ltc3728legn-1-tr Linear Technology Corporation, ltc3728legn-1-tr Datasheet - Page 15

• Manufacturer Part Number: ltc3728legn-1-tr
• Description: Dual, 550khz, 2-phase Synchronous Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
where δ is the temperature dependency of R
R
at the MOSFET’s Miller threshold voltage. V
typical MOSFET minimum threshold voltage.
Both MOSFETs have I
equation includes an additional term for transition losses,
which are highest at high input voltages. 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 diode D1 shown in Figure 1 conducts dur-
ing the dead-time between the conduction of the two
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
that could cost as much as 3% in effi ciency at high V
A 1A to 3A Schottky is generally a good compromise for
both regions of operation due to the relatively small aver-
age current. Larger diodes result in additional transition
losses due to their larger junction capacitance. Schottky
diodes should be placed in parallel with the synchronous
MOSFETs when operating in pulse-skip mode or in Burst
Mode operation.
C
The selection of C
chitecture 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 RMS current occurs
when only one controller is operating. The controller with
the highest (V
formula below to determine the maximum RMS current
requirement. Increasing the output current, drawn from
the other out-of-phase controller, will actually decrease the
IN
DR
and C
(approximately 4Ω) is the effective driver resistance
OUT
MILLER
Selection
OUT
)(I
actually provides higher effi ciency. The
IN
OUT
2
IN
is simplifi ed by the multiphase ar-
R losses while the topside N-channel
> 20V the transition losses rapidly
DS(ON)
) product needs to be used in the
vs Temperature curve, but
DS(ON)
THMIN
DS(ON)
IN
device
< 20V
is the
and
IN
.
input RMS ripple current from this maximum value (see
Figure 4). 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
effi ciency effects that need to be considered in the selec-
tion process. The capacitance value chosen should be
suffi cient to store adequate charge to keep high peak
battery currents down. 20μF to 40μF is usually suffi cient
for a 25W output supply operating at 200kHz. The ESR of
the capacitor is important for capacitor power dissipation
as well as overall battery effi ciency. All of 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 very high voltage coeffi cients 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; electrolytics’
higher ESR and dryout possibility require several to be
used. 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 a 20W to
35W power supply 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. Also consider parallel ceramic and high quality
electrolytic capacitors as an effective means of achieving
ESR and bulk capacitance goals.
In continuous mode, the source current of the top N-channel
MOSFET is a square wave of duty cycle V
large voltage transients, a low ESR input 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
I
MAX
V
OUT
LTC3728L-1
(
V
IN
OUT
V
IN
/V
V
OUT
IN
. To prevent
)
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