LT3756-1 LINER [Linear Technology], LT3756-1 Datasheet - Page 12

• Manufacturer Part Number: LT3756-1
• Description: 60VIN LED Controller with Internal PWM Generator
• Manufacturer: LINER [Linear Technology]

applicaTions inFormaTion
LT3761
INTV
The INTV
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
to the IC ground.
An internal current limit on the INTV
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
Careful choice of a lower Q
frequencies, leading to smaller magnetics. The INTV
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
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
pin could be connected to the input supply. Be aware that
a small current (less than 13μA) will load the INTV
in shutdown. This action allows the LT3761 to operate
from V
occasionally drops below the INTV
then the minimum operating V
value is determined by the dropout voltage of the linear
regulator and the INTV
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
12
INTVCC
I
INTVCC
CC
IN
can be calculated from the following equation:
Regulator Bypassing and Operation
CC
as low as 4.5V. If V
= Q
pin requires a capacitor for stable operation
G
• f
OSC
IN
, will not exceed 8V, then the INTV
CC
undervoltage lockout threshold
G
FET will allow higher switching
IN
IN
will be close to 5V. This
is normally above, but
CC
CC
regulation voltage,
CC
output protects
pin and also
CC
CC
pin
pin
CC
CC
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:
LED Current Programming
The LED current is programmed by placing an appropriate
value current sense resistor, R
string. The voltage drop across R
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
V
V
IN,FALLING
IN,RISING
Figure 1. Resistor Connection to Set
V
IN
= 2.3µA •R1 + V
= 1.22 •
Undervoltage Shutdown Threshold
LT3761
EN/UVLO
R1+R2
R2
IN,FALLING
LED
V
IN
, in series with the LED
LED
R1
R2
3761 F01
is (Kelvin) sensed
3761f