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參數(shù)資料
| 型號(hào)： | LLTC1142HV |
| 廠商： | Linear Technology Corporation |
| 元件分類(lèi)： | 基準(zhǔn)電壓源/電流源 |
| 英文描述： | RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS |
| 中文描述： | 抗輻射高效，5安培開(kāi)關(guān)穩(wěn)壓器 |
| 文件頁(yè)數(shù)： | 12/20頁(yè) |
| 文件大?。?/td> | 261K |
| 代理商： | LLTC1142HV |
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LTC1142/LTC1142L/LTC1142HV
dead-time, which could cost as much as 1% in efficiency
(although there are no other harmful effects if D1 and D2
are omitted). Therefore, D1 and D2 should be selected for
a forward voltage of less than 0.6V when conducting I
MAX
.
C
IN
and C
OUT
Selection
In continuous mode, the source current 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 capaci-
tor sized for the
maximum RMS current must be used. The
maximum RMS capacitor current is given by:
C
I
V
V
V
V
IN
MAX
OUT
IN
OUT
IN
Required I
RMS
≈
(
)
[
]
1 2
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT
/2. This simple worst case conditon is com-
monly used for design because even significant deviations
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. Several capacitors may also be
paralleled to meet size or height requirements in the
design. 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 each V
IN
line (Pins 10 and 24) 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
LTC1142:
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.
In surface mount applications multiple capacitors may
have to be parallel to meet the capacitance, ESR or RMS
current handling requirements of the application. Alumi-
num electrolytic and dry tantalum capacitors are both
available in surface mount configurations. In the case of
tantalum, it is critical that the capacitors are surge tested
for use in switching power supplies. An excellent choice
is the AVX TPS series of surface mount tantalums,
avail-
able in case heights ranging from 2mm to 4mm. For
example, if 200
μ
F/10V is called for in an application
requiring 3mm height, two AVX 100
μ
F/10V (P/N TPSD
107K010) could be used. Consult the manufacturer for
other specific recommendations.
At low supply voltages, a minimum capacitance at C
OUT
is
needed to prevent an abnormal low frequency operating
mode (see Figure 4). When C
OUT
is made too small, the
output ripple at low frequencies will be large enough to trip
the voltage comparator. This causes Burst Mode opera-
tion to be activated when the LTC1142 would normally be
in continuous operation. The output remains in regulation
at all times.
Figure 4. Minimum Value of C
OUT
Checking Transient Response
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
several cycles to respond to a step in DC (resistive) load
APPLICATIU
W
U
U
V
IN
– V
OUT
VOLTAGE (V)
0
O
μ
F
1000
800
600
400
200
0
4
1142 F04
1
2
3
5
L = 50
μ
H
R
SENSE
= 0.02
L = 25
μ
H
R
SENSE
= 0.02
L = 50
μ
H
R
SENSE
= 0.05
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