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參數(shù)資料
| 型號(hào)： | IRU3038CF |
| 廠商： | International Rectifier |
| 英文描述： | RES CHP 3.83K 1/8W 1% |
| 中文描述： | 同步PWM控制器終止電源應(yīng)用 |
| 文件頁(yè)數(shù)： | 6/18頁(yè) |
| 文件大?。?/td> | 139K |
| 代理商： | IRU3038CF |
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6
Rev. 2.0
09/12/02
IRU3038
www.irf.com
Output Capacitor Selection
The criteria to select the output capacitor is normally
based on the value of the Effective Series Resistance
(ESR). In general, the output capacitor must have low
enough ESR to meet output ripple and load transient
requirements, yet have high enough ESR to satisfy sta-
bility requirements. The ESR of the output capacitor is
calculated by the following relationship:
V
I
O
The Sanyo TPC series, Poscap capacitor is a good choice.
The 6TPC150M 150
μ
F, 6.3V has an ESR 40m
. Se-
lecting two of these capacitors in parallel, results to an
ESR of
20m
which achieves our low ESR goal.
The capacitor value must be high enough to absorb the
inductor's ripple current. The larger the value of capaci-
tor, the lower will be the output ripple voltage.
Inductor Selection
The inductor is selected based on output power, operat-
ing frequency and efficiency requirements. Low inductor
value causes large ripple current, resulting in the smaller
size, but poor efficiency and high output noise. Gener-
ally, the selection of inductor value can be reduced to
desired maximum ripple current in the inductor (
i). The
optimum point is usually found between 20% and 50%
ripple of the output current.
For the buck converter, the inductor value for desired
operating ripple current can be determined using the fol-
lowing relation:
i
V
V
IN
×
i
×
f
S
If
i = 30%(I
O
), then the output inductor will be:
L = 2.6
μ
H
The Coilcraft DO5022HC series provides a range of in-
ductors in different values, low profile suitable for large
currents, 3.3
μ
H, 10A is a good choice for this applica-
tion. This will result to a ripple approximately 26.5% of
output current.
Power MOSFET Selection
The IRU3038 uses two N-Channel MOSFETs. The se-
lections criteria to meet power transfer requirements is
based on maximum drain-source voltage (V
DSS
), gate-
source drive voltage (V
GS
), maximum output current, On-
resistance R
DS(ON)
and thermal management.
The MOSFET must have a maximum operating voltage
(V
DSS
) exceeding the maximum input voltage (V
IN
).
The gate drive requirement is almost the same for both
MOSFETs. Logic-level transistor can be used and cau-
tion should be taken with devices at very low V
GS
to pre-
vent undesired turn-on of the complementary MOSFET,
which results a shoot-through current.
The total power dissipation for MOSFETs includes con-
duction and switching losses. For the Buck converter,
the average inductor current is equal to the DC load cur-
rent. The conduction loss is defined as:
The R
DS(ON)
temperature dependency should be consid-
ered for the worst case operation. This is typically given
in the MOSFET data sheet. Ensure that the conduction
losses and switching losses do not exceed the package
ratings or violate the overall thermal budget.
Choose IRF7460 for both control MOSFET and synchro-
nous MOSFET. This device provides low on-resistance
in a compact SOIC 8-Pin package.
Where:
V
O
= Output Voltage Ripple
I
O
= Output Current
V
O
=100mV and
I
O
=4A
This results to: ESR=25m
Where:
V
IN
= Maximum Input Voltage
V
OUT
= Output Voltage
i = Inductor Ripple Current
f
S
= Switching Frequency
t = Turn On Time
D = Duty Cycle
2
2
P
COND
(Upper Switch) = I
LOAD
×
R
DS(ON)
×
D
×
P
COND
(Lower Switch) = I
LOAD
×
R
DS(ON)
×
(1 - D)
×
= R
DS(ON)
Temperature Dependency
ESR
≤
---(4)
V
IN
- V
OUT
= L
×
t
t = D
×
f
1
S
V
OUT
V
IN
L = (V
IN
- V
OUT
)
×
---(5)
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