage remains relatively constant (see Figure 2). By using the input voltage regulation loop and EA3 (see Figure 3), the LT8705 can extract maximum power, so called maximum power point tracking. On the output side, it can run as a battery charger using the output current regulation loop and EA1 to provide a constant charging current until a predetermined voltage is reached, where the output voltage regulation loop can take over (not shown). Finally, a Portable Power Supply for systems powered from battery packs, the LT8705 operating initially in buck mode to step down the battery voltage can then automatically transition through to boost mode to provide extended run time


Figure 2:


Solar cell output characteristics


as the battery voltage reduces. For more information on these


applications please refer to the LT8705 datasheet on the company’s website.


PROTECTING AGAINST FAULTS There is a need to protect high reliability systems from the propagation of faults both in the upstream direction to avoid damaging the main power bus and in the downstream direction to protect expensive electronics from faults in the power conversion chain. The LT8705 activates a voltage lockout


sequence if conditions for proper operation such as minimum VIN


are


violated or if overcurrent or overvoltage conditions are detected on input or output. After the fault condition has been removed and a predefined timeout period has ended, the converter will restart at a rate dependent upon the capacitor value assigned to the soft start pin. Thermal management is also assisted by a secondary function of the CLKOUT pin, its duty cycle being linearly proportional to the die temperature. The maximum specified junction temperature is 125ºC but an internal thermal shutdown protects the reliability of the chip, at approximately 165ºC the power switch and the internal regulators are turned off. After the chip temperature drops by approximately 5°C, the part will re-initialise and perform a soft-start before resuming normal operation.


Figure 3:


Simplified solar panel battery charger


CONCLUSIONS Increasing use of electrically-powered actuators, pumps and ancillaries provide more fuel-efficient solutions at the equipment level, but has increased the onboard electrical energy demand dramatically and highlights the importance of very efficient power conversion schemes. Designers of military and aviation


applications are often encouraged to reuse proven circuits and constrained to a narrow range of products that are approved. Therefore, it is desirable that such components are as flexible as possible so that they can be deployed in multiple applications. One product that matches these requirements is the LT8705 Synchronous 4-switch buck-boost controller. With up to 98% efficiency, wide input and output voltage range and four control loops, it provides a versatile solution for many military and aviation applications.


Linear Technology (UK) Ltd. Now part of Analog Devices www.linear.com 01628 477 066


/ ELECTRONICS ELECTRONICS | APRIL 2017 15


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