Datasheet

LTC3856
28
3856f
applicaTions inForMaTion
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
cur-
rent 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 + (42mA)(5V)(34°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 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 LDO 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 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 13) 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.
Topside MOSFET Driver Supply (C
B
, D
B
)
External bootstrap capacitors, C
B
, connected to the
BOOST pins supply the gate drive voltages for the top-
side MOSFETs. Capacitor C
B
in the Functional Diagram
is charged though external diode D
B
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 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
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.
Figure 13. Set-Up for a 5V Input
INTV
CC
LTC3856
R
VIN
1Ω
C
IN
3856 F13
C
INTVCC
4.7µF
5V
+
V
IN