參數(shù)資料
型號(hào)： LT3507AEUHF#PBF
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： SWITCHING REGULATOR, PQCC38
封裝： 5 X 7 MM, LEAD FREE, PLASTIC, QFN-38
文件頁(yè)數(shù)： 4/28頁(yè)
文件大?。?/td> 372K
代理商： LT3507AEUHF#PBF
LT3507A
12
3507af
To maintain output regulation, this peak current must be
lessthantheLT3507A’sswitchcurrentlimit,ILIM.ForSW1,
ILIM is typically 5.1A at low duty cycles and decreases
linearly to 3.7A at DC = 0.8. For SW2 and SW3, ILIM is
typically 3.3A for at low duty cycles and decreases linearly
to 2.4A at DC = 0.8.
The minimum inductance can now be calculated as:
LMIN =
1
DCMIN
2 f
VOUT + VF
ILIM –IOUT
However, it’s generally better to use an inductor larger
than the minimum value. The minimum inductor has large
ripple currents which increase core losses and require
large output capacitors to keep output voltage ripple low.
Select an inductor greater than LMIN that keeps the ripple
current below 30% of ILIM.
Theinductor’sRMScurrentratingmustbegreaterthanthe
maximum load current and its saturation current should
be greater than ILPK. For highest efficiency, the series
resistance (DCR) should be less than 0.1Ω. Table 2 lists
several vendors and series that are suitable.
Table 2. Inductors
MANUFACTURER
SERIES
INDUCTANCE
RANGE
CURRENT
RANGE
Würth
WE-HC
1H to 6.5H
6A to 15A
Coilcraft
XAL40xx
0.22H to 15H
3.3A to 21.5A
Sumida
CDRH103R
0.8H to 10H
2.8A to 8.3A
TDK
VLF
2.2H to 10H
3.8A to 7.7A
Vishay
IHLP-2525CZ-11
1H to 10H
2.5A to 9.5A
This analysis is valid for continuous mode operation
(IOUT > ILIM/2). For details of maximum output current in
discontinuous mode operation, see Linear Technology’s
Application Note AN44. Finally, for duty cycles greater
than 50% (VOUT/VIN > 0.5), a minimum inductance is
requiredtoavoidsubharmonicoscillations.Thisminimum
inductance is:
SW1:LMIN = VOUT + VF
(
) 0.4
fSW
SW2, SW3:LMIN = VOUT + VF
(
) 0.75
fSW
with LMIN in H and fSW in MHz.
Output Capacitor Selection
Theoutputcapacitorfilterstheinductorcurrenttogenerate
an output with low voltage ripple. It also stores energy in
order to satisfy transient loads and stabilize the LT3507A’s
control loop. Because the LT3507A operates at a high
frequency, minimal output capacitance is necessary. In
addition, the control loop operates well with or without
the presence of output capacitor series resistance (ESR).
Ceramic capacitors, which achieve very low output ripple
and small circuit size, are therefore an option.
You can estimate output ripple with the following
equations:
VRIPPLE =
ΔIL
8 f COUT
for ceramic capacitors
and
VRIPPLE = ΔIL ESR for electrolytic capacitors
(tantalum and aluminum)
where
ΔIListhepeak-to-peakripplecurrentintheinductor.
The RMS content of this ripple is very low so the RMS
current rating of the output capacitor is usually not of
concern. It can be estimated with the formula:
IC(RMS) =
ΔIL
12
Another constraint on the output capacitor is that it must
have greater energy storage than the inductor; if the stored
energy in the inductor transfers to the output, the resulting
voltage step should be small compared to the regulation
voltage. For a 5% overshoot, this requirement indicates:
COUT >10 L
ILIM
VOUT
2
The low ESR and small size of ceramic capacitors make
them the preferred type for LT3507A applications. Not all
ceramic capacitors are the same, however. Many of the
higher value capacitors use poor dielectrics with high
temperatureandvoltagecoefficients.Inparticular,Y5Vand
Z5U types lose a large fraction of their capacitance with
appliedvoltageandattemperatureextremes.Becauseloop
stabilityandtransientresponsedependonthevalueofCOUT,
this loss may be unacceptable. Use X7R and X5R types.
APPLICATIONS INFORMATION
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