LT3752 datasheet(24/52 Pages) AD

24 / 52 page

LT3752/LT3752-1

Rev. C

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APPLICATIONS INFORMATION

The part injection locks the internal oscillator to every ris-

ing edge of the SYNC pin. If the SYNC input is removed

at any time during normal operation the part will simply

changeswitchingfrequencybacktotheoscillatorfrequency

programmed by the RTresistor.Thisinjectionlockmethod

avoids the possible issues from a PLL method which can

potentially cause a large drop in frequency if SYNC input

is removed.

During soft-start the SYNC input is ignored until SS1 ex-

ceeds 2.2V. During soft-stop the SYNC input is completely

ignored. If the SYNC input is to be used, recall that the

programmabledutycycleclampDVSECisdependentonthe

switching frequency of the part (see section Programming

Duty Cycle Clamp). RIVSEC should be reprogrammed by

1/x for an fSYNC/fOSC ratio of x.

INTVCC Regulator Bypassing and Operation

The INTVCC pin is the output of an internal linear regula-

tor driven from VIN and provides the supply for onboard

gate drivers. The LT3752 INTVCC provides a regulated 7V

supply for gate drivers AOUT, SOUT, OUT and HOUT. The

LT3752-1 INTVCCprovidesaregulated10Vsupplyforgate

drivers AOUT, SOUT and OUT. INTVCC should be bypassed

with a 4.7µF low ESR, X7R or X5R ceramic capacitor to

power ground to ensure stability and to provide enough

charge for the gate drivers.

The INTVCC regulator has a minimum 35mA output cur-

rent limit. This current limit should be considered when

choosingtheswitchingfrequencyandcapacitanceloading

on each gate driver. Average current load on the INTVCC

pin for a single gate driver driving an external MOSFET

is given as :

IINTVCC = fOSC • QG

where:

fOSC = controller switching frequency

QG = gate charge (VGS = INTVCC)

While the INTVCC 50mA output current limit is sufficient

for LT3752/LT3752-1 applications, efficiency and internal

power dissipation should also be considered. INTVCC can

be externally overdriven by the housekeeping supply to

improve efficiency, remove power dissipation from within

the IC and provide more than 35mA output current ca-

pability. Any overdrive level should exceed the regulated

INTVCC level but not exceed 16V.

In the case of a short-circuit fault from INTVCC to ground,

each IC reduces the INTVCCoutputcurrentlimittotypically

23mA. The INTVCC regulator has an undervoltage lockout

rising threshold, UVLO(+), which prevents gate driver

switching until INTVCC reaches 4.75V (7V for LT3752-1)

andmaintainsswitchinguntilINTVCCfallsbelowaUVLO(–)

threshold of 4.6V (6.8V for LT3752-1).

For VIN levels close to or below the INTVCC regulated level,

the INTVCC linear regulator may enter dropout. The result-

ing lower INTVCC level will still allow gate driver switching

as long as INTVCC remains above INTVCC UVLO(–) levels.

See the Typical Performance Characteristics section for

INTVCC performance vs VIN and load current.

HOUSEKEEPING CONTROLLER

The LT3752/LT3752-1 include an internal constant fre-

quency, current mode, PWM controller for creating a

housekeepingsupply(seetheBlockDiagramandFigure 8).

Connected as a flyback converter with multiple outputs,

the housekeeping supply is able to efficiently provide bias

to both primary and secondary ICs. It eliminates the need

to generate bias supplies from auxiliary windings in the

main forward transformer, reducing the complexity, size

and cost of the transformer.

Figure 8. Housekeeping Supply

VIN

INTVCC

VAUX*

INTVCC

VHK

•

LTC3752/LT3752-1

INTVCC

HCOMP

HOUT

HISENSE

GND

HFB

RHISLP

RHSENSE

*OPTIONAL ISOLATED SUPPLY FOR SECONDARY SIDE

3752 F08

•

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