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LTC3783

3783f

INTVCC Regulator Bypassing and Operation

An internal, P-channel low dropout voltage regulator pro-

duces the 7V supply which powers the gate drivers and

logic circuitry within the LTC3783 as shown in Figure 3.

The INTVCC regulator can supply up to 50mA and must be

bypassed to ground immediately adjacent to the IC pins

with a minimum of 4.7

µF low ESR or ceramic capacitor.

Good bypassing is necessary to supply the high transient

currents required by the MOSFET gate driver.

For input voltages that don’t exceed 8V (the absolute

maximum rating for INTVCC is 9V), the internal low drop-

out regulator in the LTC3783 is redundant and the INTVCC

pin can be shorted directly to the VIN pin. With the INTVCC

pin shorted to VIN, however, the divider that programs the

regulated INTVCC voltage will draw 15µA from the input

supply, even in shutdown mode. For applications that

require the lowest shutdown mode input supply current,

do not connect the INTVCC pin to VIN. Regardless of

whether the INTVCC pin is shorted to VIN or not, it is always

necessary to have the driver circuitry bypassed with a

4.7

µF low ESR ceramic capacitor to ground immediately

adjacent to the INTVCC and GND pins.

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

used to drive the gate capacitance of the power MOSFET.

As a result, high input voltage applications in which a large

power MOSFET is being driven at high frequencies can

cause the LTC3783 to exceed its maximum junction tem-

perature rating. The junction temperature can be esti-

mated using the following equations:

IQ(TOT) = IQ + f • QG

PIC = VIN • (IQ + f • QG)

TJ = TA + PIC • θJA

The total quiescent current IQ(TOT) consists of the static

supply current (IQ) and the current required to charge and

discharge the gate of the power MOSFET. The 16-lead FE

package has a thermal resistance of

θJA = 38°C/W and the

DHD package has an

θJA = 43°C/W

As an example, consider a power supply with VIN = 12V

and VOUT = 25V at IOUT = 1A. The switching frequency is

300kHz, and the maximum ambient temperature is 70

°C.

The power MOSFET chosen is the Si7884DP, which has a

maximum RDS(ON) of 10mΩ (at room temperature) and a

OPERATIO

1.230V

P-CH

DRIVER

GATE

CVCC

4.7

µF

X5R

CIN

INPUT

SUPPLY

6V TO 36V

GND

PLACE AS CLOSE AS

POSSIBLE TO DEVICE PINS

3783 F03

INTVCC

VIN

GND

LOGIC

6V-RATED

POWER

MOSFET

Figure 3. Bypassing the LDO Regulator and Gate Driver Supply

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