LTC3865/LTC3865-1
22
3865fb
APPLICATIONS INFORMATION
For applications where the main input power is below 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.
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 topside MOSFETs.
Capacitor C
B
in the Functional Diagram is charged through
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 fi nal arbiter is the
total input current for the regulator. If a change is made
and the input current decreases, then the effi ciency has
improved. If there is no change in input current, then there
is no change in effi ciency.
Undervoltage Lockout
The LTC3865/LTC3865-1 have two functions that help
protect the controller in case of undervoltage conditions.
A precision UVLO comparator constantly monitors the
INTV
CC
voltage to ensure that an adequate gate-drive
voltage is present. It locks out the switching action when
INTV
CC
is below 3.3V. To prevent oscillation when there is
a disturbance on the INTV
CC
, the UVLO comparator has
550mV of precision hysteresis.
Another way to detect an undervoltage condition is to
monitor the V
IN
supply. Because the RUN pins have a
precision turn-on reference of 1.22V, one can use a resistor
divider to V
IN
to turn on the IC when V
IN
is high enough.
An extra 4.5µA of current fl ows 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
IN
undervoltage
detection, V
IN
needs to be higher than 4.5V.
C
IN
and C
OUT
Selection
The selection of C
IN
is simplifi ed by the 2-phase architec-
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 occurs
when only one controller is operating. The controller with
the highest (V
OUT
)(I
OUT
) product needs to be used in the
formula below to determine the maximum RMS capacitor
current requirement. Increasing the output current drawn
from the other controller will actually decrease the input
RMS ripple current from its maximum value. The out-of-
phase technique typically reduces the input capacitor’s RMS
ripple current by a factor of 30% to 70% when compared
to a single phase power supply solution.
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle (V
OUT
)/(V
IN
). To prevent
large voltage transients, a low ESR capacitor sized for the
INTV
CC
LTC3865
R
VIN
1Ω
C
IN
3865 F08
C
INTVCC
4.7µF
5V
+
V
IN
Figure 8. Setup for a 5V Input
