LTC3853_15 Datasheet, PDF (21/36 Pages) Linear Technology – Triple Output, Multiphase Synchronous Step-Down Controller

LTC3853
APPLICATIONS INFORMATION
Topside MOSFET Driver Supply (C_B, D_B)
C_IN and C_OUT Selection
External bootstrap capacitors, C_B, connected to the BOOST
pins supply the gate drive voltages for the topside MOS-
FETs. 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
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 using the RUN pin, V_IN needs
to be higher than 4V.
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
≈ ^IMAX
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
time. The total RMS power lost is lower when more than
one controller is operating due to the reduced overlap of
current pulses required through the input capacitor’s ESR.
This is why the input capacitor’s requirement calculated
above for the worst-case controller is adequate for the
3853fc
For more information www.linear.com/LTC3853
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