Datasheet LT8607, LT8607B (Analog Devices) - 9

LT8607/LT8607B
BLOCK DIAGRAM
VIN VIN CIN – INTERNAL 0.778V REF + 3.5V R3 1V + REG OPT EN/UV – SHDN SLOPE COMP INTVCC R4 ERROR OPT PG CVCC ±8.5% OSCILLATOR AMP BST + 200kHz TO 2.2MHz RPG + – CBST VC BURST SWITCH M1 VOUT DETECT LOGIC SW L AND VOUT CFF SHDN ANTI- TSD COUT SHOOT INTV R1 CC UVLO THROUGH M2 R2 VIN UVLO FB SHDN TSD CSS 2µA VIN UVLO GND TR/SS (MSOP ONLY) RT RT SYNC (MSOP ONLY) 8607 BD
OPERATION
The LT8607 is a monolithic constant frequency current switch is shut down, reducing the input supply current to mode step-down DC/DC converter. An oscillator with 1.7µA. In a typical application, 3.0µA will be consumed frequency set using a resistor on the RT pin turns on from the input supply when regulating with no load. The the internal top power switch at the beginning of each SYNC pin is tied low to use Burst Mode operation and can clock cycle. Current in the inductor then increases until be floated to use pulse-skipping mode. If a clock is applied the top switch current comparator trips and turns off the to the SYNC pin the part will synchronize to an external top power switch. The peak inductor current at which the clock frequency and operate in pulse-skipping mode. While top switch turns off is controlled by the voltage on the in pulse-skipping mode the oscillator operates continu- internal VC node. The error amplifier servos the VC node ously and positive SW transitions are aligned to the clock. by comparing the voltage on the VFB pin with an inter- During light loads, switch pulses are skipped to regulate nal 0.778V reference. When the load current increases it the output and the quiescent current will be several mA. causes a reduction in the feedback voltage relative to the The SYNC pin may be tied high for spread spectrum modu- reference leading the error amplifier to raise the VC volt- lation mode, and the LT8607 will operate similar to pulse- age until the average inductor current matches the new skipping mode but vary the clock frequency to reduce EMI. load current. When the top power switch turns off the The LT8607 DFN has no SYNC pin and will always operate synchronous power switch turns on until the next clock in Burst Mode operation. The LT8607B has no SYNC pin cycle begins or inductor current falls to zero. If overload and will operate in pulse-skipping mode. conditions result in excess current flowing through the Comparators monitoring the FB pin voltage wil pul the PG bottom switch, the next clock cycle will be delayed until pin low if the output voltage varies more than ±8.5% (typi- switch current returns to a safe level. cal) from the set point, or if a fault condition is present. If the EN/UV pin is low, the LT8607 is shut down and The oscillator reduces the LT8607’s operating frequency draws 1µA from the input. When the EN/UV pin is above when the voltage at the FB pin is low and the part is in Burst 1.05V, the switching regulator becomes active. Mode operation. This frequency foldback helps to control To optimize efficiency at light loads, the LT8607 enters the inductor current when the output voltage is lower than Burst Mode operation during light load situations. Between the programmed value which occurs during start-up. bursts, all circuitry associated with controlling the output Rev. C For more information www.analog.com 9 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts