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參數(shù)資料
| 型號： | IRU3034PBF |
| 廠商： | International Rectifier |
| 英文描述： | 8-PIN PWM SWITCHER CONTROLLER IC WITH CURRENT LIMITING |
| 中文描述： | 8引腳的PWM開關(guān)控制器芯片，限流 |
| 文件頁數(shù)： | 9/12頁 |
| 文件大小： | 159K |
| 代理商： | IRU3034PBF |
IRU3034 & (PbF)
9
www.irf.com
R
B
= Rt / X
X =
[
(Vo + (
Vo/2)) / V
REF
]
- 1
X=
[
(3.38+ (0.044/2)) / 1.25
]
- 1 = 1.72
R
B
= 1000 / 1.72 = 580
Ppk = I
CL
2
×
Rs = 20
2
×
0.005 = 2W
P
AVG
= 0.5
×
Ppk
×
Dsc
T
HCP
= 0.1
×
200 = 20ms
P
AVG
= 0.5
×
2
×
(0.5/20) = 25mW
R
H
= (11
×
Rt) /
Vo
R
H
= (11
×
1000) / 0.044 = 250K
The bottom resistor of the divider is then calculated us-
ing the following equations:
Where:
R
B
= Bottom resistor of the divider
V
REF
= 1.25V Typical
Select R
B
= 576
, 1%
Frequency Calculation
The IRU3034 frequency of operation is calculated using
the following formula:
Where:
Vo = Output voltage (V)
D = Duty cycle
ESR = Output capacitor ESR (V)
L = Output inductance (
μ
H)
Vo = Output ripple voltage (V)
For our example:
D
≈
(Vo + Vf) / V
IN
Where:
Vf = Forward voltage drop of the Schottky diode.
The ESR=18m
for 2 of the Sanyo 1500
μ
F,
6MV1500GX caps. If L=3.5
μ
H then, Fs is calculated as
follows:
The peak power dissipated in the CS resistor is:
However, the average power dissipated is much lower
than 2W due to the long off time caused by the hiccup
circuit of IRU3034. The average power is in fact the short
circuit period divided by the short circuit period plus the
off time or "Hiccup" period. For example, if the short
circuit lasts for Tsc=100
μ
s before the IRU3034 enters
hiccup, the average power is calculated as:
Where:
Dsc = Tsc / T
HCP
T
HCP
= Css
×
M
M = 200ms/
μ
F
Css = The soft-start capacitor
For example:
For Css=0.1
μ
F and Tsc=500
μ
s=0.5ms
Without "Hiccup" technique, the power dissipation of the
resistor is 2W.
Switcher Output Voltage Setting
The output voltage can be set using the following
equations:
Assuming, Vo=3.38V and the selected output ripple is
≈
1.3%(44mV) of the output voltage, a set of equations
are derived that selects the resistor divider and the hys-
teresis resistor:
Assuming, Rt = 1K
, 1%:
Where:
Rt = Top resistor of the resistor divider
R
H
= Hysteresis resistor connected between pins 3
and 4 of the IRU3034
Vo = Selected output ripple (typically 1% to 2% of
output voltage)
Assuming,
Vo=44mV:
Select R
H
= 249K
, 1%
Fs =
= 0.087
(3.5
×
0.044)
Fs = 87KHz
[
(3.38
×
(1-0.78)
×
0.018)
D = (3.38 + 0.5) / 5 = 0.78
Fs =
[
(Vo
×
(1-D)
×
ESR)
]
/ (L
×
Vo) (MHz)
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