lt3755-2 Linear Technology Corporation, lt3755-2 Datasheet - Page 16

• Manufacturer Part Number: lt3755-2
• Description: Lt3755/lt3755-1/lt3755-2 40vin, 75vout Led Controllers Features
• Manufacturer: Linear Technology Corporation

LT3755/LT3755-1/LT3755-2
Schottky Rectifier Selection
The power Schottky diode conducts current during the
interval when the switch is turned off. Select a diode rated
for the maximum SW voltage. If using the PWM feature for
dimming, it is important to consider diode leakage, which
increases with the temperature, from the output during the
PWM low interval. Therefore, choose the Schottky diode
with sufficiently low leakage current. Table 4 has some
recommended component vendors.
Table 4. Schottky Rectifier Manufacturers
VENDOR
On Semiconductor
Diodes, Inc.
Central Semiconductor
Sense Resistor Selection
The resistor, R
nal NMOS FET and GND should be selected to provide
adequate switch current to drive the application without
exceeding the 108mV (typical) current limit threshold on
the SENSE pin of LT3755. For buck mode applications,
select a resistor that gives a switch current at least 30%
greater than the required LED current. For buck mode,
select a resistor according to:
For buck-boost, select a resistor according to:
For boost, select a resistor according to:
These equations provide an estimate of the sense resistor
value based on reasonable assumptions about induc-
tor current ripple during steady state switching. Lower
values of sense resistor may be required in applications
where inductor ripple current is higher. Examples include
applications with current limited operation at high duty
cycle, and those with discontinuous conduction mode
applicaTions inForMaTion

R
R
R
SENSE,BUCK
SENSE,BUCK-BOOST
SENSE,BOOST
SENSE
≤
≤
0.07V
I
V
, between the source of the exter-
LED
V
IN
LED
≤
• 0.07V
(
• I
V
LED
IN
V
WEB
www.onsemi.com
www.diodes.com
www.centralsemi.com
IN
+ V
• 0.07V
LED
)
I
LED
(DCM) switching. It is always prudent to verify the peak
inductor current in the application to ensure the sense
resistor selection provides margin to the SENSE current
limit threshold.
The placement of R
the NMOS FET and GND of the LT3755. The SENSE input
to LT3755 should be a Kelvin connection to the positive
terminal of R
Inductor Selection
The inductor used with the LT3755 should have a saturation
current rating appropriate to the maximum switch current
selected with the R
based on operating frequency, input and output voltage to
provide a current mode ramp on SENSE during the switch
on-time of approximately 20mV magnitude. The following
equations are useful to estimate the inductor value for
continuous conduction mode operation:
Table 5 provides some recommended inductor vendors.
Table 5. Inductor Manufacturers
VENDOR
Sumida
Würth Elektronik
Coiltronics
Vishay
Coilcraft
Loop Compensation
The LT3755 uses an internal transconductance error ampli-
fier whose VC output compensates the control loop. The
external inductor, output capacitor and the compensation
resistor and capacitor determine the loop stability.
L
L
L
BUCK
BUCK-BOOST
BOOST
=
=
R
SENSE
R
SENSE
SENSE
V
=
V
SENSE
LED
IN
.
SENSE
(
V
• V
• 0.02V • f
LED
• V
• 0.02V • f
R
LED
resistor. Choose an inductor value
SENSE
IN
should be close to the source of
+ V
(
(
V
V
WEB
www.sumida.com
www.we-online.com
www.cooperet.com
www.vishay.com
www.coilcraft.com
IN
LED
IN
)
OSC
• V
– V
• 0.02V • f
OSC
– V
LED
LED
IN
• V
)
)
IN
OSC
37551fd