参数资料
型号: LTC3862HGN#PBF
厂商: LINEAR TECHNOLOGY CORP
元件分类: 稳压器
英文描述: DUAL SWITCHING CONTROLLER, 500 kHz SWITCHING FREQ-MAX, PDSO24
封装: 0.150 INCH, LEAD FREE, PLASTIC, SSOP-24
文件页数: 24/40页
文件大小: 613K
代理商: LTC3862HGN#PBF
LTC3862
30
3862fb
For the bulk capacitance, which we assume contributes
1% to the total output ripple, the minimum required ca-
pacitance is approximately:
C
I
nV
f
OUT
OMAX
OUT
()
.
001
For many designs it will be necessary to use one type of
capacitor to obtain the required ESR, and another type
to satisfy the bulk capacitance. 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.
The voltage rating of the output capacitor must be greater
than the maximum output voltage, with sufcient derating
to account for the maximum capacitor temperature.
Because the ripple current in the output capacitor is a
square wave, the ripple current requirements for this
capacitor depend on the duty cycle, the number of phases
and the maximum output current. Figure 23 illustrates the
normalized output capacitor ripple current as a function of
duty cycle. In order to choose a ripple current rating for
the output capacitor, rst establish the duty cycle range,
based on the output voltage and range of input voltage.
Referring to Figure 23, choose the worst-case high nor-
malized ripple current, as a percentage of the maximum
load current.
The output ripple current is divided between the various
capacitors connected in parallel at the output voltage.
Although ceramic capacitors are generally known for low
ESR (especially X5R and X7R), these capacitors suffer
from a relatively high voltage coefcient. Therefore, it is
not safe to assume that the entire ripple current ows in
the ceramic capacitor. Aluminum electrolytic capacitors are
generally chosen because of their high bulk capacitance,
but they have a relatively high ESR. As a result, some
amount of ripple current will ow in this capacitor. If the
ripple current owing into a capacitor exceeds its RMS
rating, the capacitor will heat up, reducing its effective
capacitance and adversely affecting its reliability. After
the output capacitor conguration has been determined
using the equations provided, measure the individual
capacitor case temperatures in order to verify good
thermal performance.
Input Capacitor Selection
The input capacitor voltage rating in a boost converter
should comfortably exceed the maximum input voltage.
Although ceramic capacitors can be relatively tolerant of
overvoltage conditions, aluminum electrolytic capacitors
are not. Be sure to characterize the input voltage for any
possible overvoltage transients that could apply excess
stress to the input capacitors.
APPLICATIONS INFORMATION
Figure 23. Normalized Output Capacitor
Ripple Current (RMS) for a Boost Converter
0.1
I ORIPPLE
/I
OUT
0.9
3862 F23
0.3
0.5
0.7
0.8
0.2
0.4
0.6
3.25
3.00
2.75
2.50
2.25
2.00
1.75
1.50
1.25
1.00
0.75
0.50
0.25
0
DUTY CYCLE OR (1-VIN/VOUT)
1-PHASE
2-PHASE
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