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參數(shù)資料
| 型號： | LTC3618IFE#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 8 A DUAL SWITCHING CONTROLLER, 2700 kHz SWITCHING FREQ-MAX, PDSO24 |
| 封裝： | 4.40 MM, LEAD FREE, PLASTIC, TSSOP-24 |
| 文件頁數(shù)： | 8/24頁 |
| 文件大小： | 603K |
| 代理商： | LTC3618IFE#PBF |
LTC3618
16
3618f
This formula has a maximum at VIN = 2VOUT, where IRMS =
IOUT/2.Thissimpleworst-caseconditioniscommonlyused
for design because even signicant deviations do not offer
much relief. Note that ripple current ratings from capacitor
manufacturers are often based on only 2000 hours of life
which makes it advisable to further derate the capacitor,
or 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.
Output Capacitor COUT Selection
The selection of COUT is typically driven by the required
ESR to minimize voltage ripple and load step transients
(low-ESR ceramic capacitors are discussed in the next
section). Typically, once the ESR requirement is satised,
the capacitance is adequate for ltering. The output ripple
ΔVOUT is determined by:
VOUT
IL ESR+
1
8 fSW COUT
where fSW=operatingfrequency,COUT=outputcapacitance
and ΔIL = ripple current in the inductor. The output ripple
is highest at maximum input voltage since ΔIL increases
with input voltage.
In surface mount applications, multiple capacitors may
be paralleled to meet the capacitance, ESR or RMS cur-
rent handling requirement of the application. Aluminum
electrolytic, special polymer, ceramic and dry tantalum
capacitors are all available in surface mount packages.
Tantalum capacitors have the highest capacitance density,
but can have higher ESR and must be surge tested for
use in switching power supplies. Aluminum electrolytic
capacitors have signicantly higher ESR, but can often
be used in extremely cost-sensitive applications provided
that consideration is given to ripple current ratings and
long term reliability.
Ceramic Input and Output Capacitors
Ceramic capacitors have the lowest ESR and can be cost
effective, but also have the lowest capacitance density,
high voltage and temperature coefcients, and exhibit
audible piezoelectric effects. In addition, the high-Q of
ceramic capacitors along with trace inductance can lead
to signicant ringing.
Ceramic capacitors are tempting for switching regulator
use because of their very low ESR. Great care must be
taken when using only ceramic input and output capacitors.
Ceramic caps are prone to temperature effects which re-
quire the designer to check loop stability over the operating
temperature range. To minimize their large temperature
and voltage coefcients, only X5R or X7R ceramic capaci-
tors should be used.
When a ceramic capacitor is used at the input, and the
power is being supplied through long wires, such as from
a wall adapter, a load step at the output can induce ringing
at the VIN pin. At best, this ringing can couple to the output
and be mistaken as loop instability. At worst, the ringing
at the input can be large enough to damage the part.
Since the ESR of a ceramic capacitor is so low, the input
and output capacitor must instead fulll a charge storage
requirement. During a load step, the output capacitor must
instantaneously supply the current to support the load
until the feedback loop raises the switch current enough
to support the load. The time required for the feedback
loop to respond is dependent on the compensation com-
ponents and the output capacitor size. Typically, three to
four cycles are required to respond to a load step, but
only in the rst cycle does the output drop linearly. The
output droop, VDROOP, is usually about two to three times
the linear drop of the rst cycle. Thus, a good place to
start is with the output capacitor size of approximately:
COUT ≈
2.5
ΔI
OUT
fSW VDROOP
More capacitance may be required depending on the duty
cycle and load step requirements. In most applications, the
input capacitor is merely required to supply high frequency
bypassing, since the impedance to the supply is very low.
APPLICATIONS INFORMATION
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PDF描述 |
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| LTC3618EFE#TRPBF | 8 A DUAL SWITCHING CONTROLLER, 2700 kHz SWITCHING FREQ-MAX, PDSO24 |
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| LTC3620EDC#TRMPBF | SWITCHING REGULATOR, 50 kHz SWITCHING FREQ-MAX, PDSO8 |
| LTC3620EDC-1#PBF | SWITCHING REGULATOR, 50 kHz SWITCHING FREQ-MAX, PDSO8 |
| LTC3620EDC-1#TRPBF | SWITCHING REGULATOR, 50 kHz SWITCHING FREQ-MAX, PDSO8 |
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