LT3757A View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LT3757A

Boost, Flyback, SEPIC and Inverting Controller

Linear Technology 

LT3757/LT3757A

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. If a low input voltage operation is

expected (e.g., supplying power from a lithium-ion battery

or a 3.3V logic supply), low threshold MOSFETs should

be used. The LT3757 contains an undervoltage lockout

comparator A8 and an overvoltage lockout comparator

A9 for the INTVCC supply. The INTVCC undervoltage (UV)

threshold is 2.7V (typical), with 100mV hysteresis, to

ensure that the MOSFETs have sufficient gate drive voltage

before turning on. The logic circuitry within the LT3757 is

also powered from the internal INTVCC supply.

The INTVCC overvoltage (OV) 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 OV thresh-

old, the GATE pin will be forced to GND and the soft-start

operation will be triggered.

The INTVCC regulator must be bypassed to ground imme-

diately adjacent to the IC pins with a minimum of 4.7µF cera-

mic 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 LT3757 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

10

The LT3757 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 LT3757 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 LT3757 can drive at

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

QG vs VIN at different frequencies to guarantee a minimum

4.5V INTVCC is shown in Figure 2.

As illustrated in Figure 2, a trade-off between the operating

frequency and the size of the power MOSFET may be needed

in order to maintain a reliable IC junction temperature.

300

250

300kHz

200

150

100

1MHz

50

0

0 5 10 15 20 25 30 35 40

VIN (V)

3757 F02

Figure 2. Recommended Maximum QG vs VIN at Different

Frequencies to Ensure INTVCC Higher Than 4.5V

3757afd

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