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APPLICATIONS INFORMATION
The following list summarizes the four possible connec-
tions for EXTV
:
CC
1. EXTV
left open (or grounded). This will cause INTV
CC
to be powered from the internal 5.5V regulator resulting
in an efficiency penalty at high input voltages.
2. EXTV
connected directly to V
CC
connection for a 5V regulator and provides the highest
efficiency.
3. EXTV
connected to an external supply. If a 5V external
CC
supply is available, it may be used to power EXTV
providing it is compatible with the MOSFET gate drive
requirements.
4. EXTV
connected to an output-derived boost network.
CC
For 3.3V and other low voltage regulators, efficiency
gains can still be realized by connecting EXTV
output-derived voltage that has been boosted to greater
than 4.7V.
For applications where the main input power is below 5V,
tie the V
and INTV
pins together and tie the combined
IN
CC
pins to the 5V input with a 1Ω or 2.2Ω resistor as shown
in Figure 11 to minimize the voltage drop caused by the
gate charge current. This will override the INTV
regulator and will prevent INTV
due to the dropout voltage. Make sure the INTV
is at or exceeds the R
DS(ON)
which is typically 4.5V for logic level devices.
V
IN
LTC3875
INTV
CC
Figure 11. Setup for a 5V Input
Topside MOSFET Driver Supply (C
External bootstrap capacitor, C
pin supplies the gate drive voltages for the topside MOSFET.
Capacitor C
in the Functional Diagram is charged though
B
external diode D
from INTV
B
When the topside MOSFET is to be turned on, the driver
places the C
voltage across the gate source of the
B
. This is the normal
OUT
to an
CC
linear
CC
from dropping too low
CC
voltage
CC
test voltage for the MOSFET
R
VIN
1Ω
5V
+
C
INTVCC
C
IN
4.7µF
3875 F11
, D
)
B
B
, connected to the BOOST
B
when the SW pin is low.
CC
For more information
MOSFET. This enhances the MOSFET and turns on the
topside switch. The switch node voltage, SW, rises to V
and the BOOST pin follows. With the topside MOSFET on,
CC
the boost voltage is above the input supply:
V
= V
+ V
BOOST
IN
where V
is the diode forward voltage drop.
D B
The value of the boost capacitor, C
that of the total input capacitance of the topside MOSFET(s).
The reverse breakdown of the external Schottky diode
must be greater than V
CC
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.
Undervoltage Lockout
The LTC3875 has two functions that help protect the
controller in case of undervoltage conditions. A precision
UVLO comparator constantly monitors the INTV
to ensure that an adequate gate-drive voltage is present.
It locks out the switching action when INTV
3.7V. To prevent oscillation when there is a disturbance
on the INTV
, the UVLO comparator has 500mV of preci-
CC
sion hysteresis.
Another way to detect an undervoltage condition is to
monitor the V
supply. Because the RUN pins have a
IN
precision turn-on reference of 1.22V, one can use a resistor
divider to V
to turn on the IC when V
IN
An extra 4.5µA of current flows out of the RUN pin once
the RUN pin voltage passes 1.22V. One can program the
hysteresis of the run comparator by adjusting the values
of the resistive divider. For accurate V
detection, V
needs to be higher than 4.5V.
IN
C
and C
Selection
IN
OUT
The selection of C
IN
ture and its impact on the worst-case RMS current drawn
through the input network (battery/fuse/capacitor). It can
be shown that the worst-case capacitor RMS current oc-
curs when only one controller is operating. The controller
with the highest (V
www.linear.com/LTC3875
LTC3875
– V
INTVCC
D B
, needs to be 100 times
B
. When adjusting the gate
IN(MAX)
is high enough.
IN
undervoltage
IN
is simplified by the 2-phase architec-
)(I
) product needs to be used
OUT
OUT
IN
voltage
CC
is below
CC
3875fb
27
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