LT3758EDD View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LT3758EDD

High Input Voltage, Boost, Flyback, SEPIC and Inverting Controller

Linear Technology 

LT3758/LT3758A

Applications Information

INTVCC Regulator Bypassing and Operation

An internal, low dropout (LDO) voltage regulator produces

the 7.2V INTVCC supply which powers the gate driver, as

shown in Figure 1. The LT3758 contains an undervoltage

lockout comparator A8 and an overvoltage lockout com-

parator A9 for the INTVCC supply. The INTVCC undervoltage

(UV) threshold is 4.5V (typical), with 0.5V hysteresis, to

ensure that the MOSFETs have sufficient gate drive voltage

before turning on. The logic circuitry within the LT3758 is

also powered from the internal INTVCC supply.

The INTVCC overvoltage threshold is set to be 17.5V

(typical) to protect the gate of the power MOSFET. When

INTVCC is below the UV threshold, or above the overvolt-

age threshold, the GATE pin will be forced to GND and the

soft-start operation will be triggered.

The INTVCC regulator must be bypassed to ground im-

mediately adjacent to the IC pins with a minimum of 4.7µF

ceramic capacitor. Good bypassing is necessary to supply

the high transient currents required by the MOSFET gate

driver.

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 concern

when a large power MOSFET is being driven at a high fre-

quency and the VIN voltage is high. It is important to limit

the power dissipation through selection of MOSFET and/

or operating frequency so the LT3758 does not exceed its

maximum junction temperature rating. The junction tem-

perature 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)

IQ = VIN operation IQ = 1.6mA

IDRIVE = average gate drive current = f • QG

f = switching frequency

QG = power MOSFET total gate charge

The LT3758 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 43°C/W for the DD package and 40°C/W

for the MSE package. For an ambient board temperature of

TA = 70°C and maximum junction temperature of 125°C,

the maximum IDRIVE (IDRIVE(MAX)) of the DD package can

be calculated as:

IDRIVE(MAX )

=

(TJ −

(θJA •

TA)

VIN )

−

IQ

=

1.28W

VIN

−

1.6mA

The LT3758 has an internal INTVCC IDRIVE current limit

function to protect the IC from excessive on-chip power

dissipation. The IDRIVE current limit decreases as the VIN

increases (see the INTVCC Minimum Output Current vs VIN

graph in the Typical Performance Characteristics section).

If IDRIVE reaches the current limit, INTVCC voltage will fall

and may trigger the soft-start.

Based on the preceding equation and the INTVCC Minimum

Output Current vs VIN graph, the user can calculate the

maximum MOSFET gate charge the LT3758 can drive at

a given VIN and switch frequency. A plot of the maximum

QG vs VIN at different frequencies to guarantee a minimum

4.7V INTVCC is shown in Figure 2.

140

120

100

80

60

40

20

0

1

300kHz

1MHz

10

VIN (V)

100

3758 F02

Figure 2. Recommended Maximum QG vs VIN at Different

Frequencies to Ensure INTVCC Higher Than 4.7V

3758afd

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