參數(shù)資料
型號(hào)： LTC3615IFE#PBF
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： SWITCHING REGULATOR, PDSO24
封裝： 4.40 MM, LEAD FREE, PLASTIC, TSSOP-24
文件頁(yè)數(shù)： 10/32頁(yè)
文件大小： 585K
代理商： LTC3615IFE#PBF
LTC3615/LTC3615-1
18
3615fa
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:
ΔΔ
VI
ESR
fC
OUT
L
SW
OUT
≤
+
1
8
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
have to be paralleled to meet the capacitance, ESR or RMS
current 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.
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 capacitors
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:
C
I
fV
OUT
SW
DROOP
≈
25
.
Δ
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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