Datasheet

LT3761
19
3761f
applicaTions inFormaTion
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 as described in the
section on power MOSFET selection. If using the PWM
feature for dimming, it may be 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. The
diode current and V
F
should be considered when select-
ing the diode to be sure that power dissipation does not
exceed the rating of the diode. The power dissipated by
the diode in a converter is:
P
D
= I
D
• V
F
• (1-D
MAX
)
It is prudent to measure the diode temperature in steady
state to ensure that its absolute maximum ratings are not
exceeded.
Table 5. Schottky Rectifier Manufacturers
MANUFACTURER WEB
Vishay www.vishay.com
Central Semiconductor www.centralsemi.com
Diodes, Inc. www.diodes.com
Sense Resistor Selection
The resistor, R
SENSE
, between the source of the exter-
nal NMOS FET and GND should be selected to provide
adequate switch current to drive the application without
exceeding the 105mV (typical) current limit threshold on
the SENSE pin of LT3761. For a boost converter, select a
resistor value according to:
R
SENSE,BOOST
≤
V
IN
• 0.07V
V
LED
• I
LED
For buck-boost mode and SEPIC, select a resistor ac-
cording to:
R
SENSE,BUCK-BOOST
≤
V
IN
• 0.07V
V
IN
+ V
LED
( )
I
LED
For buck mode, select a resistor according to:
R
SENSE,BUCK
≤
0.07V
I
LED
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
(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
SENSE
should be close to the source of
the NMOS FET and GND of the LT3761. The SENSE input
to LT3761 should be a Kelvin connection to the positive
terminal of R
SENSE
. Verify the power on the resistor to
ensure that it does not exceed the rated maximum.
Inductor Selection
The inductor used with the LT3761 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 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 (use the minimum
value for V
IN
and maximum value for V
LED
):
L
BUCK
=
R
SENSE
• V
LED
V
IN
– V
LED
( )
V
IN
• 0.02V • f
OSC
L
BUCK-BOOST
=
R
SENSE
• V
LED
• V
IN
V
LED
+ V
IN
( )
• 0.02V • f
OSC
L
BOOST
=
R
SENSE
• V
IN
V
LED
– V
IN
( )
V
LED
• 0.02V • f
OSC