LT3479EFE-TRPBF データシートの表示（PDF） - Linear Technology

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LT3479EFE-TRPBF

3A, Full Featured DC/DC Converter with Soft-Start and Inrush Current Protection

Linear Technology 

LT3479

OPERATION

The LT3479 uses a fixed frequency, current mode control

scheme to provide excellent line and load regulation. Op-

eration can be best understood by referring to the Block

Diagram. The start of each oscillator cycle sets the SR latch

and turns on power switch Q1. The signal at the inverting

input of the PWM comparator (SLOPE) is proportional to

the sum of the switch current and oscillator ramp. When

SLOPE exceeds VC (the output of the feedback amplifier),

the PWM comparator resets the latch and turns off the

power switch. In this manner, the feedback amplifier and

PWM comparators set the correct peak current level to

keep the output in regulation.

The LT3479 also features a soft-start function. During

start-up, 10μA of current charges the external soft-start

capacitor. The SS pin directly limits the rate of voltage rise

on the VC pin, which in turn limits the peak switch cur-

rent. The switch current is constantly monitored and not

allowed to exceed the nominal value of 3A. If the switch

current reaches 3A, the SR latch is reset regardless of the

output of the PWM comparator. Current limit protects the

power switch and external components.

Soft-start plays an important role in applications where the

switch will reach levels of 30V or higher. During startup,

an overshoot in the switch current together with the pres-

ence of high switch voltage can overstress the switch. A

properly used soft-start feature will greatly improve the

robustness of such designs.

In addition to soft-start, inrush current protection protects

the LT3479 against shorts and line transients. During such

faults, the inductor current can momentarily exceed 3A and

damage the switch. Through an internal 8.5mΩ resistor

placed in series with the inductor, the inrush current protec-

tion comparator measures the inductor current. If it exceeds

5A, a soft-start cycle is initiated. The LT3479 will remain in

the soft-start condition until the fault has passed.

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