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參數(shù)資料
| 型號： | IRU3073 |
| 廠商： | International Rectifier |
| 英文描述： | SYNCHRONOUS PWM CONTROLLER WITH OVER-CURRENT PROTECTION / LDO CONTROLLER |
| 中文描述： | 同步PWM控制器，帶有過電流保護(hù)/ LDO控制器 |
| 文件頁數(shù)： | 13/21頁 |
| 文件大?。?/td> | 236K |
| 代理商： | IRU3073 |
IRU3073
13
Rev. 1.0
09/17/03
www.irf.com
LDO Section
Output Voltage Programming
Output voltage for LDO is programmed by reference volt-
age and external voltage divider. The Fb2 pin is the in-
verting input of the error amplifier, which is internally ref-
erenced to 0.8V. The divider is ratioed to provide 0.8V at
the Fb2 pin when the output is at its desired value. The
output voltage is defined by using the following equation
V
OUT2
= V
REF
×
1+
( )
For:
V
OUT2
= 1.6V
V
REF
= 0.8V
R
10
= 1K
Results to R
7
=1K
Figure 13 - Programming the output voltage for LDO.
LDO Power MOSFET Selection
The first step in selecting the power MOSFET for the
linear regulator is to select the maximum R
DS(ON)
based
on the input to the dropout voltage and the maximum
load current.
R
DS(ON)
=V
IN(LDO)
- V
OUT2
I
OUT2
For:
V
IN(LDO)
= 2.5V
V
OUT2
= 1.6V
I
OUT2
= 2A
Results to: R
DS(ON)(MAX)
= 0.45
Note that since the MOSFET R
DS(ON)
increases with tem-
perature, this number must be divided by ~1.5 in order
to find the R
DS(ON)(MAX)
at room temperature. The IRLR2703
has a maximum of 0.065
R
DS(ON)
at room temperature,
which meets our requirements.
R
7
R
10
Layout Consideration
The layout is very important when designing high fre-
quency switching converters. Layout will affect noise
pickup and can cause a good design to perform with
less than expected results.
Start to place the power components. Make all the con-
nections in the top layer with wide, copper filled areas.
The inductor, output capacitor and the MOSFET should
be close to each other as possible. This helps to reduce
the EMI radiated by the power traces due to the high
switching currents through them. Place input capacitor
directly to the drain of the high-side MOSFET. To reduce
the ESR, replace the single input capacitor with two par-
allel units. The feedback part of the system should be
kept away from the inductor and other noise sources
and be placed close to the IC. In multilayer PCB, use
one layer as power ground plane and have a separate
control circuit ground (analog ground), to which all sig-
nals are referenced. The goal is to localize the high cur-
rent path to a separate loop that does not interfere with
the more sensitive analog control function. These two
grounds must be connected together on the PC board
layout at a single point.
Figure 14 - Switching Frequency vs. Rt.
0
50
100
150
200
250
300
350
400
450
500
0
50
100
150
200
250
300
350
400
450
500
550
Rt (K
)
F
Fb2
IRU3073
V
OUT2
R
10
R
7
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