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參數(shù)資料
| 型號： | LT3724 |
| 廠商： | Linear Technology Corporation |
| 元件分類： | 基準(zhǔn)電壓源/電流源 |
| 英文描述： | RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS |
| 中文描述： | 抗輻射高效，5安培開關(guān)穩(wěn)壓器 |
| 文件頁數(shù)： | 14/24頁 |
| 文件大小： | 240K |
| 代理商： | LT3724 |
LT3724
14
3724f
APPLICATIOU
Step-Down Converter: Input Capacitor Selection
A local input bypass capacitor is required for buck con-
verters because the input current is pulsed with fast rise
and fall times. The input capacitor selection criteria are
based on the bulk capacitance and RMS current capability.
The bulk capacitance will determine the supply input ripple
voltage. The RMS current capability is used to keep from
overheating the capacitor.
The bulk capacitance is calculated based on maximum
input ripple,
V
IN
:
W
U
U
C
I
V
V
f
V
IN BULK
(
OUT MAX
(
OUT
IN
SW
IN MIN
(
)
)
)
=
V
IN
is typically chosen at a level acceptable to the user.
100mV-200mV is a good starting point. Aluminum elec-
trolytic capacitors are a good choice for high voltage, bulk
capacitance due to their high capacitance per unit area.
The capacitor’s RMS current is:
I
I
V
V
V
(
V
CIN RMS
(
OUT
OUT
IN
OUT
IN
)
(
–
)
)
=
2
If applicable, calculate it at the worst case condition, V
IN
=
2V
OUT
. The RMS current rating of the capacitor is specified
by the manufacturer and should exceed the calculated
I
CIN(RMS)
. Due to their low ESR (Equivalent Series Resis-
tance), ceramic capacitors are a good choice for high
voltage, high RMS current handling. Note that the ripple
current ratings from aluminum electrolytic capacitor manu-
facturers are based on 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.
The combination of aluminum electrolytic capacitors and
ceramic capacitors is an economical approach to meeting
the input capacitor requirements. The capacitor voltage
rating must be rated greater than V
IN(MAX)
. Multiple ca-
pacitors may also be paralleled to meet size or height
requirements in the design. Locate the capacitor very
close to the MOSFET switch and use short, wide PCB
traces to minimize parasitic inductance.
Step-Down Converter: Output Capacitor Selection
The output capacitance, C
OUT
, selection is based on the
design’s output voltage ripple,
V
OUT
, and transient load
requirements.
V
OUT
is a function of
I
L
and the C
OUT
ESR. It is calculated by:
=
+
V
I
ESR
f
C
OUT
L
SW
OUT
(
8
)
1
The maximum ESR required to meet a
V
OUT
design
requirement can be calculated by:
ESR MAX
(
V
V
V
IN MAX
(
V
OUT
SW
OUT
OUT
)
(
)L f
)
–
)
=
1
Worst-case
V
OUT
occurs at highest input voltage. Use
paralleled multiple capacitors to meet the ESR require-
ments. Increasing the inductance is an option to lower the
ESR requirements. For extremely low
V
OUT
, an additional
LC filter stage can be added to the output of the supply.
Application Note 44 has some good tips on sizing an
additional output filter.
Output Voltage Programming
A resistive divider sets the DC output voltage according to
the following formula:
R
R
V
.
V
OUT
2
11 231
1
=
–
The external resistor divider is connected to the output of
the converter as shown in Figure 2. Tolerance of the
feedback resistors will add additional error to the output
voltage.
Example: V
OUT
= 12V; R1 = 10k
R
k
V
V
k
use
k
2
10
12
.
1 231
1
87 48
.
86 6
1
=
=
.
%
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