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參數(shù)資料
| 型號： | IRU3004CW |
| 廠商： | International Rectifier |
| 元件分類： | 基準電壓源/電流源 |
| 英文描述： | 5-BIT PROGRAMMABLE SYNCHRONOUS BUCK CONTROLLER IC WITH DUAL LDO CONTROLLER |
| 中文描述： | 5位可編程同步降壓控制器IC具有雙LDO控制器 |
| 文件頁數(shù)： | 11/17頁 |
| 文件大?。?/td> | 104K |
| 代理商： | IRU3004CW |
IRU3004
11
Rev. 1.7
07/16/02
www.irf.com
Output Inductor Selection
The output inductance must be selected such that un-
der low line and the maximum output voltage condition,
the inductor current slope times the output capacitor
ESR is ramping up faster than the capacitor voltage is
drooping during a load current step. However, if the in-
ductor is too small, the output ripple current and ripple
voltage become too large. One solution to bring the ripple
current down is to increase the switching frequency,
however that will be at the cost of reduced efficiency and
higher system cost. The following set of formulas are
derived to achieve the optimum performance without
many design iterations.
The maximum output inductance is calculated using the
following equation:
(
)
Where:
V
IN(MIN)
= Minimum input voltage
Vo = 2.8V ,
I = 14.2A
Assuming that the programmed switching frequency is
set at 200KHz, an inductor is designed using the
Micrometals’ powder iron core material. The summary
of the design is outlined below:
The selected core material is Powder Iron, the selected
core is T50-52D from Micro Metal wound with 8 turns of
#16 AWG wire, resulting in 3
μ
H inductance with
≈
3m
of DC resistance.
Assuming L=3
μ
H and Fsw=200KHz (switching fre-
quency), the inductor ripple current and the output ripple
voltage is calculated using the following set of equations:
In our example for Vo=2.8V and 14.2A load, assuming
IRL3103 MOSFET for both switches with maximum on-
resistance of 19m
, we have:
1
200000
Vsw = Vsync = 14.2
×
0.019 = 0.27V
Power Component Selection
Assuming IRL3103 MOSFETs as power components,
we will calculate the maximum power dissipation as fol-
lows:
For high-side switch the maximum power dissipation
happens at maximum Vo and maximum duty cycle.
For synchronous MOSFET, maximum power dissipa-
tion happens at minimum Vo and minimum duty cycle.
Heat Sink Selection
Selection of the heat sink is based on the maximum
allowable junction temperature of the MOSFETS. Since
we previously selected the maximum R
DS(on)
at 125
8
C,
then we must keep the junction below this temperature.
Selecting TO-220 package gives
θ
JC
=1.8
8
C/W (from the
venders’ data sheet) and assuming that the selected
heat sink is black anodized, the heat-sink-to-case ther-
mal resistance is
θ
CS
=0.05
8
C/W, the maximum heat sink
temperature is then calculated as:
3
2.8 + 0.27
5 - 0.27 + 0.27
T
ON
= 0.61
×
5 = 3.1
μ
s
T = = 5
μ
s
D
≈
= 0.61
T
OFF
= 5 - 3.1 = 1.9
μ
s
Ir = (2.8 + 0.27)
×
1.9
Vo = 1.94
×
0.006 = 0.011V = 11mV
Ts = T
J
- P
D
×
(
θ
JC
+
θ
CS
)
Ts = 125 - 3.82
×
(1.8 + 0.05) = 118
8
C
L = 0.006
×
9000
×
= 3.7
μ
H
2
×
14.2
(
)
4.75 - 2.8
L = ESR
×
C
×
V
IN(MIN)
- Vo
(MAX)
2
×
I
T
≡
Switching Period
D
≡
Duty Cycle
Vsw
≡
High side Mosfet ON Voltage
R
DS
≡
Mosfet On Resistance
Vsync
≡
Synchronous MOSFET ON Voltage
Ir
≡
Inductor Ripple Current
Vo
≡
Output Ripple Voltage
1
Fsw
Vsw = Vsync = Io
×
R
DS
T
ON
= D
×
T
T
OFF
= T - T
ON
Ir = (Vo + Vsync)
×
T
OFF
Vo =
Ir
×
ESR
L
D
≈
Vo + Vsync
V
IN
- Vsw + Vsync
T =
D
MAX
≈
(4.75 - 0.27 + 0.27)
P
DH
= D
MAX
×
Io
2
×
R
DS(MAX)
P
DH
= 0.65
×
14.2
2
×
0.029 = 3.8W
R
DS(MAX)
= Maximum R
DS(ON)
of the MOSFET (125
8
C)
(2.8 + 0.27)
D
MIN
≈
(2 + 0.27)
(5.25 - 0.27 + 0.27)
P
DS
= (1 - D
MIN
)
×
Io
2
×
R
DS(MAX)
P
DS
= (1 - 0.43)
×
14.2
2
×
0.029 = 3.33W
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