Datasheet

LT3761
12
3761f
applicaTions inFormaTion
INTV
CC
Regulator Bypassing and Operation
The INTV
CC
pin requires a capacitor for stable operation
and to store the charge for the large GATE switching cur-
rents. Choose a 10V rated low ESR, X7R ceramic capacitor
for best performance. A 1μF capacitor will be adequate
for many applications. Place the capacitor close to the IC
to minimize the trace length to the INTV
CC
pin and also
to the IC ground.
An internal current limit on the INTV
CC
output protects
the LT3761 from excessive on-chip power dissipation.
The minimum value of this current should be considered
when choosing the switching NMOS and the operating
frequency.
I
INTVCC
can be calculated from the following equation:
I
INTVCC
= Q
G
• f
OSC
Careful choice of a lower Q
G
FET will allow higher switching
frequencies, leading to smaller magnetics. The INTV
CC
pin
has its own undervoltage disable set to 4.1V (typical) to
protect the external FETs from excessive power dissipa-
tion caused by not being fully enhanced. If the INTV
CC
pin
drops below the UVLO threshold, the GATE and PWMOUT
pins will be forced to 0V and the soft-start pin will be reset.
If the input voltage, V
IN
, will not exceed 8V, then the INTV
CC
pin could be connected to the input supply. Be aware that
a small current (less than 13μA) will load the INTV
CC
in shutdown. This action allows the LT3761 to operate
from V
IN
as low as 4.5V. If V
IN
is normally above, but
occasionally drops below the INTV
CC
regulation voltage,
then the minimum operating V
IN
will be close to 5V. This
value is determined by the dropout voltage of the linear
regulator and the INTV
CC
undervoltage lockout threshold
mentioned above.
Programming the Turn-On and Turn-Off Thresholds
with the EN/UVLO Pin
The power supply undervoltage lockout (UVLO) value can
be accurately set by the resistor divider to the EN/UVLO pin.
A small 2.3μA pull-down current is active when EN/UVLO
EN/UVLO
LT3761
V
IN
R2
3761 F01
R1
Figure 1. Resistor Connection to Set
V
IN
Undervoltage Shutdown Threshold
is below the threshold. The purpose of this current is to
allow the user to program the rising hysteresis. The fol-
lowing equations should be used to determine the value
of the resistors:
V
IN,FALLING
=1.22•
R1
+
R2
R2
V
IN,RISING
= 2.3µA•R1 + V
IN,FALLING
LED Current Programming
The LED current is programmed by placing an appropriate
value current sense resistor, R
LED
, in series with the LED
string. The voltage drop across R
LED
is (Kelvin) sensed
by the ISP and ISN pins. A half watt resistor is usually
a good choice. To give the best accuracy, sensing of the
current should be done at the top of the LED string. If this
option is not available then the current may be sensed
at the bottom of the string, or in the source of the PWM
disconnect NFET driven by the PWMOUT signal. A unique
case of GND sensing is the inverting converter shown in
the applications where the LED current is sensed in the
cathode of the power Schottky rectifier. This configuration
allows the LED anode to be grounded for heat sinking. In
this case, it is important to lowpass filter the discontinu-
ous current signal. Input bias currents for the ISP and ISN
inputs are shown in the typical performance characteristics
and should be considered when placing a resistor in series
with the ISP or ISN pins.
The CTRL pin should be tied to a voltage higher than 1.2V
to get the full-scale 250mV (typical) threshold across the
sense resistor. The CTRL pin can also be used to dim the