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LT3756-1 Datasheet(PDF) 11 Page - Linear Technology |
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LT3756-1 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 32 page  LT3760 11 3760fc INTVCC Regulator Bypassing and Operation The INTVCC pin is the output of an internal linear regula- tor driven from VIN and is the supply for the LT3760 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 40mA to limit power dissipation inside the I.C. Thiscurrentlimitshouldbeconsideredwhenchoosingthe N-channel power MOSFET and the switching frequency. The average current load on the INTVCC pin due to the LT3760 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 LT3760 boost converter. It is possible to drive the INTVCC pin from a variety of external sources in order to remove power dissipation from the LT3760 and/or to remove the INTVCC current limitation of 40mA. 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 = 1 - 1 VOUT VIN • 1 fOSC • VIN 0.5 • VOUT VIN • ILEDx • 8 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 8 strings of 10 LEDs each driven with 40mA, and choos- ing VIN = 12V, VOUT = (3.75V •10) +1V = 38.5V, ILEDx = 40mA and fOSC = 1MHz the value for L is calculated as L = (1 - 1 3.2 ) • 1 106 • 12V 0.5 • 3.2 • 40mA • 8 = 16.5µH |
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