LT3580(RevC) 데이터 시트보기 (PDF) - Linear Technology

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LT3580
(Rev.:RevC)

Boost/Inverting DC/DC Converter with 2A Switch, Soft-Start, and Synchronization

Linear Technology 

LT3580

OPERATION

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.

3580fc

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