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参数资料
| 型号： | LTC3872ETS8#PBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 650 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封装： | 1 MM HEIGHT, LEAD FREE, PLASTIC, MO-193, TSOT-23, 8 PIN |
| 文件页数： | 4/22页 |
| 文件大小： | 245K |
| 代理商： | LTC3872ETS8#PBF |
LTC3872
12
3872fb
For a 1% contribution to the total ripple voltage, the ESR
of the output capacitor can be determined using the fol-
lowing equation:
ESRCOUT ≤
0.01 VO
IIN(PEAK)
where:
IIN(PEAK)= 1+
χ
2
IO(MAX)
1– DMAX
For the bulk C component, which also contributes 1% to
the total ripple:
COUT ≥
IO(MAX)
0.01 VO f
Formanydesignsitispossibletochooseasinglecapacitor
type that satisfies both the ESR and bulk C requirements
forthedesign.Incertaindemandingapplications,however,
the ripple voltage can be improved significantly by con-
necting two or more types of capacitors in parallel. For
example, using a low ESR ceramic capacitor can minimize
the ESR step, while an electrolytic capacitor can be used
to supply the required bulk C.
Once the output capacitor ESR and bulk capacitance have
been determined, the overall ripple voltage waveform
should be verified on a dedicated PC board (see Board
Layout section for more information on component place-
ment). Lab breadboards generally suffer from excessive
series inductance (due to inter-component wiring), and
these parasitics can make the switching waveforms look
significantly worse than they would be on a properly
designed PC board.
The output capacitor in a boost regulator experiences
high RMS ripple currents, as shown in Figure 7. The RMS
output capacitor ripple current is:
IRMS(COUT) ≈IO(MAX)
VO – VIN(MIN)
VIN(MIN)
Note that the ripple current ratings from capacitor manu-
facturers 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 placed in parallel
to meet size or height requirements in the design.
Manufacturers such as Nichicon, United Chemicon and
Sanyoshouldbeconsideredforhighperformancethrough-
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo has the lowest product of
ESR and size of any aluminum electrolytic, at a somewhat
higher price.
In surface mount applications, multiple capacitors may
have to be placed in parallel in order to meet the ESR or
RMS current handling requirements of the application.
Aluminum electrolytic and dry tantalum capacitors are
VIN
L
D
SW
6a. Circuit Diagram
6b. Inductor and Input Currents
COUT
VOUT
RL
IIN
IL
6c. Switch Current
ISW
tON
6d. Diode and Output Currents
6e. Output Voltage Ripple Waveform
IO
ID
VOUT
(AC)
tOFF
VESR
RINGING DUE TO
TOTAL INDUCTANCE
(BOARD + CAP)
VCOUT
Figure 6. Switching Waveforms for a Boost Converter
applicaTions inForMaTion
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