Datasheet

LTC3833
19
3833f
APPLICATIONS INFORMATION
the high transient currents required by the MOSFET gate
drivers.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3833 to be
exceeded, especially if the LDO is active and provides
INTV
CC
. Power dissipation for the IC in this case is high-
est and is approximately equal to V
IN
• I
INTVCC
. The gate
charge current is dependent on operating frequency as
discussed in the Efficiency Considerations section. The
junction temperature can be estimated by using the equa-
tions given in Note 2 of the Electrical Characteristics. For
example, when using the LDO, LTC3833’s INTV
CC
current
is limited to less than 38mA from a 38V supply at T
A
=70°C
in the FE package:
T
J
= 70°C + (38mA)(38V)(38°C/W) ≈ 125°C
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode at maximum
V
IN
.
When the voltage applied to EXTV
CC
pin rises above 4.6V,
the INTV
CC
LDO is turned off and the EXTV
CC
is connected
to INTV
CC
with an internal switch. This switch remains on
as long as the voltage applied to EXTV
CC
remains above
4.4V. Using the EXTV
CC
allows the MOSFET driver and
control power to be derived from the LTC3833’s switching
regulator output during normal operation and from the
LDO when the output is out of regulation (e.g., start-up,
short circuit). If more than 50mA
RMS
current is required
through EXTV
CC
, then an external Schottky diode can be
added between the EXTV
CC
and INTV
CC
pins. Do not apply
more than 6V to the EXTV
CC
pin and make sure that this
external voltage source is less than V
IN
.
Significant efficiency and thermal gains can be realized
by powering INTV
CC
from the switching regulator output,
since the V
IN
current resulting from the driver and control
currents will be scaled by a factor of (Duty Cycle)/(Switcher
Efficiency).
Tying the EXTV
CC
pin to a 5V supply reduces the junction
temperature in the previous example from 125°C to:
T
J
= 70°C + (38mA)(5V)(38°C/W) ≈ 77°C
However, for 3.3V and other low voltage outputs, addi-
tional circuitry is required to derive EXTV
CC
power from
the regulator output.
The following list summarizes the four possible connec-
tions for EXTV
CC
:
1. EXTV
CC
left open (or grounded). This will cause INTV
CC
to be powered from the internal 5.3V LDO resulting
in an efficiency penalty of up to 10% at high input
voltages.
2. EXTV
CC
connected directly to switching regulator output
V
OUT
> 4.6V. This provides the highest efficiency.
3. EXTV
CC
connected to an external supply. If a 4.6V or
greater external supply is available, it may be used to
power EXTV
CC
providing that the external supply is
sufficient enough for MOSFET gate drive requirements.
4. EXTV
CC
connected to an output-derived boost network.
For 3.3V and other low voltage converters, efficiency
gains can still be realized by connecting EXTV
CC
to an
output-derived voltage that has been boosted to greater
than 4.6V.
For applications where the main input power is less than
5.3V, tie the V
IN
and INTV
CC
pins together and tie the com-
bined pins to the V
IN
input with an optional 1Ω or 2.2Ω
resistor as shown in Figure 5 to minimize the voltage drop
caused by the gate charge current. This will override the
INTV
CC
LDO and will prevent INTV
CC
from dropping too low
due to the dropout voltage. Make sure the INTV
CC
voltage
exceeds the R
DS(ON)
test voltage for the external MOSFET
which is typically at 4.5V for logic-level devices.
INTV
CC
LTC3833
V
IN
C
VCC
R
VIN
3833 F05
V
IN
C
IN
Figure 5. Setup for V
IN
≤ 5V