LT3755 LINER [Linear Technology], LT3755 Datasheet - Page 16

• Manufacturer Part Number: LT3755
• Description: 40VIN, 75VOUT LED Controller
• Manufacturer: LINER [Linear Technology]

Table 3. MOSFET Manufacturers
VENDOR
Vishay Siliconix
Fairchild
International Rectifi er
Schottky Rectifi er 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 suffi ciently low leakage current. Table 4 has some
recommended component vendors.
Table 4. Schottky Rectifi er 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:
LT3755/LT3755-1
APPLICATIONS INFORMATION
16
R
R
R
SENSE,BUCK
SENSE,BUCK-BOOST
SENSE,BOOST
SENSE
≤
≤
0 07
PHONE
402-563-6866
972-910-8000
310-252-7105
PHONE
888-743-7826
805-446-4800
631-435-1110
I
V
.
, between the source of the exter-
V
LED
IN
LED
≤
V
• .
0 07
(
•
V
I
IN
LED
V
IN
V
+
• . 0 07
V
WEB
www.onsemi.com
www.diodes.com
www.centralsemi.com
LED LED
WEB
www.vishay.com
www.fairchildsemi.com
www.irf.com
)
I
V
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 (T
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-
fi er whose VC output compensates the control loop. The
external inductor, output capacitor and the compensation
resistor and capacitor determine the loop stability.
The inductor and output capacitor are chosen based on
performance, size and cost. The compensation resistor
and capacitor at VC are selected to optimize control loop
response and stability. For typical LED applications, a
2.2nF compensation capacitor at VC is adequate, and
L
L
L
BUCK
BUCK-BOOST
BOOST
SW
=
=
= 1/f
T
SENSE
SW
T
SW
OSC
=
•
SENSE
•
R
.
SENSE
T
PHONE
408-321-9660
605-886-4385
561-998-4100
402-563-6866
847-639-6400
):
R
SENSE
SW
SENSE
(
V
V
V
LED
resistor. Choose an inductor value
•
LED
IN
should be close to the source of
R
•
SENSE
• . 0 02
+
•
V
• . 0 02
V
LED
V
IN
IN
(
(
V
)
•
V
V
V
• 0. . 02V
V
LED
IN
LED
–
WEB
www.sumida.com
www.we-online.com
www.cooperet.com
www.vishay.com
www.coilcraft.com
–
V
•
V
LED
V
IN
IN
)
)
37551fa