LT3477EUF#TRPBF Linear Technology, LT3477EUF#TRPBF Datasheet - Page 9

• 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: -

APPLICATIONS INFORMATION
Setting Positive Output Voltages
To set a positive output voltage, select the values of R1 and
R2 (see Figure 2) according to the following equation:
Setting Negative Output Voltages
To set a negative output voltage, select the values of R3 and
R4 (see Figure 3) according to the following equation:
Selecting R
Using the following formula to choose the correct current
sense resistor value (for constant current or fail-safe
operation).
R
Figure 3. Negative Output Voltage Feedback Connections
V
V
Figure 2. Positive Output Voltage Feedback Connections
OUT
OUT
SENSE
=1.235V 1+
= –1.235V
=
SENSE
100
I
SENSE
mV
/Current Sense Adjustment
R3
R4
LT3477
LT3477
R2
R1
V
FBN
FBP
V
REF
FBN
FBP
REF
–V
OUT
V
R3
R4
3477 F03
OUT
R1
R2
3477 F02
For designs needing an adjustable current level, the
I
second current sense amplifi ers, respectively. With the
I
current sense voltage is 100mV (appearing between the
I
DC voltage less than 600mV to the I
will decrease the current sense voltage according to the
following formula:
For example, if 309mV is applied to the I
R
200mA to 100mA. The adjustability allows the regulated
current to be reduced without changing the current sense
resistor (e.g., to adjust brightness in an LED driver or to
reduce the charge current in a battery charger).
Considerations When Sensing Input Current
In addition to regulating the DC output current for cur-
rent-source applications, the constant-current loop of
the LT3477 can also be used to provide an accurate input
current limit. Boost converters cannot provide output
short-circuit protection, but the surge turn-on current can
be drastically reduced using the LT3477 current sense at
the input. SEPICs, however, have an output that is DC-
isolated from the input, so an input current limit not only
helps soft-start the output but also provides excellent
short-circuit protection.
When sensing input current, the sense resistor should be
placed in front of the inductor (between the decoupling
capacitor and the inductor). This will regulate the average
inductor current and maintain a consistent inductor ripple
current, which will, in turn, maintain a well regulated input
current. Do not place the sense resistor between the input
source and the input decoupling capacitor, as this may allow
the inductor ripple current to vary widely (even though the
average input current and the average inductor current will
still be regulated). Since the inductor current is a triangular
waveform (not a DC waveform like the output current)
ADJ1
ADJ1
SP1
SENSE
I
SENSE
and I
and I
and I
is 0.5Ω, the current sense will be reduced from
SN2
ADJ2
=
ADJ2
R
100
or I
SENSE
pins tied higher than 650mV, the nominal
pins are provided for the fi rst and the
mV
SP2
and I
•
618
V
IADJ
SN2
mV
pins). Applying a positive
ADJ1
and I
LT3477
ADJ1
ADJ2
pin and
pins
3477fc
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