lt3479efe-trpbf Linear Technology Corporation, lt3479efe-trpbf Datasheet - Page 5

• Manufacturer Part Number: lt3479efe-trpbf
• Description: 3a, Full Featured Dc/dc Converter With Soft-start And Inrush Current Protection
• Manufacturer: Linear Technology Corporation

OPERATIO
BLOCK DIAGRA
The LT3479 uses a fixed frequency, current mode control
scheme to provide excellent line and load regulation.
Operation 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 pro-
portional to the sum of the switch current and oscillator
ramp. When SLOPE exceeds V
back 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
R
C
T
IN
FBP
FBN
V
SHDN
V
R
REF
IN
T
U
+
–
OSCILLATOR
REFERENCE
1.25V
AMPLIFIER
FEEDBACK
W
C
t
C
(the output of the feed-
SS
SS
t
SLOPE
V
ICON
C
R
C
+
–
C
C
Σ
COMPARATOR
INRUSH CURRENT
COMPARATOR
PWM
PROTECTION
on the V
The switch current is constantly monitored and not al-
lowed 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.
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 resis-
tor placed in series with the inductor, the inrush current
protection 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.
pwmout
+
–
C
pin, which in turn limits the peak switch current.
C
S
MASTER
LATCH
R
+
–
V
CURRENT LIMIT
S
COMPARATOR
S
36mV
Q
8.5mΩ
L
DRIVER
L1
+
–
Q1
SW
GND
D1
3479 BD
LT3479
R1
R2
FB
C1
3479fa
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