Datasheet LT8611 (Analog Devices) - 11

LT8611
OPERATION
The LT8611 is a monolithic, constant frequency, current To optimize efficiency at light loads, the LT8611 operates mode step-down DC/DC converter. An oscillator, with in Burst Mode operation in light load situations. Between frequency set using a resistor on the RT pin, turns on bursts, all circuitry associated with controlling the output the internal top power switch at the beginning of each switch is shut down, reducing the input supply current to clock cycle. Current in the inductor then increases until 1.7μA. In a typical application, 2.5μA will be consumed the top switch current comparator trips and turns off the from the input supply when regulating with no load. The top power switch. The peak inductor current at which SYNC pin is tied low to use Burst Mode operation and can the top switch turns off is controlled by the voltage on be tied to a logic high to use pulse-skipping mode. If a the internal VC node. The error amplifier servos the VC clock is applied to the SYNC pin the part will synchronize to node by comparing the voltage on the VFB pin with an an external clock frequency and operate in pulse-skipping internal 0.97V reference. When the load current increases mode. While in pulse-skipping mode the oscillator operates it causes a reduction in the feedback voltage relative to continuously and positive SW transitions are aligned to the reference leading the error amplifier to raise the VC the clock. During light loads, switch pulses are skipped voltage until the average inductor current matches the new to regulate the output and the quiescent current will be load current. When the top power switch turns off, the several hundred µA. synchronous power switch turns on until the next clock To improve efficiency across all loads, supply current to cycle begins or inductor current falls to zero. If overload internal circuitry can be sourced from the BIAS pin when conditions result in more than 3.3A flowing through the biased at 3.3V or above. Else, the internal circuitry will draw bottom switch, the next clock cycle will be delayed until current from V switch current returns to a safe level. IN. The BIAS pin should be connected to VOUT if the LT8611 output is programmed at 3.3V or above. The LT8611 includes a current control and monitoring Comparators monitoring the FB pin voltage will pull the loop using the ISN, ISP, IMON and ICTRL pins. The ISP/ PG pin low if the output voltage varies more than ±9% ISN pins monitor the voltage across an external sense (typical) from the set point, or if a fault condition is present. resistor such that the VISP-VISN does not exceed 50mV by limiting the peak inductor current controlled by the VC The oscillator reduces the LT8611’s operating frequency node. The current sense amplifier inputs (ISP/ISN) are rail- when the voltage at the FB pin is low. This frequency to-rail such that input, output, or other system currents foldback helps to control the inductor current when the may be monitored and regulated. The IMON pin outputs output voltage is lower than the programmed value which a ground-referenced voltage equal to 20 times the voltage occurs during start-up or overcurrent conditions. When between the ISP-ISN pins for monitoring system currents. a clock is applied to the SYNC pin or the SYNC pin is The ICTRL pin can be used to override the internal 50mV held DC high, the frequency foldback is disabled and the limit between the ISP, ISN pin to a lower set point for the switching frequency will slow down only during overcur- current control loop. rent conditions. If the EN/UV pin is low, the LT8611 is shut down and draws 1µA from the input. When the EN/UV pin is above 1V, the switching regulator will become active. 8611fa For more information www.linear.com/LT8611 11 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Typical Application Related Parts