LT3477EUF#TRPBF Linear Technology, LT3477EUF#TRPBF Datasheet - Page 7

• Manufacturer Part Number: LT3477EUF#TRPBF
• Description: IC LED DRVR HP CONST CURR 20-QFN
• Manufacturer: Linear Technology
• Type: High Power, Constant Currentr

Specifications of LT3477EUF#TRPBF

• Constant Current: Yes
• Constant Voltage: Yes
• Topology: Flyback, PWM, SEPIC, Step-Down (Buck), Step-Up (Boost)
• Number Of Outputs: 1
• Internal Driver: Yes
• Type - Primary: General Purpose
• Type - Secondary: White LED
• Frequency: 200kHz ~ 3.5MHz
• Voltage - Supply: 2.5 V ~ 25 V
• Mounting Type: Surface Mount
• Package / Case: 20-QFN
• Operating Temperature: -40°C ~ 85°C
• Current - Output / Channel: 3A
• Internal Switch(s): Yes
• Efficiency: 93%
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -

OPERATION
The LT3477 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 in Figure 1. The start of each oscillator cycle sets
the SR latch and turns on power switch Q1. The signal
at the noninverting input of the PWM comparator (A4
SLOPE) is proportional to the sum of the switch current
and oscillator ramp. When SLOPE exceeds V
of the feedback amplifi er), 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 regula-
tion. Amplifi er A3 drives A4 inverting input. A3 has three
inputs, one from the voltage feedback loop and the other
two from the current feedback loop. Whichever feedback
input is higher takes precedence, forcing the converter into
either a constant-current or a constant-voltage mode. The
LT3477 is designed to transition cleanly between the two
modes of operation. Current sense amplifi er IA1 senses
the voltage between the I
a pre-gain to amplifi er A1. When the voltage between I
and I
to the inverting input of A1 and the converter is in con-
stant-current mode. If the current sense voltage exceeds
100mV, the output of IA1 will increase causing the output
of A3 to decrease, thus reducing the amount of current
SN1
reaches 100mV, the output of IA1 provides V
SP1
and I
SN1
pins and provides
C
(the output
ADJ
SP1
delivered to the output. In this manner the current sense
voltage is regulated to 100mV. The current sense level is
also pin adjustable by I
625mV will overwrite V
thus providing current level control. The second current
sense amplifi er, IA2, works the same as the fi rst current
sense amplifi er IA1. Both current sense amplifi ers provide
rail-to-rail current sense operation. Similarly, for positive
output voltage operation where FBP is tied to V
the FBN pin increases above V
decrease to reduce the peak current level and regulate
the output (constant-voltage mode). For negative output
voltage operation where FBN is tied to GND, if the FBP pin
decreases below GND level, the output of A3 will decrease
to reduce the peak current level and regulate the output
(constant-voltage mode).
The LT3477 also features a soft-start function. During
start-up, 9μA 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
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.
C
pin, which in turn limits the peak switch cur-
ADJ
ADJ1
voltage that’s set internally,
. Forcing I
REF
, the output of A3 will
ADJ1
LT3477
to less than
REF
3477fc
7
, if