LTC3728EUH Linear Technology, LTC3728EUH Datasheet - Page 17
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LTC3728EUH
Manufacturer Part Number
LTC3728EUH
Description
IC SW REG SYNC STP-DN DUAL 32QFN
Manufacturer
Linear Technology
Series
PolyPhase®r
Type
Step-Down (Buck)r
Datasheet
1.LTC3728EGPBF.pdf
(36 pages)
Specifications of LTC3728EUH
Internal Switch(s)
No
Synchronous Rectifier
Yes
Number Of Outputs
2
Voltage - Output
0.8 ~ 5.5 V
Current - Output
3A
Frequency - Switching
250kHz ~ 550kHz
Voltage - Input
3.5 ~ 36 V
Operating Temperature
-40°C ~ 85°C
Mounting Type
Surface Mount
Package / Case
32-WFQFN Exposed Pad
Lead Free Status / RoHS Status
Contains lead / RoHS non-compliant
Power - Output
-
Available stocks
Company
Part Number
Manufacturer
Quantity
Price
Company:
Part Number:
LTC3728EUH
Manufacturer:
LT
Quantity:
10 000
Part Number:
LTC3728EUH
Manufacturer:
LT
Quantity:
20 000
Company:
Part Number:
LTC3728EUH#PBF
Manufacturer:
LINEAR
Quantity:
3
Part Number:
LTC3728EUH#TR
Manufacturer:
LINEAR/凌特
Quantity:
20 000
APPLICATIONS INFORMATION
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. C
characteristics. The constant k = 1.7 can be used to esti-
mate the contributions of the two terms in the main switch
dissipation equation.
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
IN
and C
OUT
RSS
Selection
OUT
actually provides higher effi ciency. The
)(I
IN
RSS
OUT
IN
is simplifi ed by the multiphase ar-
> 20V the transition losses rapidly
DS(ON)
is usually specifi ed in the MOSFET
) product needs to be used in the
vs Temperature curve, but
DS(ON)
IN
device
< 20V
IN
.
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 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: ceramic
voltage coeffi cients are very high 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
OUT
LTC3728
/V
IN
. To prevent
17
3728fg