參數(shù)資料
型號(hào)： LTC1148HVIS-5
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS
中文描述： SWITCHING CONTROLLER, 250 kHz SWITCHING FREQ-MAX, PDSO14
封裝： 0.150 INCH, PLASTIC, SOP-14
文件頁(yè)數(shù)： 10/20頁(yè)
文件大?。?/td> 250K
代理商： LTC1148HVIS-5
10
LTC1148
LTC1148-3.3/LTC1148-5
114835fd
APPLICATIU
level threshold MOSFETs (V
GS(TH)
< 2.5V) are strongly
recommended. The LTC1148/LTC1148HV series supply
voltage must always be less than the absolute maximum
V
GS
ratings for the MOSFETs.
The maximum output current I
MAX
determines the R
DS(ON)
requirement for the two MOSFETs. When the LTC1148
series is operating in continuous mode, the simplifying
assumption can be made that one of the two MOSFETs is
always conducting the average load current. The duty
cycles for the two MOSFETs are given by:
W
U
U
P-Ch Duty Cycle =V
OUT
V
IN
N-Ch Duty Cycle =(V
IN
– V
OUT
)
V
IN
From the duty cycles the required R
DS(ON)
for each MOS-
FET can be derived:
P-Ch R
DS(ON)
=
V
IN
(P
P
)
V
OUT
(I
MAX2
)(1 +
δ
P
)
N-Ch R
DS(ON)
=
V
IN
(P
N
)
(V
IN
– V
OUT
)(I
MAX2
)(1 +
δ
N
)
where P
P
and P
N
are the allowable power dissipations and
d
P
and d
N
are the temperature dependencies of R
DS(ON)
.
P
P
and P
N
will be determined by efficiency and/or thermal
requirements (see Efficiency Considerations). (1 + d) is
generally given for a MOSFET in the form of a normalized
R
DS(ON)
vs temperature curve, but d = 0.007/
°
C can be
used as an approximation for low voltage MOSFETs.
The Schottky diode D1 shown in Figure 1 only conducts
during the dead-time between the conduction of the two
power MOSFETs. D1’s sole purpose in life is to prevent the
body diode of the N-channel MOSFET from turning on and
storing charge during the dead time, which could cost as
much as 1% in efficiency (although there are no other
harmful effects if D1 is omitted). Therefore, D1 should be
selected for a forward voltage of less than 0.7V when
conducting I
MAX
.
C
IN
and C
OUT
Selection
In continuous mode, the source of the P-channel MOSFET
is a square wave of duty cycle V
OUT
/V
IN
. To prevent large
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum
RMS capacitor current is given by:
C
IN
Required I
RMS
≈
I
MAX
[V
OUT
(V
IN
–
V
OUT
)]
1/2
V
IN
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT
/2. This simple worst-case condition is com-
monly used for design because even significant devia-
tions do not offer much relief. Note that capacitor
manufacturer’s 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. Always consult the
manufacturer if there is any question. An additional 0.1
μ
F
to 1
μ
F ceramic capacitor is also required on V
IN
Pin 3 for
high frequency decoupling.
The selection of C
OUT
is driven by the required effective
series resistance (ESR). The ESR of C
OUT
must be less
than twice the value of R
SENSE
for proper operation of the
LTC1148 series:
C
OUT
Required ESR < 2R
SENSE
Optimum efficiency is obtained by making the ESR equal
to R
SENSE
. As the ESR is increased up to 2R
SENSE
, the
efficiency degrades by less than 1%. If the ESR is greater
than 2R
SENSE
, the voltage ripple on the output capacitor
will prematurely trigger Burst Mode
operation, resulting in
disruption of continuous mode and an efficiency hit which
can be several percent.
Manufacturers such as Nichicon and United Chemicon
should be considered for high performance capacitors.
The OS-CON semiconductor dielectric capacitor available
from Sanyo has the lowest ESR/size ratio of any aluminum
electrolytic at a somewhat higher price. Once the ESR
requirement for C
OUT
has been met, the RMS current
rating generally far exceeds the I
RIPPLE(P-P)
requirement.
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