LT3759 APPLICATIONS INFORMATION regulates the INTVCC to 3.75V. VIN LDO is turned off when cern when a large power MOSFET is being driven at a the INTVCC voltage is greater than 3.75V (typical). Both high frequency and the VIN voltage is high. It is important LDO’s can be turned off if the INTVCC pin is driven by a to limit the power dissipation with proper selection of a supply of 4.75V or higher but less than 8V (the INTVCC MOSFET and/or an operating frequency so the LT3759 maximum voltage rating is 8V). A table of the LDO sup- does not exceed its maximum junction temperature rating. ply and output voltage combination is shown in Table 1. The junction temperature TJ can be estimated using the following equations: Table 1. LDO’s Supply and Output Voltage Combination (AssumingThat the LDO Dropout Voltage is 0.15V)SUPPLY VOLTAGESLDO OUTPUT TJ = TA +PIC • θJA LDO STATUSV(Note 7)INDRIVEINTVCC TA = ambient temperature VIN ≤ 3.9V VDRIVE < VIN VIN – 0.15V #1 Is ON θ V JA = junction-to-ambient thermal resistance DRIVE = VIN VIN – 0.15V #1 #2 are ON VIN < VDRIVE < 4.9V VDRIVE – 0.15V #2 Is ON PIC = IC power consumption = VIN • (IQ + IDRIVE) 4.9V ≤ VDRIVE ≤ 42V 4.75V #2 Is ON (Assume the DRIVE pin is connected to VIN Supply) 3.9V < VIN ≤ 42V VDRIVE < 3.9V 3.75V #1 Is ON V I DRIVE = 3.9V 3.75V #1 #2 are ON Q = VIN operation IQ = 1.8mA 3.9V < VDRIVE < 4.9V VDRIVE – 0.15V #2 Is ON IDRIVE = average gate drive current = f • QG 4.9V ≤ VDRIVE ≤ 42V 4.75V #2 Is ON f = switching frequency Note 7: #1 is VIN LDO and #2 is DRIVE LDO QG = power MOSFET total gate charge The DRIVE pin provides flexibility to power the gate driver The LT3759 uses packages with an exposed pad for en- and the internal loads from a supply that is available only hanced thermal conduction. With proper soldering to the when the switcher is enabled and running. If not used, the exposed pad on the underside of the package and a full DRIVE pin should be tied to VIN. copper plane underneath the device, thermal resistance The INTV (θ CC pin must be bypassed to ground immediately JA) will be about 40°C/W for the MSE package. adjacent to the INTVCC pin with a minimum of 4.7µF ceramic The LT3759 has an internal INTVCC IDRIVE current limit capacitor. Good bypassing is necessary to supply the high function to protect the IC from excessive on-chip power transient currents required by the MOSFET gate driver. dissipation. If IDRIVE reaches the current limit, INTVCC If a low input voltage operation is expected (V voltage will fall and may trigger the soft-start. IN is 3V or less), low threshold MOSFETs should be used. The LT3759 There is a trade-off between the operating frequency and contains an undervoltage lockout comparator A8 for the the size of the power MOSFET (QG) in order to maintain internal INTVCC supply. The INTVCC undervoltage (UV) a reliable IC junction temperature. Prior to lowering the threshold is 1.3V (typical), with 100mV hysteresis, to operating frequency, however, be sure to check with ensure that the MOSFETs have sufficient gate drive voltage power MOSFET manufacturers for their most recent low before turning on. The logic circuitry within the LT3759 QG, low RDS(ON) devices. Power MOSFET manufacturing is also powered from the internal INTVCC supply. When technologies are continually improving, with newer and INTVCC is below the UV threshold, the GATE pin will be better performance devices being introduced almost yearly. forced to GND and the soft-start operation will be triggered. Operating Frequency and Synchronization In an actual application, most of the IC supply current is used to drive the gate capacitance of the power MOSFET. The choice of operating frequency may be determined The on-chip power dissipation can be a significant con- by on-chip power dissipation, otherwise it is a trade-off between efficiency and component size. Low frequency 3759fc 10 For more information www.linear.com/3759 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Related Parts
