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LT8580

8580fa

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Start-Up Operation

Several functions are provided to enable a very clean

start-up for the LT8580.

• First, the SHDN pin voltage is monitored by an internal

voltage reference to give a precise turn-on voltage level.

Anexternalresistor(orresistordivider)canbeconnected

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 280k

resistor pulls the SS pin up to ~2.1V. 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,thefrequencyfoldbackcircuitreducestheswitch-

ing frequency when the FBX pin is in a nominal range

of 300mV to 920mV. 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

FBX voltage is pulled outside of this range, the switching

frequency returns to normal.

Current Limit and Thermal Shutdown Operation

The LT8580 has a current limit circuit not shown in the

BlockDiagram.Theswitchcurrentisconstantlymonitored

and not allowed to exceed the maximum switch current at

a given duty cycle (see the Electrical Characteristics 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

approximately165°C,theSR2latchissetregardlessofthe

state of the amplifier (A1/A2). When the part temperature

falls below approximately 160°C, 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 LT8580.

operaTion

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