LTC3773EG-PBF LINER [Linear Technology], LTC3773EG-PBF Datasheet - Page 17

• Manufacturer Part Number: LTC3773EG-PBF
• Description: Triple Output Synchronous 3-Phase DC/DC Controller with Up/Down Tracking
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
where δ is the temperature dependency of R
is the effective top driver resistance (approximately 2Ω
at V
change in drain potential in the particular application.
V
power MOSFET data sheet at the specifi ed drain current.
C
curve from the MOSFET data sheet and the technique
described above.
Both MOSFETs have I
equation includes an additional term for transition losses,
which peak at the highest input voltage. For V
the high current effi ciency generally improves with larger
MOSFETs, while for V
increase to the point that the use of a higher R
with lower C
synchronous MOSFET losses are greatest at high input
voltage when the top switch duty factor is low or during
a short-circuit when the synchronous switch is on close
to 100% of the period.
The term (1 + δ) is generally given for a MOSFET in the
form of a normalized R
δ = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
The Schottky diodes in Figure 1 conduct during the dead
time between the conduction of the two large power
MOSFETs. This prevents the body diode of the bottom
MOSFET from turning on, storing charge during the dead
time and requiring a reverse recovery period which could
cost as much as several percent in effi ciency. A 2A to 8A
Schottky is generally a good compromise for both regions
of operation due to the relatively small average current.
Larger diodes result in additional transition loss due to
their larger junction capacitance.
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 oc-
curs when only one controller is operating. The controller
with the highest (V
determine the maximum RMS capacitor current require-
MILLER
TH(IL)
IN
and C
GS
= V
is the typical gate threshold voltage shown in the
is the calculated capacitance using the gate charge
OUT
MILLER
MILLER
Selection
), and V
IN
OUT
actually provides higher effi ciency. The
is simplifi ed by the 3-phase architec-
2
IN
R losses while the topside N-channel
)(I
> 12V the transition losses rapidly
DS(ON)
OUT
IN
is the drain potential and the
) product needs to be used to
vs temperature curve, but
DS(ON)
DS(ON)
IN
< 12V,
device
, R
DR
ment. Increasing the output current drawn from the other
controller will actually decrease the input RMS ripple cur-
rent 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.
The type of input capacitor, value and ESR rating have ef-
fi ciency effects that need to be considered in the selection
process. The capacitance value chosen should be suffi cient
to store adequate charge to keep high peak battery currents
down. The ESR of the capacitor is important for capacitor
power dissipation as well as overall battery effi ciency. All
the power (RMS ripple current • ESR) not only heats up
the capacitor but wastes power from the battery.
Medium voltage (20V to 35V) ceramic, tantalum, OS-CON
and switcher-rated electrolytic capacitors can be used as
input capacitors, but each has drawbacks: ceramics have
high voltage coeffi cients of capacitance and may have
audible piezoelectric effects; tantalums need to be surge
rated; OS-CONs suffer from higher inductance, larger
case size and limited surface mount applicability; and
electrolytics’ higher ESR and dry out possibility require
several to be used. Sanyo OS-CON SVP, SVPD series; Sanyo
POSCAP TQC series or aluminum electrolytic capacitors
from Panasonic WA series or Cornell Dubilier SPV series,
in parallel with a couple of high performance ceramic ca-
pacitors, can be used as an effective means of achieving
low ESR and large bulk capacitance. Multiphase systems
allow the lowest amount of capacitance overall. As little
as one 22μF or two to three 10μF ceramic capacitors are
an ideal choice in 20W to 35W power supplies due to their
extremely low ESR. Even though the capacitance at 20V
is substantially below their rating at zero-bias, very low
ESR loss makes ceramics an ideal candidate for highest
effi ciency battery operated systems.
In continuous mode, the source current of the top N-chan-
nel MOSFET is a square wave of duty cycle V
prevent large voltage transients, a low ESR input capaci-
tor sized for the maximum RMS current of one channel
must be used. The maximum RMS capacitor current is
given by:
I
RMS
I
OUT(MAX)
V
OUT
(V
V
IN
IN
– V
OUT
)
LTC3773
OUT
/V
17
IN
3773fb
. To