LTC3850-1_15 Datasheet, PDF (21/38 Pages) Linear Technology – Dual, 2-Phase Synchronous Step-Down Switching Controller

LTC3850/LTC3850-1
APPLICATIONS INFORMATION
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 LTC3850 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
3V. 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, V_IN needs to be higher than 4V.
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
Re
quired
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 LTC3850, ceramic capacitors
can also be used for C_IN. Always consult the manufacturer
if there is any question.
The benefit of the LTC3850 2-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 both controller channels switched on at the same
time. The total RMS power lost is lower when both control-
lers are 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 dual controller
design. Also, the input protection fuse resistance, battery
resistance, and PC board trace resistance losses are also
reduced due to the reduced peak currents in a 2-phase
system. The overall benefit of a multiphase design will
only be fully realized when the source impedance of the
power supply/battery is included in the efficiency testing.
The sources of the top MOSFETs should be placed within
1cm of each other and share a common C_IN(s). Separating
the sources and C_IN may produce undesirable voltage and
current resonances at V_IN.
A small (0.1µF to 1µF) bypass capacitor between the chip
V_IN pin and ground, placed close to the LTC3850, is also
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