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LT3754

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:

OUT

OSC

OUT

LEDx

−

••

.•

•

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

μH

−

(

)•

•

.•.•

•

05 32 20

16 5

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