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LTC1693

Driver Electrical Isolation

The LTC1693-1 and LTC1693-2 incorporate two individual

drivers in a single package that can be separately connected

to GND and VCC connections. Figure 2 shows a circuit with

an LTC1693-2, its top driver left floating while the bottom

driver is powered with respect to ground. Similarly Figure

3 shows a simplified circuit of a LTC1693-1 which is driv-

ing MOSFETs with different ground potentials. Because

there is 1G

Ω of isolation between these drivers in a single

package, ground current on the secondary side will not

recirculate to the primary side of the circuit.

Power Dissipation

To ensure proper operation and long term reliability, the

LTC1693 must not operate beyond its maximum tempera-

ture rating. Package junction temperature can be calcu-

lated by:

TJ = TA + PD(θJA)

where:

TJ = Junction Temperature

TA = Ambient Temperature

PD = Power Dissipation

θJA = Junction-to-Ambient Thermal Resistance

Power dissipation consists of standby and switching

power losses:

PD = PSTDBY + PAC

where:

PSTDBY = Standby Power Losses

PAC = AC Switching Losses

The LTC1693 consumes very little current during standby.

This DC power loss per driver at VCC = 12V is only

(360

µA)(12V) = 4.32mW.

AC switching losses are made up of the output capacitive

load losses and the transition state losses. The capactive

load losses are primarily due to the large AC currents

needed to charge and discharge the load capacitance

during switching. Load losses for the CMOS driver driving

a pure capacitive load COUT will be:

Load Capacitive Power (COUT) = (COUT)(f)(VCC)2

The power MOSFET’s gate capacitance seen by the driver

output varies with its VGS voltage level during switching.

A power MOSFET’s capacitive load power dissipation can

be calculated by its gate charge factor, QG. The QG value

Figure 2. Simplified LTC1693-2 Floating Driver Application

Figure 3. Simplified LTC1693-1 Application

with Different Ground Potentials

OUT1

IN1

GND1

VCC1

VCC2

V +

••

V +

LTC1693-1

OTHER

PRIMARY-SIDE

CIRCUITS

OTHER

SECONDARY-SIDE

CIRCUITS

OUT2

IN2

GND2

1693 F03

APPLICATIONS INFORMATION

OUT1

IN1

GND1

VCC1

VCC2

V +

VIN

LTC1693-2

OUT2

IN2

GND2

1693 F02

•

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