Datasheet

9
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 V
CC
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 1GΩ 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:
T
J
= T
A
+ PD(θ
JA
)
where:
T
J
= Junction Temperature
T
A
= Ambient Temperature
PD = Power Dissipation
θ
JA
= 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 V
CC
= 12V is only
(360µA)(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 C
OUT
will be:
Load Capacitive Power (C
OUT
) = (C
OUT
)(f)(V
CC
)
2
The power MOSFET’s gate capacitance seen by the driver
output varies with its V
GS
voltage level during switching.
A power MOSFET’s capacitive load power dissipation can
be calculated by its gate charge factor, Q
G
. The Q
G
value
Figure 2. Simplified LTC1693-2 Floating Driver Application
Figure 3. Simplified LTC1693-1 Application
with Different Ground Potentials
OUT1
IN1
GND1
V
CC1
V
CC2
V
+
••
V
+
LTC1693-1
OTHER
PRIMARY-SIDE
CIRCUITS
OTHER
SECONDARY-SIDE
CIRCUITS
OUT2
IN2
GND2
1693 F03
APPLICATIONS INFORMATION
WUU
U
OUT1
IN1
GND1
V
CC1
V
CC2
V
+
V
IN
LTC1693-2
N1
OUT2
IN2
GND2
N2
1693 F02
•