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LT3759H

Wide Input Voltage Range Boost/SEPIC/Inverting Controller

Linear Technology 

LT3759

APPLICATIONS INFORMATION

regulates the INTVCC to 3.75V. VIN LDO is turned off when

the INTVCC voltage is greater than 3.75V (typical). Both

LDO’s can be turned off if the INTVCC pin is driven by a

supply of 4.75V or higher but less than 8V (the INTVCC

maximum voltage rating is 8V). A table of the LDO sup-

ply and output voltage combination is shown in Table 1.

Table 1. LDO’s Supply and Output Voltage Combination (Assuming

That the LDO Dropout Voltage is 0.15V)

SUPPLY VOLTAGES

LDO OUTPUT

VIN

VIN ≤ 3.9V

3.9V < VIN ≤ 42V

DRIVE

VDRIVE < VIN

VDRIVE = VIN

VIN < VDRIVE < 4.9V

4.9V ≤ VDRIVE ≤ 42V

VDRIVE < 3.9V

VDRIVE = 3.9V

3.9V < VDRIVE < 4.9V

4.9V ≤ VDRIVE ≤ 42V

INTVCC

VIN – 0.15V

VIN – 0.15V

VDRIVE – 0.15V

4.75V

3.75V

3.75V

VDRIVE – 0.15V

4.75V

LDO STATUS

(Note 7)

#1 Is ON

#1 #2 are ON

#2 Is ON

#2 Is ON

#1 Is ON

#1 #2 are ON

#2 Is ON

#2 Is ON

Note 7: #1 is VIN LDO and #2 is DRIVE LDO

The DRIVE pin provides flexibility to power the gate driver

and the internal loads from a supply that is available only

when the switcher is enabled and running. If not used, the

DRIVE pin should be tied to VIN.

The INTVCC pin must be bypassed to ground immediately

adjacent to the INTVCC pin with a minimum of 4.7µF ceramic

capacitor. Good bypassing is necessary to supply the high

transient currents required by the MOSFET gate driver.

If a low input voltage operation is expected (VIN is 3V or

less), low threshold MOSFETs should be used. The LT3759

contains an undervoltage lockout comparator A8 for the

internal INTVCC supply. The INTVCC undervoltage (UV)

threshold is 1.3V (typical), with 100mV hysteresis, to

ensure that the MOSFETs have sufficient gate drive voltage

before turning on. The logic circuitry within the LT3759

is also powered from the internal INTVCC supply. When

INTVCC is below the UV threshold, the GATE pin will be

forced to GND and the soft-start operation will be triggered.

In an actual application, most of the IC supply current is

used to drive the gate capacitance of the power MOSFET.

The on-chip power dissipation can be a significant con-

cern when a large power MOSFET is being driven at a

high frequency and the VIN voltage is high. It is important

to limit the power dissipation with proper selection of a

MOSFET and/or an operating frequency so the LT3759

does not exceed its maximum junction temperature rating.

The junction temperature TJ can be estimated using the

following equations:

TJ = TA +PIC • θJA

TA = ambient temperature

θJA = junction-to-ambient thermal resistance

PIC = IC power consumption = VIN • (IQ + IDRIVE)

(Assume the DRIVE pin is connected to VIN Supply)

IQ = VIN operation IQ = 1.8mA

IDRIVE = average gate drive current = f • QG

f = switching frequency

QG = power MOSFET total gate charge

The LT3759 uses packages with an exposed pad for en-

hanced thermal conduction. With proper soldering to the

exposed pad on the underside of the package and a full

copper plane underneath the device, thermal resistance

(θJA) will be about 40°C/W for the MSE package.

The LT3759 has an internal INTVCC IDRIVE current limit

function to protect the IC from excessive on-chip power

dissipation. If IDRIVE reaches the current limit, INTVCC

voltage will fall and may trigger the soft-start.

There is a trade-off between the operating frequency and

the size of the power MOSFET (QG) in order to maintain

a reliable IC junction temperature. Prior to lowering the

operating frequency, however, be sure to check with

power MOSFET manufacturers for their most recent low

QG, low RDS(ON) devices. Power MOSFET manufacturing

technologies are continually improving, with newer and

better performance devices being introduced almost yearly.

Operating Frequency and Synchronization

The choice of operating frequency may be determined

by on-chip power dissipation, otherwise it is a trade-off

between efficiency and component size. Low frequency

3759fc

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For more information www.linear.com/3759

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