LTC3728LX Linear Technology, LTC3728LX Datasheet - Page 15

• Manufacturer Part Number: LTC3728LX
• Description: 2-Phase Synchronous Regulators
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
where
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
high current efficiency 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+ ) is generally given for a MOSFET in the form
of a normalized R
voltage MOSFETs.
The Schottky diode D1 shown in Figure 1 conducts during
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 efficiency at high V
Schottky is generally a good compromise for both regions
of operation due to the relatively small average current.
Larger diodes result in additional transition losses due to
their larger junction capacitance.
C
The selection of C
tecture 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 input RMS ripple current from this maximum value
(see Figure 4). The out-of-phase technique typically re-
duces the input capacitor’s RMS ripple current by a factor
IN
DR
= 0.005/ C can be used as an approximation for low
and C
(approximately 4 ) is the effective driver resistance
is the temperature dependency of R
OUT
MILLER
Selection
OUT
IN
)(I
actually provides higher efficiency. The
U
DS(ON)
is simplified by the multiphase archi-
OUT
2
IN
R losses while the topside N-channel
> 20V the transition losses rapidly
) product needs to be used in the
U
vs Temperature curve, but
W
IN
DS(ON)
THMIN
. A 1A to 3A
IN
DS(ON)
U
< 20V the
device
is the
and
of 30% to 70% when compared to a single phase power
supply solution.
The type of input capacitor, value and ESR rating have
efficiency effects that need to be considered in the selec-
tion process. The capacitance value chosen should be
sufficient to store adequate charge to keep high peak
battery currents down. 20 F to 40 F is usually sufficient
for a 25W output supply operating at 200kHz. The ESR of
the capacitor is important for capacitor power dissipation
as well as overall battery efficiency. 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: ceramic voltage
coefficients are very high and may have audible piezoelec-
tric 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 22 F or two to three 10 F ceramic capaci-
tors 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 efficiency battery operated systems. Also con-
sider parallel ceramic and high quality electrolytic capaci-
tors as an effective means of achieving ESR and bulk
capacitance goals.
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 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
monly used for design because even significant deviations
do not offer much relief. Note that capacitor manufacturer’s
RMS
C
IN
= I
Re
OUT
quiredI
/2. This simple worst case condition is com-
LTC3728L/LTC3728LX
RMS
I
MAX
V
OUT
w w w . D a t a S h e e t 4 U . c
V
IN
IN
V
IN
= 2V
V
OUT
OUT
OUT
/ 1 2
, where
/V
15
IN
3728lxfa
. To