LT3755 LINER [Linear Technology], LT3755 Datasheet - Page 11

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

APPLICATIONS INFORMATION
INTV
The INTV
and to store the charge for the large GATE switching cur-
rents. Choose a 10V rated low ESR, X7R or X5R ceramic
capacitor for best performance. The value of the capacitor
is determined primarily by the stability of the regulator
rather than the gate charge, Q
4.7μF capacitor will be adequate for many applications.
Place the capacitor close to the IC to minimize the trace
length to the INTV
An internal current limit on the INTV
the LT3755 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
ing frequencies, leading to smaller magnetics. The INTV
pin has its own undervoltage disable (UVLO) set to 4.3V
(typical) to protect the external FETs from excessive power
dissipation caused by not being fully enhanced. If the
INTV
and PWMOUT pins will be forced to 0V and the soft-start
pin will be reset.
If the input voltage, V
INTV
aware that a small current (typically less than 10μA) will
load the INTV
occasionally drops below the INTV
then the minimum operating V
value is determined by the dropout voltage of the linear
regulator and the 4.5V (4.3V typical) INTV
lockout threshold mentioned above.
INTVCC
I
INTVCC
CC
CC
CC
can be calculated from the following equation:
Regulator Bypassing and Operation
pin should be connected to the input supply. Be
pin drops below the UVLO threshold, the GATE
CC
= Q
pin requires a capacitor for stable operation
CC
G
• f
in shutdown. If V
OSC
CC
pin and also to the IC ground.
IN
, will not exceed 7V, then the
G
G
FET will allow higher switch-
, of the switching NMOS —a
IN
IN
will be close to 6V . This
is normally above, but
CC
CC
regulation voltage,
CC
output protects
undervoltage
CC
Programming the Turn-On and Turn-Off Thresholds
with the SHDN/UVLO Pin
The falling UVLO value can be accurately set by the resis-
tor divider. A small 2μA pull-down current is active when
SHDN/UVLO is below the 1.24V threshold. The purpose
of this current is to allow the user to program the rising
hysteresis. The following equations should be used to
determine the values of the resistors:
LED Current Programming
The LED current is programmed by placing an appropriate
value current sense resistor between the ISP and ISN pins.
Typically, 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, but take
caution that the minimum ISN value does not fall below
3V, which is the lower limit of the LED current regulation
function. The CTRL pin should be tied to a voltage higher
than 1.1V to get the full-scale 100mV (typical) threshold
across the sense resistor. The CTRL pin can also be used
to dim the LED current to zero, although relative accuracy
decreases with the decreasing voltage sense threshold.
When the CTRL pin voltage is less than 1.1V, the LED
current is:
I
V
V
LED
IN FALLING
IN RISING HYST
,
,
=
V
CTRL
R
=
LED
1 24
− 100
.
=
SHDN/UVLO
•
LT3755
2
10
•
μA
R
mV
LT3755/LT3755-1
Figure 1
1
R
• • R1
+
2
R
2
V
IN
R1
R2
37551 F01
11
37551fa