LT3479 LINER [Linear Technology], LT3479 Datasheet - Page 7

• Manufacturer Part Number: LT3479
• Description: 3A, Full Featured DC/DC Converter with Soft-Start and Inrush Current Protection
• Manufacturer: LINER [Linear Technology]

OPERATION
The LT3479 uses a fi xed 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 V
the PWM comparator resets the latch and turns off the
power switch. In this manner, the feedback amplifi er 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 V
rent. The switch current is constantly monitored and not
allowed to exceed the nominal value of 3A. If the switch
C
pin, which in turn limits the peak switch cur-
C
(the output of the feedback amplifi er),
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.
LT3479
3479fb
7