Datasheet

LT3796/LT3796-1
21
3796fa
For more information www.linear.com/3796
APPLICATIONS INFORMATION
in Table 4. The MOSFETs used in the application circuits
in this data sheet have been found to work well with the
LT3796/LT3796-1. Consult factory applications for other
recommended MOSFETs.
Table 4. MOSFET Manufacturers
VENDOR WEB
Vishay Siliconix www.vishay.com
Fairchild www.fairchildsemi.com
International Rectifier www.irf.com
Infineon www.infineon.com
High Side PMOS Disconnect Switch Selection
A high side PMOS disconnect switch with a minimum V
TH
of –1V to –2V is recommended in most LT3796/LT3796-1
applications to optimize or maximize the PWM dimming
ratio and protect the LED string from excessive heating
during fault conditions as well. The PMOS disconnect
switch is typically selected for drain-source voltage V
DS
,
and continuous drain current I
D
. For proper operations,
V
DS
rating must exceed the open LED regulation voltage
set by the FB1 pin, and I
D
rating should be above I
LED
.
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 5 has some
recommended component vendors.
Table 5. Schottky Rectifier Manufacturers
VENDOR WEB
On Semiconductor www.onsemi.com
Diodes, Inc www.diodes.com
Central Semiconductor www.centralsemi.com
Rohm Semiconductor www.rohm.com
Sense Resistor Selection
The resistor, R
SENSE
, between the source of the external
N-channel MOSFET and GND should be selected to provide
adequate switch current to drive the application without
exceeding the 113mV (typical) current limit threshold on
the SENSE pin of LT3796/LT3796-1. For buck mode ap-
plications, 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:
R
SENSE(BUCK)
≤
0.07V
I
LED
For buck-boost mode, select a resistor according to:
R
SENSE(BUCK − BOOST)
≤
V
IN
• 0.07V
(V
IN
+V
LED
)I
LED
For boost, select a resistor according to:
R
SENSE(BOOST)
≤
V
IN
• 0.07V
V
LED
• I
LED
The placement of R
SENSE
should be close to the source
of the NMOS FET and GND of the LT3796/LT3796-1. The
SENSE input to LT3796/LT3796-1 should be a Kelvin con-
nection to the positive terminal of R
SENSE
.
70mV is used in the equations above to give some margin
below the 113mV (typical) sense current limit threshold.
Inductor Selection
The inductor used with the LT3796/LT3796-1 should have
a saturation current rating appropriate to the maximum
switch current selected with the R
SENSE
resistor. Choose
an inductor value based on operating frequency, input and
output voltage to provide a current mode signal on SENSE