Datasheet LTC1693 (Linear Technology) - 9

LTC1693
U U W U APPLICATIONS INFORMATION Driver Electrical Isolation
driver is powered with respect to ground. Similarly Figure 3 shows a simplified circuit of a LTC1693-1 which is driv- The LTC1693-1 and LTC1693-2 incorporate two individual ing MOSFETs with different ground potentials. Because drivers in a single package that can be separately connected there is 1GΩ of isolation between these drivers in a single to GND and VCC connections. Figure 2 shows a circuit with package, ground current on the secondary side will not an LTC1693-2, its top driver left floating while the bottom recirculate to the primary side of the circuit. V
Power Dissipation
LTC1693-2 IN VCC1 To ensure proper operation and long term reliability, the LTC1693 must not operate beyond its maximum tempera- IN1 OUT1 N1 ture rating. Package junction temperature can be calcu- lated by: GND1 T • J = TA + PD(θJA) where: VCC2 TJ = Junction Temperature V + TA = Ambient Temperature IN2 OUT2 N2 PD = Power Dissipation θJA = Junction-to-Ambient Thermal Resistance GND2 Power dissipation consists of standby and switching 1693 F02 power losses:
Figure 2. Simplified LTC1693-2 Floating Driver Application
PD = PSTDBY + PAC where: OTHER OTHER PSTDBY = Standby Power Losses PRIMARY-SIDE SECONDARY-SIDE CIRCUITS CIRCUITS PAC = AC Switching Losses • • The LTC1693 consumes very little current during standby. LTC1693-1 VCC1 This DC power loss per driver at VCC = 12V is only V + (360µA)(12V) = 4.32mW. IN1 OUT1 AC switching losses are made up of the output capacitive load losses and the transition state losses. The capactive GND1 load losses are primarily due to the large AC currents VCC2 needed to charge and discharge the load capacitance V + during switching. Load losses for the CMOS driver driving IN2 OUT2 a pure capacitive load COUT will be: Load Capacitive Power (COUT) = (COUT)(f)(VCC)2 GND2 The power MOSFET’s gate capacitance seen by the driver 1693 F03 output varies with its VGS voltage level during switching. A power MOSFET’s capacitive load power dissipation can
Figure 3. Simplified LTC1693-1 Application with Different Ground Potentials
be calculated by its gate charge factor, QG. The QG value 9