Datasheet

LTC3855
25
3855f
applicaTions inForMaTion
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.
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 1.2V, 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 flows out of the RUN pin once
the RUN pin voltage passes 1.2V. 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 simplified 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
maximum RMS current of one channel must be used. The
maximum RMS capacitor current is given by:
C
IN
Required I
RMS
≈
I
MAX
V
IN
V
OUT
( )
V
IN
– V
OUT
( )




1/2
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
=
I
OUT
/2. This simple worst-case condition is commonly used
for design because even significant deviations do not of-
fer much relief. Note that capacitor manufacturers’ ripple
current ratings are often based on only 2000 hours of life.
This makes it advisable to further derate the capacitor, or
to choose a capacitor rated at a higher temperature than
required. Several capacitors may be paralleled to meet
size or height requirements in the design. Due to the high
operating frequency of the LTC3855, ceramic capacitors
Figure 10. Setup for a 5V Input
INTV
CC
LTC3855
R
VIN
1Ω
C
IN
3855 F07
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
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 capa-
citance of the topside MOSFET(s). The reverse break-
down 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.
Undervoltage Lockout
The LTC3855 has 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 3.2V.
To prevent oscillation when there is a disturbance on the
INTV
CC
, the UVLO comparator has 600mV of precision
hysteresis.