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LT3754 Datasheet(PDF) 11 Page - Linear Technology

Part No. LT3754
Description  16-Channel × 50mA LED Driver
Download  28 Pages
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LT3754 Datasheet(HTML) 11 Page - Linear Technology

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LT3754
11
3754f
INTVCC Regulator Bypassing and Operation
The INTVCC pin is the output of an internal linear regulator
driven from VIN and is the supply for the LT3754 gate
driver. The INTVCC 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 VIN levels the INTVCC pin provides
a regulated 7V supply. Make sure INTVCC voltage does
not exceed the VGS rating of the external MOSFET driven
by the GATE pin. For low VIN levels the INTVCC level will
depend on VIN and the voltage drop of the regulator. The
INTVCC regulator has an undervoltage lockout which
prevents gate driver switching until INTVCC reaches 3.8V
and maintains switching until INTVCC falls below 3.4V.
This feature prevents excessive power dissipation in the
external MOSFET by ensuring a minimum gate drive level
to keep RDS(ON) low. The INTVCC regulator has a current
limit of 44mA 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 INTVCC pin due to the
LT3754 gate driver can be calculated as:
IINTVCC = Qg • fOSC
where Qg is the gate charge (at VGS = INTVCC) specified
for the MOSFET and fosc is the switching frequency of the
LT3754 boost converter. It is possible to drive the INTVCC
pin from a variety of external sources in order to remove
power dissipation from the LT3754 and/or to remove the
INTVCC current limitation of 44mA. An external supply for
INTVCC should never exceed the VIN pin voltage or the
maximum INTVCC pin rating of 13V. If INTVCC is shorted
to the VIN pin, VIN 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 I2R 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
V
V
f
V
V
V
I
OUT
IN
OSC
IN
OUT
IN
LEDx
=
1
11
05
16
••
.•
where:
VOUT = (N • VF) + 1V
(N = number of LEDs per string),
VF = LED forward voltage drop,
ILEDx = LED current per string
Example: For a 12W LED driver application requiring 16
strings of 10 LEDs each driven with 20mA, and choos-
ing VIN = 12V, VOUT = (3.75V • 10) + 1V = 38.5V, ILEDx =
20mA and fOSC = 1MHz the value for L is calculated as
L
V
mA
μH
=
=
(
.
)•
.•.•
.
1
1
32
1
10
12
05 32 20
16
16 5
6


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