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LTDCJ Datasheet(PDF) 8 Page - Linear Technology |
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LTDCJ Datasheet(HTML) 8 Page - Linear Technology |
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8 / 28 page  LT3580 8 3580fc Start-Up Operation Several functions are provided to enable a very clean start-up for the LT3580. • First, the SHDN pin voltage is monitored by an internal voltage reference to give a precise turn-on voltage level. An external resistor (or resistor divider) can be connected from the input power supply to the SHDN pin to provide a user-programmable undervoltage lockout function. • Second, the soft-start circuitry provides for a gradual ramp-up of the switch current. When the part is brought out of shutdown, the external SS capacitor is first discharged (providing protection against SHDN pin glitches and slow ramping), then an integrated 275k resistor pulls the SS pin up to ~2.2V. By connecting an external capacitor to the SS pin, the voltage ramp rate on the pin can be set. Typical values for the soft-start capacitor range from 100nF to 1μF. • Finally, the frequency foldback circuit reduces the switch- ing frequency when the FB pin is in a nominal range of 350mV to 900mV. This feature reduces the minimum duty cycle that the part can achieve thus allowing better control of the switch current during start-up. When the FB voltage is pulled outside of this range, the switching frequency returns to normal. Current Limit and Thermal Shutdown Operation The LT3580 has a current limit circuit not shown in the Block Diagram. The switch current is consistently moni- tored and not allowed to exceed the maximum switch current at a given duty cycle (see the Electrical Charac- teristics table). If the switch current reaches this value, the SR latch (SR1) is reset regardless of the state of the comparator (A1/A2). Also not shown in the Block Diagram is the thermal shutdown circuit. If the temperature of the part exceeds approximately 165°C, the SR2 latch is set regardless of the state of the comparator (A1/A2). A full soft-start cycle will then be initiated. The current limit and thermal shutdown circuits protect the power switch as well as the external components connected to the LT3580. OPERATION |
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