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.