Figure 3 shows a simplified diagram of how the four power switches are connected to the inductor, VIN, VOUT and ground.
Figure 3: Simplified diagram of the four MOSFET switches driven by the LT8705
is significantly higher than VOUT
, the part will run in buck (step-down) mode. In this region, M3 is always off and M4 is always on unless reverse current is detected while in Burst Mode operation or discontinuous mode. At the start of every cycle, synchronous switch M2 is turned on first and the inductor current is sensed by an internal amplifier. A slope compensation ramp is added to the sensed voltage, which is then compared to a reference voltage. After the sensed inductor current falls below the reference, switch M2 is turned off and M1 (synchronous rectifier) is turned on for the remainder of the cycle. Switches M1 and M2 will alternate, behaving like a typical synchronous buck regulator.
get close to each other, the duty cycle decreases until the minimum duty cycle of the converter in buck mode is reached and the part moves into the buck-boost region and all four MOSFETs are switching.
is significantly higher than VIN
, the part will run in boost (step-up) mode. In this region M1 is always on and switch M2 is always off. At the start of every cycle, switch M3 is turned on first and the inductor current is sensed by an internal amplifier. After the sensed inductor current rises above the reference voltage, switch M3 is turned off and switch M4 is turned on for the remainder of the cycle. Switches M3and M4 will alternate, behaving like a typical synchronous boost regulator.
The LT8705 activates a fault sequence under certain operating conditions. If any of these conditions occur, such as an over current or over voltage condition, the internal switching and clock output are disabled. At the same time, a timeout sequence commences where the soft start function needs to be reinitialized. If the fault persist, like during an over current condition, the soft start function will not be allowed to restart the converter. 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 on the LT8705. The timeout period relieves the part and other downstream power components from electrical and thermal stress.
Robotic surgical systems allow for several types of minimally evasive major surgeries, which can reduce the hospital stay time, provide a faster return to normal activities and the potential for better clinical outcome. Powering these types of systems with distributive power architecture enables the use of a 48V nominal battery backed up bus voltage that powers downstream point-of-load regulators for all subsystems. Linear Technology’s LT8705 synchronous buck-boost DC/DC controller can simplify the power supply design with its ability to efficiently charge a battery with a float voltage that can be above, below or equal to the input voltage.
About the author
Bruce Haug, senior product marketing engineer, Linear Technology Corporation
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