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| 型号： | IRU3138CSTRPBF |
| 厂商： | International Rectifier |
| 文件页数： | 8/18页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM VM 14-SOIC |
| 标准包装： | 2,500 |
| PWM 型： | 电压模式 |
| 输出数： | 1 |
| 频率 - 最大： | 440kHz |
| 占空比： | 90% |
| 电源电压： | 4.25 V ~ 25 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | 0°C ~ 70°C |
| 封装/外壳： | 14-SOIC（0.154"，3.90mm 宽） |
| 包装： | 带卷 (TR) |
IRU3138
? I O
I RMS = I OUT D × (1-D) ---(9)
Where:
D is the Duty Cycle, D=V OUT /V IN.
I RMS is the RMS value of the input capacitor current.
I OUT is the output current for each channel.
For V IN =5V, I OUT =12A and D=0.36, the I RMS =5.7A
For higher efficiency, a low ESR capacitor is recom-
mended. Choose three Poscap from Sanyo 6TPC150M
(6.3V, 150 μ F, 40m ? ) with a maximum allowable ripple
current of 5.7A.
Inductor Selection
The inductor is selected based on operating frequency,
requirements, yet have high enough ESR to satisfy sta-
bility requirements. The ESR of the output capacitor is
calculated by the following relationship:
? V O
ESR ≤ ---(10)
Where:
? V O = Output Voltage Ripple
? i = Inductor Ripple Current
? V O = 50mV and ? I ? 22% of 12A = 2.64A
This results to: ESR=18.9m ?
The Sanyo TPC series, Poscap capacitor is a good choice.
The 6TPC330M, 330 μ F, 6.3V has an ESR 40m ? . Se-
lecting three of these capacitors in parallel, results to an
ESR of ? 13.3m ? which achieves our low ESR goal.
transient performance and allowable output voltage ripple.
The capacitor value must be high enough to absorb the
Low inductor value results to faster response to step
load (high ? i/ ? t) and smaller size but will cause larger
output ripple due to increase of inductor ripple current.
As a rule of thumb, select an inductor that produces a
ripple current of 10-40% of full load DC.
For the buck converter, the inductor value for desired
operating ripple current can be determined using the fol-
lowing relation:
inductor's ripple current. The larger the value of capaci-
tor, the lower will be the output ripple voltage.
Power MOSFET Selection
The IRU3138 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.
V IN - V OUT = L ×
? i
? t
; ? t = D ×
1
f S
;D=
V OUT
V IN
The MOSFET must have a maximum operating voltage
L = (V IN - V OUT ) ×
V OUT
V IN ×? i × f S
---(11)
(V DSS ) exceeding the maximum input voltage (V IN ).
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
If ? i = 25%(I O ), then the output inductor will be:
L = 0.91 μ H
The Panasonic PCCN6B series provides a range of in-
ductors in different values, low profile suitable for large
currents, 1.1 μ H, 16A is a good choice for this applica-
tion. This will result to a ripple approximately 22% of
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:
P COND (Upper Switch) = I LOAD2 × R DS(ON) × D ×?
P COND (Lower Switch) = I LOAD2 × R DS(ON) × (1 - D) ×?
? = R DS(ON) Temperature Dependency
output current.
The R DS(ON) temperature dependency should be consid-
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
www.irf.com
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.
8
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