ltc3853euj Linear Technology Corporation, ltc3853euj Datasheet - Page 21

• Manufacturer Part Number: ltc3853euj
• Description: Triple Output, Multiphase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Topside MOSFET Driver Supply (C
External bootstrap capacitors, C
pins supply the gate drive voltages for the topside MOSFETs.
Capacitor C
external diode, D
When one of the topside MOSFETs is to be turned on,
the driver places the C
of the desired MOSFET. This enhances the MOSFET and
turns on the topside switch. The switch node voltage, SW,
rises to V
MOSFET on, the boost voltage is above the input supply:
V
C
citance of the topside MOSFET(s). The reverse breakdown
of the external Schottky diode must be greater than V
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 LTC3853 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
3.35V. To prevent oscillation when there is a disturbance
on the INTV
sion hysteresis.
Another way to detect an undervoltage condition is to moni-
tor the V
turn-on reference of 1.2V, one can use a resistor divider
to V
4.5μA of current fl ows 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
RUN pin, V
BOOST
B
, needs to be 100 times that of the total input capa-
IN
to turn on the IC when V
= V
IN
IN
supply. Because the RUN pins have a precision
IN
IN
B
CC
and the BOOST pin follows. With the topside
in the Functional Diagram is charged though
+ V
, the UVLO comparator has 500mV of preci-
needs to be higher then 4V.
INTVCC
B
, from INTV
IN
. The value of the boost capacitor,
B
undervoltage detection using the
voltage across the gate source
CC
IN
B
, connected to the BOOST
when the SW pin is low.
is high enough. An extra
B
, D
B
)
CC
CC
is below
IN(MAX)
voltage
.
C
The selection of C
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
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
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:
This formula has a maximum at V
= I
used for design because even signifi cant 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
if there is any question.
The benefi t 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
C
IN
IN
OUT
and C
Re
/2. This simple worst-case condition is commonly
quired I
OUT
Selection
OUT
RMS
)(I
IN
is simplifi ed by the 3-phase architec-
OUT
IN
≈
. Always consult the manufacturer
) product needs to be used in the
I
MAX
V
IN
⎡ ⎣
(
V
OUT
IN
OUT
= 2V
)(
)/(V
V
LTC3853
IN
OUT
IN
–
). To prevent
, where I
V
OUT
21
)
⎤ ⎦ ⎦
1 2 /
RMS
3853f