Datasheet

LTC3850/LTC3850-1
20
38501fc
exceeded. The INTV
CC
current, which is dominated by the
gate charge current, may be supplied by either the 5V linear
regulator or EXTV
CC
. When the voltage on the EXTV
CC
pin
is less than 4.7V, the linear regulator is enabled. Power
dissipation for the IC in this case is highest and is 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 equations given in Note 3 of the
Electrical Characteristics. For example, the LTC3850 INTV
CC
current is limited to less than 24mA from a 24V supply in
the GN package and not using the EXTV
CC
supply:
T
J
= 70°C + (24mA)(24V)(95°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 (MODE/PLLIN =
SGND) at maximum V
IN
. When the voltage applied to EXTV
CC
rises above 4.7V, the INTV
CC
linear regulator is turned off
and the EXTV
CC
is connected to the INTV
CC
. The EXTV
CC
remains on as long as the voltage applied to EXTV
CC
remains
above 4.5V. Using the EXTV
CC
allows the MOSFET driver
and control power to be derived from one of the LTC3850’s
switching regulator outputs during normal operation and
from the INTV
CC
when the output is out of regulation
(e.g., start-up, short-circuit). If more current is required
through the EXTV
CC
than is specified, 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 EXTV
CC
< V
IN
.
Significant efficiency and thermal gains can be realized by
powering INTV
CC
from the 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 + (24mA)(5V)(95°C/W) = 81°C
However, for 3.3V and other low voltage outputs, addi-
tional circuitry is required to derive INTV
CC
power from
the 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 5V regulator
resulting in an efficiency penalty of up to 10% at high
input voltages.
2. EXTV
CC
connected directly to V
OUT
. This is the
normal connection for a 5V regulator and provides
the highest efficiency.
3. EXTV
CC
connected to an external supply. If a 5V
external supply is available, it may be used to power
EXTV
CC
providing it is compatible with the MOSFET
gate drive requirements.
4. EXTV
CC
connected to an output-derived boost net-
work. For 3.3V and other low voltage regulators,
efficiency gains can still be realized by connecting
EXTV
CC
to an output-derived voltage that has been
boosted to greater than 4.7V.
For applications where the main input power is 5V, tie
the V
IN
and INTV
CC
pins together and tie the combined
pins to the 5V input with a 1Ω or 2.2Ω resistor as shown
in Figure 8 to minimize the voltage drop caused by the
gate charge current. This will override the INTV
CC
linear
regulator and will prevent INTV
CC
from dropping too low
due to the dropout voltage. Make sure the INTV
CC
voltage
is at or exceeds the R
DS(ON)
test voltage for the MOSFET
which is typically 4.5V for logic level devices.
APPLICATIONS INFORMATION
Figure 8. Setup for a 5V Input
INTV
CC
LTC3850
R
VIN
1Ω
C
IN
38501 F08
4.7µF
5V
CINTV
CC
+
V
IN
Topside MOSFET Driver Supply (C
B
, DB)
External bootstrap capacitors C
B
connected to the BOOST
pins supply the gate drive voltages for the topside MOSFETs.
Capacitor C
B
in the Functional Diagram is charged though
external diode DB from INTV
CC
when the SW pin is low.
When one of the topside MOSFETs is to be turned on,
the driver places the C
B
voltage across the gate source
of the desired MOSFET. This enhances the MOSFET and