Datasheet LT8580 (Analog Devices) - 8

LT8580
OPERATION Start-Up Operation
of 300mV to 920mV. This feature reduces the minimum Several functions are provided to enable a very clean duty cycle that the part can achieve thus allowing better start-up for the LT8580. control of the switch current during start-up. When the FBX voltage is pulled outside of this range, the switching • First, the SHDN pin voltage is monitored by an internal frequency returns to normal. voltage reference to give a precise turn-on voltage level. An external resistor (or resistor divider) can be connected
Current Limit and Thermal Shutdown Operation
from the input power supply to the SHDN pin to provide The LT8580 has a current limit circuit not shown in the a user-programmable undervoltage lockout function. Block Diagram. The switch current is constantly monitored • Second, the soft-start circuitry provides for a gradual and not allowed to exceed the maximum switch current at ramp-up of the switch current. When the part is brought a given duty cycle (see the Electrical Characteristics table). out of shutdown, the external SS capacitor is first If the switch current reaches this value, the SR latch (SR1) discharged (providing protection against SHDN pin is reset regardless of the state of the comparator (A1/ glitches and slow ramping), then an integrated 280k A2). Also, not shown in the Block Diagram is the thermal resistor pulls the SS pin up to ~2.1V. By connecting an shutdown circuit. If the temperature of the part exceeds external capacitor to the SS pin, the voltage ramp rate approximately 165°C, the SR2 latch is set regardless of the on the pin can be set. Typical values for the soft-start state of the amplifier (A1/A2). When the part temperature capacitor range from 100nF to 1µF. falls below approximately 160°C, a full soft-start cycle will then be initiated. The current limit and thermal shutdown • Finally, the frequency foldback circuit reduces the switch- circuits protect the power switch as well as the external ing frequency when the FBX pin is in a nominal range components connected to the LT8580. Rev. B 8 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts