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參數(shù)資料
| 型號： | IRU3007CW |
| 廠商： | International Rectifier |
| 英文描述： | 5-BIT PROGRAMMABLE SYNCHRONOUS BUCK, NON-SYNCHRONOUS,ADJUSTABLE LDO AND 200mA ON-BOARD LDO |
| 中文描述： | 5位可編程同步降壓，非同步，可調(diào)LDO和200mA的板上穩(wěn)壓器 |
| 文件頁數(shù)： | 12/17頁 |
| 文件大小： | 100K |
| 代理商： | IRU3007CW |
12
Rev. 2.1
08/20/02
IRU3007
www.irf.com
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
T = 1 / 200000 = 5
μ
s
Vsw = Vsync = 14.2
×
0.019 = 0.27V
D
≈
(2.8 + 0.27) / (5 - 0.27 + 0.27) = 0.61
T
ON
= 0.61
×
5 = 3.1
μ
s
T
OFF
= 5 - 3.1 = 1.9
μ
s
Ir = (2.8 + 0.27)
×
1.9 / 3 = 1.94A
Vo = 1.94
×
0.006 = 0.011V = 11mV
Vf = 0.5V
D
MAX
≈
(3.3 + 0.5) / (4.75 - 0.27 + 0.5) = 0.76
of the 1500
μ
F, 6MV1500GX type Sanyo capacitors. With
Rs=5m
, the maximum ESR becomes 9.5m
which is
equivalent to
≈
4 caps. Another important consideration
is that if a trace is being used to implement the resistor,
the power dissipated by the trace increases the case
temperature of the output capacitors which could seri-
ously affect the life span of the output capacitors.
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 made too small, the output ripple current and
ripple voltage will become too large. One solution to bring
the ripple current down is to increase the switching fre-
quency, however that will be at the cost of reduced effi-
ciency and higher system cost. The following set of for-
mulas are derived to achieve optimum performance with-
out many design iterations.
The maximum output inductance is calculated using the
following equation:
(V
IN(MIN)
- Vo
(MAX)
)
Where:
V
IN(MIN)
= Minimum input voltage
For Vo = 2.8V and
I = 14.2A, we get:
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
≈
3 m
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:
L = ESR
×
C
×
(2
×
I)
(2
×
14.2)
L = 0.006
×
9000
×
(4.75 - 2.8)
μ
H
In our example for Vo = 2.8V and 14.2 A load, assuming
IRL3103 MOSFET for both switches with maximum on
resistance of 19m
, we have:
Power Component Selection
Vcore
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.
R
DS(MAX)
=Maximum R
DS(ON)
of the MOSFET at 125
8
C
For synch MOSFET, maximum power dissipation hap-
pens at minimum Vo and minimum duty cycle.
3.3V Supply
Again, for high side switch the maximum power dissipa-
tion happens at maximum Vo and maximum duty cycle.
The duty cycle equation for non synchronous replaces
the forward voltage of the diode with the Synch MOSFET
on voltage. In equations below:
T = 1 / Fsw
Vsw = Vsync = Io
×
R
DS
D
≈
(Vo + Vsync) / (V
IN
- Vsw + Vsync)
T
ON
= D
×
T
T
OFF
= T - T
ON
Ir = (Vo + Vsync)
×
T
OFF
/ L
Vo =
Ir
×
ESR
D
MAX
≈
(2.8 + 0.27) / (4.75 - 0.27 + 0.27) = 0.65
P
DH
= D
MAX
×
Io
2
×
R
DS(MAX)
P
DH
= 0.65
×
14.2
2
×
0.029 = 3.8W
D
MIN
≈
(2 + 0.27) / (5.25 - 0.27 + 0.27) = 0.43
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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