ltc3850gn-2 Linear Technology Corporation, ltc3850gn-2 Datasheet - Page 20

• Manufacturer Part Number: ltc3850gn-2
• Description: Dual, 2-phase Synchronous Step-down Switching Controller
• Manufacturer: Linear Technology Corporation

LTC3850-2
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 DB from INTV
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 break-
down of the external Schottky diode must be greater
than V
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 LTC3850-2 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
3V. To prevent oscillation when there is a disturbance on
the INTV
sion hysteresis.
Another way to detect an undervoltage condition is to
monitor the V
precision turn-on reference of 1.2V, one can use a resistor
divider to V
An extra 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
detection, V
20
BOOST
B
needs to be 100 times that of the total input capa-
IN(MAX)
= V
CC
IN
IN
, the UVLO comparator has 500mV of preci-
B
IN
and the BOOST pin follows. With the topside
IN
in the Functional Diagram is charged though
+ V
. When adjusting the gate drive level, the
to turn on the IC when V
IN
needs to be higher than 4V.
INTVCC
supply. Because the RUN pins have a
B
. The value of the boost capacitor
voltage across the gate source
CC
B
when the SW pin is low.
connected to the BOOST
B
, DB)
IN
IN
is high enough.
undervoltage
CC
CC
is below
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 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
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 LTC3850-2, ceramic capacitors
can also be used for C
if there is any question.
The benefi t of the LTC3850-2 2-phase operation can be
calculated 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
IN
OUT
C
and C
IN
/2. This simple worst-case condition is commonly
Required I
OUT
Selection
OUT
)(I
IN
RMS
is simplifi ed by the 2-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
IN
OUT
IN
– V
). To prevent
, where I
OUT
)
⎤ ⎦
1/2
38502f
RMS