Datasheet

LT3760
11
3760fc
INTV
CC
Regulator Bypassing and Operation
The INTV
CC
pin is the output of an internal linear regula-
tor driven from V
IN
and is the supply for the LT3760 gate
driver. The INTV
CC
pin should be bypassed with a 10V
rated 4.7µF low ESR, X7R or X5R ceramic capacitor to
ensure stability and to provide enough charge for the gate
driver. For high enough V
IN
levels the INTV
CC
pin provides
a regulated 7V supply. Make sure INTV
CC
voltage does
not exceed the V
GS
rating of the external MOSFET driven
by the GATE pin. For low V
IN
levels the INTV
CC
level will
depend on V
IN
and the voltage drop of the regulator. The
INTV
CC
regulator has an undervoltage lockout which
prevents gate driver switching until INTV
CC
reaches 3.8V
and maintains switching until INTV
CC
falls below 3.4V.
This feature prevents excessive power dissipation in the
external MOSFET by ensuring a minimum gate drive level
to keep R
DS(ON)
low. The INTV
CC
regulator has a current
limit of 40mA to limit power dissipation inside the I.C.
This current limit should be considered when choosing the
N-channel power MOSFET and the switching frequency.
The average current load on the INTV
CC
pin due to the
LT3760 gate driver can be calculated as:
I
INTVCC
= Q
g
• f
OSC
where Q
g
is the gate charge (at V
GS
= INTV
CC
) specified
for the MOSFET and fosc is the switching frequency of the
LT3760 boost converter. It is possible to drive the INTV
CC
pin from a variety of external sources in order to remove
power dissipation from the LT3760 and/or to remove the
INTV
CC
current limitation of 40mA. An external supply for
INTV
CC
should never exceed the V
IN
pin voltage or the
maximum INTV
CC
pin rating of 13V. If INTV
CC
is shorted
to the V
IN
pin, V
IN
operational range is 4.5V to 13V.
applicaTions inForMaTion
Inductor
A list of inductor manufacturers is given in Table 1. How-
ever, there are many other manufacturers and inductors
that can be used. Consult each manufacturer for more
detailed information and their entire range of parts. Ferrite
cores should be used to obtain the best efficiency. Choose
an inductor that can handle the necessary peak current
without saturating. Also ensure that the inductor has a
low DCR (copper-wire resistance) to minimize I
2
R power
losses. Values between 2.2µH and 33µH will suffice for
most
applications. The typical inductor value required for
a given application (assuming 50% inductor ripple current
for example) can be calculated as:
L =
1-
1
V
OUT
V
IN
•
1
f
OSC
• V
IN
0.5 •
V
OUT
V
IN
• I
LEDx
• 8
where:
V
OUT
= (N • V
F
) + 1V
(N = number of LEDs per string),
V
F
= LED forward voltage drop,
I
LEDx
= LED current per string
Example: For a 12W LED driver application requiring 8
strings of 10 LEDs each driven with 40mA, and choos-
ing V
IN
= 12V, V
OUT
= (3.75V • 10) + 1V = 38.5V, I
LEDx
=
40mA and f
OSC
= 1MHz the value for L is calculated as
L =
(1-
1
3.2
) •
1
10
6
• 12V
0.5 • 3.2 • 40mA • 8
= 16.5µH