Datasheet

LTC3866
23
3866fb
APPLICATIONS INFORMATION
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
LDO 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
re-
mains above 4.5V. Using the EXTV
CC
allows the MOSFET
driver and control power to be derived from an efficient
switching regulator output during normal operation. 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 EXTV
CC
, 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 + (39mA)(5V)(37°C/W) = 77°C
However, for low voltage outputs, additional circuitry is
required to derive INTV
CC
power from the output.
The following list summarizes the three possible connec-
tions for EXTV
CC
:
1. EXTV
CC
left open (or grounded). This will cause
INTV
CC
to be powered from the internal LDO resulting
in an efficiency penalty of up to 10% at high input
voltages.
2. 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.
3. EXTV
CC
connected to an output-derived boost network.
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 10 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.
Figure 10. Setup for a 5V Input
R
VIN
1Ω
C
IN
3866 F10
5V
C
INTVCC
4.7µF
+
INTV
CC
LTC3866
V
IN
Topside MOSFET Driver Supply (C
B
, D
B
)
External bootstrap capacitor, C
B
, connected to the BOOST
pin supplies the gate drive voltages for the topside MOS-
FET. Capacitor C
B
in the Functional Diagram is charged
though external diode D
B
from INTV
CC
when the SW pin
is low. When the topside MOSFET is to be turned on, the
driver places the C
B
voltage across the gate source of the
MOSFET. This enhances the MOSFET and turns on the
topside switch. The switch node voltage, SW, rises to V
IN
and the BOOST pin follows. With the topside MOSFET on,
the boost voltage is above the input supply:
V
BOOST
= V
IN
+ V
INTVCC
– V
DB
The value of the boost capacitor, C
B
, needs to be 100 times
that of the total input capacitance of the topside MOSFET(s).
The reverse breakdown of the external Schottky diode
must be greater than V
IN(MAX)
. When adjusting the gate
drive level, the final arbiter is the total input current for
the regulator. If a change is made and the
input current
decreases, then the efficiency has improved. If there is
no change in input current, then there is no change in
efficiency.
Setting Output Voltage
The LTC3866 output voltage is set by an external feedback
resistive divider carefully placed across the DIFFOUT pin,