Datasheet

LTC3853
21
3853fa
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 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.
Undervoltage Lockout
The LTC3853 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
3.35V. To prevent oscillation when there is a disturbance
on the INTV
CC
, the UVLO comparator has 500mV of preci-
sion hysteresis.
Another way to detect an undervoltage condition is to moni-
tor 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 using the
RUN pin, V
IN
needs to be higher then 4V.
C
IN
and C
OUT
Selection
The selection of C
IN
is simplified by the 3-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 controllers 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
offer 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 LTC3853, ceramic capacitors
can also be used for C
IN
. Always consult the manufacturer
if there is any question.
The benefit of the LTC3853 3-phase operation can be cal-
culated by using the equation above for the higher power
controller and then calculating the loss that would have
resulted if all controller channels switched on at the same
APPLICATIONS INFORMATION