LT8494 OPERATION The LT8494 is a constant-frequency, current mode The LT8494 contains a power good comparator which SEPIC/boost/flyback regulator. Operation can be best trips when the FB pin is above 92% of its regulated value. understood by referring to the Block Diagram. In the Block The PG output is an open-drain transistor that is off when Diagram, the adjustable oscillator, with frequency set by the output is in regulation, al owing an external resistor to the external RT resistor, enables an RS latch, turning on pull the PG pin high (See Applications Information section the internal power switch. An amplifier and comparator for details). monitor the switch current flowing through an internal Several functions are provided to enable a very clean sense resistor, turning the switch off when this current start-up for the LT8494. reaches a level determined by the voltage at VC. An error amplifier adjusts the VC voltage by measuring the output • First, the SWEN pin voltage is monitored by an internal voltage through an external resistor divider tied to the FB voltage reference to give a precise turn-on threshold. pin. If the error amplifier’s output voltage (VC) increases, An external resistor divider can be connected from more current is delivered to the output; if the VC voltage the input power supply to the SWEN pin to provide a decreases, less current is delivered. An active clamp on user-programmable undervoltage lockout function. the VC voltage provides current limit. An internal regulator • Second, the soft-start circuitry provides for a grad- provides power to the control circuitry. ual ramp-up of the switch current. When the part is In order to improve efficiency, the NPN power switch brought out of shutdown, the external SS capacitor is driver (see Block Diagram) supplies NPN base current first discharged, and then an integrated 256k resistor from whichever of VIN and BIAS has the lower supply pulls the SS pin up to ~2.1V. By connecting an exter- voltage. However, if either of them is below 2.4V or above nal capacitor to the SS pin, the voltage ramp rate on 34V (typical values), the power switch draws current from the pin can be set. Typical values for the soft-start the other pin. If both supply pins are below 2.4V or above capacitor range from 100nF to 1µF. 34V then switching activity is stopped. • Finally, the frequency foldback circuit reduces the To further optimize efficiency, the LT8494 automatically maximum switching frequency when the FB pin is enters Burst Mode operation in light load situations. below 1V. This feature reduces the minimum duty Between bursts, all circuitry associated with controlling cycle that the part can achieve thus allowing better the output switch is shut down, reducing the VIN/BIAS control of the switch current during start-up. pin supply currents to be less than 3µA typically (see Electrical Characteristics). 8494fa For more information www.linear.com/LT8494 9 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Application Related Parts
