Datasheet LT8618, LT8618-3.3 (Analog Devices) - 10

link to page 9 LT8618/LT8618-3.3
OPERATION
The LT8618 is a monolithic constant-frequency current To improve efficiency across all loads, supply current to mode step-down DC/DC converter. Operation is best internal circuitry can be sourced from the BIAS pin when understood by referring to the Block Diagram. An internal biased at 3.2V or above. Else, the internal circuitry will oscillator turns on the integrated top power switch at the draw current from VIN. The BIAS pin should be connected beginning of each clock cycle. Current in the inductor then to VOUT if the LT8618 output is programmed to a voltage increases until the top switch current comparator trips between 3.3V and 25V. and turns off the top power switch. The peak inductor cur- Comparators monitoring the FB (LT8618) or OUT rent at which the top switch turns off is controlled by the (LT8618-3.3) pin voltage will pull the PG pin low if the voltage on the internal VC node. The error amplifier servos output voltage varies more than ±7.5% (typical) from the the VC node by comparing the voltage on the FB node with set point, or if a fault condition is present. an internal reference. When the load current increases, it causes a reduction in the feedback voltage relative to the In the LT8618, the oscillator reduces its operating fre- reference leading the error amplifier to raise the VC volt- quency when the voltage at the FB (LT8618) or OUT age until the average inductor current matches the new (LT8618-3.3) pin is low. This frequency foldback helps load current. When the top power switch turns off, the to control the inductor current when the output voltage synchronous power switch turns on until the next clock is lower than the programmed value which occurs during cycle begins or inductor current falls to zero. If overload start-up. conditions result in excess current flowing through the If the EN/UV pin is low, the LT8618 is shut down and bottom switch, the next clock cycle will be delayed until draws 1µA from the input. When the EN/UV pin is above switch current returns to a safe level. 1.05V, the switching regulator becomes active. To optimize efficiency, the LT8618 enters Burst Mode operation at light loads. Between bursts, al circuitry asso- ciated with controlling the output switch is shut down reducing the input supply current to 1.7µA. In a typical application with a 48V input, 2.5µA will be consumed from the input supply when regulating with no load. Rev. 0 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Typical Application Related Parts